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  • Co-Seismic Internal Deformations in a Spherical Layered Earth Model
    Geophys. J. Int. (IF 2.777) Pub Date : 2020-02-17
    Liu T, Fu G, She Y, et al.

    SUMMARYUsing a numerical integral method, we deduced a set of formulae for the co-seismic internal deformation in a spherically symmetric Earth model, simultaneously taking self-gravitation, compressibility and realistically stratified structure of the Earth into account. Using these formulae, we can calculate the internal deformation at an arbitrary depth caused by an arbitrary seismic source. To demonstrate the correctness of our formulae, we compared our numerical solutions of radial functions with analytical solutions reported by Dong & Sun (2017) based on a homogeneous Earth model; we found that two sets of results agree well with each other. Our co-seismic internal Green's functions in the near field agree well with the results calculated by the formulae of Okada (1992), which also verifies our Green's functions. Finally, we calculated the Coulomb stress changes on the Japanese Islands and Northeast China induced by the Tohoku-Oki Mw 9.0 earthquake using the methods described above. We found that the effect of layered structure plays a leading role on the near field, while curvature occupies a dominant position on the deep region of the far field. Through a comparison of the Coulomb stress changes at a depth of 10 km on a layered Earth model calculated by our method along with the corresponding results of Okada (1992), we found that the discrepancy between them in near field was ∼31.5 per cent, and that of far field was > 100 per cent of the signals.

    更新日期:2020-02-18
  • Laboratory study on the effects of fault waviness on granodiorite stick-slip instabilities
    Geophys. J. Int. (IF 2.777) Pub Date : 2020-02-18
    Zhuo Y, Guo Y, Chen S, et al.

    SUMMARYThe effects of fault waviness on the fault slip modes are unclear. Laboratory study on the effects of the centimeter-scale fault contact distribution, which is mainly controlled by the fault waviness, on granodiorite stick-slip instabilities may help to unveil some aspects of the problem. The fast and slow stick-slip motions were separately generated in two granodiorite samples of the same roughness but different fault contact distributions in the centimeter scale in the laboratory. The experimental results show the following: (1) the fault with the small contact area and heterogeneous contact distribution generates fast stick-slip instabilities, while the fault with the large contact area and homogeneous contact distribution produces slow stick-slip events; (2) the nucleation processes of the fast stick-slip events are characterized by abrupt changes once the nucleation zones expand to the critical nucleation length that is observed to be shorter than the fault length, while the slow stick-slip events appear as a gradual evolution of the nucleation zones leading to total fault sliding. These indicate that, unlike the micron-scale fault contact distribution controlled by roughness, which depends on the grain size of the abrasives used for lapping the fault surface, the centimeter-scale fault contact distribution, which depends mainly on the waviness of the fault surface profile, also plays an important role in the fault slip modes. In addition, the effects of the fault waviness on the fault friction properties are preliminarily analyzed based on the rate- and state- dependent friction law.

    更新日期:2020-02-18
  • On the measurement of seismic traveltime changes in the time–frequency domain with wavelet cross-spectrum analysis
    Geophys. J. Int. (IF 2.777) Pub Date : 2019-10-30
    Mao S, Mordret A, Campillo M, et al.

    SUMMARYThe spatial distribution of temporal variations in seismic wavespeed is key to understanding the sources and physical mechanisms of various geophysical processes. The imaging of wavespeed changes requires accurate measurements of traveltime delays with both high lapse-time and frequency resolutions. However, traditional methods for time-shift estimation suffer from their limited resolutions. In this paper we propose a new approach, the wavelet method, to measure the traveltime changes in the time–frequency domain. This method is based on wavelet cross-spectrum analysis, and can provide optimal time–frequency joint resolution while being computationally efficient. It can deal not only with coda but also dispersive surface waves even in the presence of cycle skipping. Using synthetic coda, we show that the wavelet method can retrieve traveltime shifts more stably and accurately than traditional methods. An application at Salton Sea Geothermal Field indicates that the wavelet method is less affected by spectral smearing and better discriminates dv/v variations at different frequencies. Furthermore, upon investigations on synthetic coda, we illustrate that the bias on dv/v measurements due to changes in source frequency content is likely to be negligible, either with traditional methods or with the new wavelet method. The wavelet method sheds lights on applications of seismic interferometry that aim to locate changes in space.

    更新日期:2020-02-18
  • Heterogeneous oceanic mass distribution in GRACE observations and its leakage effect
    Geophys. J. Int. (IF 2.777) Pub Date : 2020-01-10
    Yi S, Heki K.

    SUMMARYSignal leakage between the land and ocean is a challenge in using Gravity Recovery and Climate Experiment (GRACE) observation data to study global mass redistributions. Although the leakage occurs in both directions, more attention has been paid to the land-to-ocean leakage and less to the ocean-to-land leakage. Here, we show that the ocean-to-land leakage is non-uniform and non-negligible and propose a new forward modelling method to fully consider bi-directional leakages with the help of the global Ocean ReAnalysis System ORAS5. This observation-driven model could significantly reduce the variations in ocean grids and thus decrease the ocean-to-land leakage. The results with different treatment of the ocean signal leakage are compared. We find that failing to consider the ocean-to-land leakage will cause an underestimation of ∼20 per cent in the seasonal variation and will introduce a bias of several giga-tons in the secular trend. Although the uniform and non-uniform model have similar results in the global average of seasonal mass variations, the non-uniform ocean model is necessary in most places, especially near the Arctic Ocean, the Sea of Japan and the Gulf of Carpentaria. Despite these achievements, we also point out that there is still much room for improvement in ocean mass models, particularly in long-term trends. Our results indicate the importance of the ocean-to-land leakage correction in the mass estimation in coastal land areas using the GRACE data.

    更新日期:2020-02-18
  • A rapid four-dimensional resistivity data inversion method using temporal segmentation
    Geophys. J. Int. (IF 2.777) Pub Date : 2020-01-10
    Liu B, Pang Y, Mao D, et al.

    SUMMARY4-D electrical resistivity tomography (ERT), an important geophysical method, is widely used to observe dynamic processes within static subsurface structures. However, because data acquisition and inversion consume large amounts of time, rapid changes that occur in the medium during a single acquisition cycle are difficult to detect in a timely manner via 4-D inversion. To address this issue, a scheme is proposed in this paper for restructuring continuously measured data sets and performing GPU-parallelized inversion. In this scheme, multiple reference time points are selected in an acquisition cycle, which allows all of the acquired data to be sequentially utilized in a 4-D inversion. In addition, the response of the 4-D inversion to changes in the medium has been enhanced by increasing the weight of new data being added dynamically to the inversion process. To improve the reliability of the inversion, our scheme uses actively varied time-regularization coefficients, which are adjusted according to the range of the changes in model resistivity; this range is predicted by taking the ratio between the independent inversion of the current data set and historical 4-D inversion model. Numerical simulations and experiments show that this new 4-D inversion method is able to locate and depict rapid changes in medium resistivity with a high level of accuracy.

    更新日期:2020-02-18
  • Pseudo-linear-array analysis of passive surface waves based on beamforming
    Geophys. J. Int. (IF 2.777) Pub Date : 2020-01-11
    Liu Y, Xia J, Cheng F, et al.

    SUMMARYLinear arrays are usually deployed for passive surface-wave investigations because of their high efficiency and convenience. In populated urban areas, it is almost impossible to set up a 2-D array in terms of the restriction from the existing infrastructures. The limited azimuthal coverage, however, lacks the ability to attenuate velocity overestimation caused by directional noise sources. We came up with a novel idea to compensate the azimuthal coverage by adding two more offline receivers to a conventional linear array, which is called pseudo-linear-array analysis of passive surface waves (PLAS). We used a beamforming algorithm to capture noise sources distribution and extract accurate dispersion curves. We used array response function to explain the superiority of the pseudo-linear array over the linear array and present the basic workflow of PLAS. Synthetic tests and field examples demonstrated the feasibility of PLAS to measure unbiased dispersion image. Comparison with mostly used passive surface wave methods (refraction microtremor, multichannel analysis of passive surface waves, spatial autocorrelation method, frequency–wavenumber analysis) suggested that PLAS can serve as an alternative passive surface wave method, especially in urban areas with restricted land accessibility and short-time acquisition demands.

    更新日期:2020-02-18
  • Atmospheric infrasound generation by ocean waves in finite depth: unified theory and application to radiation patterns
    Geophys. J. Int. (IF 2.777) Pub Date : 2020-01-11
    De Carlo M, Ardhuin F, Le Pichon A.

    SUMMARYBetween 0.1 and 0.5 Hz, infrasound signals recorded in the atmosphere are dominated by ocean-generated noise called microbaroms. Microbaroms propagate through the atmosphere over thousands of kilometres due to low absorption and efficient ducting between the ground and the stratopause. Different theoretical models have been developed to characterize the source of microbaroms, all based on the second-order nonlinear interaction of ocean waves. While early theories considered an infinite ocean depth and a source radiation depending on the acoustic wave elevation angle, other works have approximated the radiation pattern as a monopole, and found a considerable effect of the water depth. This paper reviews these models and extends the previous theories to the combined effects of both finite depth ocean and source directivity in both elevation and azimuth angles. It is found that the water depth has a negligible effect for the near-horizontally propagating acoustic waves that should dominate the measured microbarom records. Another important result is that the microbarom azimuthal variation can be highly directive locally, but it generally becomes isotropic when integrated over a realistic source region.

    更新日期:2020-02-18
  • A new method of improving global geopotential models regionally using GNSS/levelling data
    Geophys. J. Int. (IF 2.777) Pub Date : 2020-02-03
    Liang W, Pail R, Xu X, et al.

    SUMMARYIn this paper, a new method for regionally improving global geopotential models (GGMs) with global navigation satellite system (GNSS)/levelling data is proposed. In this method, the GNNS/levelling data are at first converted to disturbing potential data with inverse Bruns’ formula. Then the systematic errors in disturbing potential data are removed with a three-parameter correction surface. Afterwards, the disturbing potential data on the Earth's surface are downward continued to the surface of an inner sphere with inverse Poisson's integral equation. Global disturbing potential data on the whole sphere could be achieved with combination of the downward continued data and the GGM-derived data. At last, the final regionally improved geopotential model (RIGM) could be recovered from the disturbing potential data using least-squares method. Four RIGM models for Qingdao (QD) are determined based on four different sets of GNSS/levelling data points to validate the capability of the method. The standard deviation of height anomaly errors of RIGM-QDs are nearly 25 and 30 per cent on average smaller than Earth Gravity Model 2008 (EGM2008) on checkpoints and data points, respectively. This means that the RIGM-QDs fit better to the GNSS/levelling network in this area than EGM2008. The results show that the proposed method is successful at improving GGMs in regional area with regional GNSS/levelling data.

    更新日期:2020-02-18
  • Impact of model inconsistencies on reconstructions of past mantle flow obtained using the adjoint method
    Geophys. J. Int. (IF 2.777) Pub Date : 2020-01-20
    Colli L, Bunge H, Oeser J.

    SUMMARYThe adjoint method is a powerful technique to compute sensitivities (Fréchet derivatives) with respect to model parameters, allowing one to solve inverse problems where analytical solutions are not available or the cost to determine many times the associated forward problem is prohibitive. In Geodynamics it has been applied to the restoration problem of mantle convection—that is, to reconstruct past mantle flow states with dynamic models by finding optimal flow histories relative to the current model state—so that poorly known mantle flow parameters can be tested against observations gleaned from the geological record. By enabling us to construct time dependent earth models the adjoint method has the potential to link observations from seismology, geology, mineral physics and palaeomagnetism in a dynamically consistent way, greatly enhancing our understanding of the solid Earth system. Synthetic experiments demonstrate for the ideal case of no model error and no data error that the adjoint method restores mantle flow over timescales on the order of a transit time (≈100 Myr). But in reality unavoidable limitations enter the inverse problem in the form of poorly known model parameters and uncertain state estimations, which may result in systematic errors of the reconstructed flow history. Here we use high-resolution, 3-D spherical mantle circulation models to perform a systematic study of synthetic adjoint inversions, where we insert on purpose a mismatch between the model used to generate synthetic data and the model used for carrying out the inversion. By considering a mismatch in rheology, final state and history of surface velocities we find that mismatched model parameters do not inhibit misfit reduction: the adjoint method still produces a flow history that fits the estimated final state. However, the recovered initial state can be a poor approximation of the true initial state, where reconstructed and true flow histories diverge exponentially back in time and where for the more divergent cases the reconstructed initial state includes physically implausible structures, especially in and near the thermal boundary layers. Consequently, a complete reduction of the cost function may not be desirable when the goal is a best fit to the initial condition. When the estimated final state is a noisy low-pass version of the true final state choosing an appropriate misfit function can reduce the generation of artefacts in the initial state. While none of the model mismatches considered in this study, taken singularly, results in a complete failure of the recovered flow history, additional work is needed to assess their combined effects.

    更新日期:2020-02-18
  • Propagation of distinct Love-wave pulses from regional to teleseismic distances in continental and oceanic environments
    Geophys. J. Int. (IF 2.777) Pub Date : 2020-01-16
    Furumura T, Kennett B.

    SUMMARYSurface waves are usually dispersive with long wave trains and steady decay of amplitude with distance. However, if the group velocity is nearly constant for a span of periods a strong pulse is produced that retains its amplitude for large distances. This situation arises for the fundamental mode of Love waves in the period band 40–500 s for crust and mantle structures with a positive gradient of S wave speed in the uppermost mantle. Such a distinct Love-wave pulse with limited dispersion observed at teleseismic distance is termed the G wave in honour of Gutenberg. The long-period G-wave pulse caused by large earthquakes carries a large amount of energy to substantial distances, with significant effects across the globe, for example event triggering. A similar G-type Love-wave pulse with a much shorter-period of 10–20 s is generated for crustal structures without thick sediment. Such pulses produce anomalously large ground displacement at near-regional distances with, for example an overestimate of surface wave magnitude. We investigate the generation and propagation mechanism of the G-type Love-wave pulses in the crust and upper-mantle with the analysis of observed strong motion records from the Mw 6.2 2016 Central Tottori earthquake and the Mw 9.0 2011 Off Tohoku earthquake in Japan, in conjunction with 3-D finite-difference simulation of seismic wave propagation and analysis of dispersion curves.

    更新日期:2020-02-18
  • Fast collocation for Moho estimation from GOCE gravity data: the Iran case study
    Geophys. J. Int. (IF 2.777) Pub Date : 2020-01-14
    Heydarizadeh Shali H, Sampietro D, Safari A, et al.

    SUMMARYThe study of the discontinuity between crust and mantle beneath Iran is still an open issue in the geophysical community due to its various tectonic features created by the collision between the Iranian and Arabian Plate. For instance in regions such as Zagros, Alborz or Makran, despite the number of studies performed, both by exploiting gravity or seismic data, the depth of the Moho and also interior structure is still highly uncertain. This is due to the complexity of the crust and to the presence of large short wavelength signals in the Moho depth. GOCE observations are capable and useful products to describe the Earth’s crust structure either at the regional or global scale. Furthermore, it is plausible to retrieve important information regarding the structure of the Earth’s crust by combining the GOCE observations with seismic data and considering additional information. In the current study, we used as observation a grid of second radial derivative of the anomalous gravitational potential computed at an altitude of 221 km by means of the space-wise approach, to study the depth of the Moho. The observations have been reduced for the gravitational effects of topography, bathymetry and sediments. The residual gravity has been inverted accordingly to a simple two-layer model. In particular, this guarantees the uniqueness of the solution of the inverse problem which has been regularized by means of a collocation approach in the frequency domain. Although results of this study show a general good agreement with seismically derived depths with a root mean square deviation of 6 km, there are some discrepancies under the Alborz zone and also Oman sea with a root mean square deviation up 10 km for the former and an average difference of 3 km for the latter. Further comparisons with the natural feature of the study area, for instance, active faults, show that the resulting Moho features can be directly associated with geophysical and tectonic blocks.

    更新日期:2020-02-18
  • Noise-based ballistic wave passive seismic monitoring – Part 2: surface waves
    Geophys. J. Int. (IF 2.777) Pub Date : 2020-02-18
    Mordret A, Courbis R, Brenguier F, et al.

    SUMMARYWe develop a new method to monitor and locate seismic velocity changes in the subsurface using seismic noise interferometry. Contrary to most ambient noise monitoring techniques, we use the ballistic Rayleigh waves computed from 30 d records on a dense nodal array located above the Groningen gas field (the Netherlands), instead of their coda waves. We infer the daily relative phase velocity dispersion changes as a function of frequency and propagation distance with a cross-wavelet transform processing. Assuming a 1-D velocity change within the medium, the induced ballistic Rayleigh wave phase shift exhibits a linear trend as a function of the propagation distance. Measuring this trend for the fundamental mode and the first overtone of the Rayleigh waves for frequencies between 0.5 and 1.1 Hz enables us to invert for shear wave daily velocity changes in the first 1.5 km of the subsurface. The observed deep velocity changes (±1.5 per cent) are difficult to interpret given the environmental factors information available. Most of the observed shallow changes seem associated with effective pressure variations. We observe a reduction of shear wave velocity (–0.2 per cent) at the time of a large rain event accompanied by a strong decrease in atmospheric pressure loading, followed by a migration at depth of the velocity decrease. Combined with P-wave velocity changes observations from a companion paper, we interpret the changes as caused by the diffusion of effective pressure variations at depth. As a new method, noise-based ballistic wave passive monitoring could be used on several dynamic (hydro-)geological targets and in particular, it could be used to estimate hydrological parameters such as the hydraulic conductivity and diffusivity.

    更新日期:2020-02-18
  • Noise-based ballistic wave passive seismic monitoring. Part 1: body waves
    Geophys. J. Int. (IF 2.777) Pub Date : 2020-02-18
    Brenguier F, Courbis R, Mordret A, et al.

    SUMMARYUnveiling the mechanisms of earthquake and volcanic eruption preparation requires improving our ability to monitor the rock mass response to transient stress perturbations at depth. The standard passive monitoring seismic interferometry technique based on coda waves is robust but recovering accurate and properly localized P- and S-wave velocity temporal anomalies at depth is intrinsically limited by the complexity of scattered, diffracted waves. In order to mitigate this limitation, we propose a complementary, novel, passive seismic monitoring approach based on detecting weak temporal changes of velocities of ballistic waves recovered from seismic noise correlations. This new technique requires dense arrays of seismic sensors in order to circumvent the bias linked to the intrinsic high sensitivity of ballistic waves recovered from noise correlations to changes in the noise source properties. In this work we use a dense network of 417 seismometers in the Groningen area of the Netherlands, one of Europe's largest gas fields. Over the course of 1 month our results show a 1.5 per cent apparent velocity increase of the P wave refracted at the basement of the 700-m-thick sedimentary cover. We interpret this unexpected high value of velocity increase for the refracted wave as being induced by a loading effect associated with rainfall activity and possibly canal drainage at surface. We also observe a 0.25 per cent velocity decrease for the direct P-wave travelling in the near-surface sediments and conclude that it might be partially biased by changes in time in the noise source properties even though it appears to be consistent with complementary results based on ballistic surface waves presented in a companion paper and interpreted as a pore pressure diffusion effect following a strong rainfall episode. The perspective of applying this new technique to detect continuous localized variations of seismic velocity perturbations at a few kilometres depth paves the way for improved in situ earthquake, volcano and producing reservoir monitoring.

    更新日期:2020-02-18
  • Transfer functions between teleseismic data components
    Geophys. J. Int. (IF 2.777) Pub Date : 2020-02-17
    Frederiksen A.

    SummaryDifferent data components of teleseismic waveforms are related by transfer functions that depend only on receiver-side structure. This is the common basis of a number of teleseismic techniques, including receiver functions and shear-wave splitting analysis. Common trace misfits used in these analysis techniques are shown to be equivalent to band-limited comparisons of real and synthetic transfer functions. The data deconvolution used in receiver function analysis leads to reduced structural resolution compared to direct trace-based misfits such as cross-convolution, with direct transfer function modelling of a data trace having the particular advantage of a physically meaningful misfit. Having established that the inter-trace transfer function contains all available structural information, the sensitivity of transfer functions to structure is examined for a series of teleseismic scenarios. Transfer functions for the teleseismic P coda show a strong sensitivity to shallow low-velocity structures such as sedimentary basins; the Sp precursors used in S receiver functions are less affected. Examination of transfer functions for shear-wave splitting shows that response complexities occur at frequencies too high to be observable in teleseismic studies, and that the dominant control on the response is the splitting intensity.

    更新日期:2020-02-18
  • Gradient based First-order and second- order Filters for the Demarcation of Continental-Oceanic Boundaries using Satellite Gravity data
    Geophys. J. Int. (IF 2.777) Pub Date : 2020-02-14
    Maurya V, Fontes S, Oliveira V, Jr, et al.

    SUMMARYDemarcating the continental-oceanic boundary (COB) is extremely important as it contributes to an understanding of the tectonic environments of marginal basins through plate reconstructions. In general, the COB is estimated by jointly interpreting the features of the basement geomorphology, bathymetry, and geophysical datasets, including gravity, magnetic, seismic, and geochemical data. We propose a novel methodology named the tilt-theta angles correlation (TTAC), a second-order filtering approach to estimate the COB using high-resolution satellite gravity data. This method computes the angular difference or correlation angle between the vectors computed from the gradients of the tilt and theta derivative-based first-order filters. Oceanic regions generally exhibit small correlation angles (∼0°), thereby indicating a direct correlation; in contrast, continental regions depict large correlation angles (∼180°) and thus display an inverse correlation. The TTAC method marks the COB at the spatial position where the correlation angles abruptly shift from inverse to direct. Tests using synthetic data and the larger spectral energy content of TTAC for long wavelengths over tilt, theta and minus of the sign of vertical derivative (-SiVD) filters attest the effectiveness of the new methodology over first-order derivative based filters. Additionally, tests utilizing real data over both the West African Transform Margin (WATM) and across the South Atlantic Conjugate Margin (SACM) provide favourable results compared with the previous methodologies used to demarcate the COB. The COB estimates derived using the new methodology are consistent with the COB estimates obtained during recent plate-reconstruction studies. We also compare our results with an earlier interpretation along a conjugate margin encompassing magma-poor and magma-rich regions. In the complex rifted environments of the SACM, in which volcanic rocks are dominant, the TTAC methodology substantially improves the previous COB estimates, thereby establishing it as an efficient edge marker by simultaneously reducing small-scale geological noise and enhancing regional-scale geological contributions.

    更新日期:2020-02-14
  • Magnetometric resistivity tomography using chaos polynomial expansion
    Geophys. J. Int. (IF 2.777) Pub Date : 2020-02-14
    Vu M, Jardani A, Revil A, et al.

    SUMMARYWe present an inversion algorithm to reconstruct the spatial distribution of the electrical conductivity from the analysis of magnetometric resistivity (MMR) data acquired at the ground surface. We first review the theoretical background of magnetometric resistivity connecting the generation of a magnetic field in response to the injection of a low-frequency current source and sink in the ground given a known distribution of electrical conductivity in the subsurface of the Earth. The forward modelling is based on sequentially solving the Poisson equation for the electrical potential distribution and the magnetostatic (Biot and Savart) equation for the magnetic field. Then, we introduce a Gauss-Newton inversion algorithm in which the logarithm of the electrical conductivity field is parameterized by using the chaos polynomial expansion in order to reduce the number of model parameters. To illustrate how the method works, the algorithm is successfully applied on four synthetic models with 3D heterogeneous distribution of the electrical conductivity. Finally, we apply our algorithm to a field case study in which seepage was known to be occurring along an embankment of a headrace channel to a power station.

    更新日期:2020-02-14
  • Crustal structure of the Azores Archipelago from Rayleigh wave ellipticity data
    Geophys. J. Int. (IF 2.777) Pub Date : 2020-02-14
    Ferreira A, Marignier A, Attanayake J, et al.

    Determining the crustal structure of ocean island volcanoes is important to understand the formation and tectonic evolution of the oceanic lithosphere and tectonic swells in marine settings, and to assess seismic hazard in the islands. The Azores Archipelago is located near a triple junction system and is possibly under the influence of a mantle plume, being at the locus of a wide range of geodynamical processes. However, its crustal structure is still poorly constrained and debated due to the limited seismic coverage of the region and the peculiar linear geometry of the islands. To address these limitations, in this study we invert teleseismic Rayleigh wave ellipticity measurements for 1-D shear-wave speed (VS) crustal models of the Azores Archipelago. Moreover, we test the reliability of these new models by using them in independent moment tensor inversions of local seismic data and demonstrate that our models improve the waveform fit compared to previous models. We find that data from the westernmost seismic stations used in this study require a shallower Moho depth (∼10 km) than data from stations in the eastern part of the archipelago (∼13-16 km). This apparent increase in the Moho depth with increasing distance from the mid-Atlantic ridge (MAR) is expected. However, the rate at which Moho deepens away from the MAR is greater than that predicted from a half-space cooling model, suggesting that local tectonic perturbations have modified crustal structure. The 1-D VS models obtained beneath the westernmost seismic stations also show higher wave speeds than for the easternmost stations, which correlates well with the ages of the islands except Santa Maria Island. We interpret the relatively low VS profile found beneath Santa Maria Island as resulting from underplating, which agrees with previous geological studies of the island. Compared to a recent receiver function study of the region, the shallow structure (top ∼2 km) in our models shows lower shear wave speed, which may have important implications for future hazard studies of the region. More generally, the new seismic crustal models we present in this study will be useful to better understand the tectonics, seismicity, moment tensors and strong ground motions in the region.

    更新日期:2020-02-14
  • Frequency-dependent anisotropy due to two orthogonal sets of mesoscale fractures in porous media
    Geophys. J. Int. (IF 2.777) Pub Date : 2020-02-14
    Shuai D, Stovas A, Wei J, et al.

    Seismic anisotropy can occur in rocks which have complicated internal structures and thin layering. Wave-induced fluid flow (WIFF) is one of the major cause of elastic wave dispersion and anisotropy. The principle goal of this paper is to combine the effects of WIFF and layer-induced anisotropy in orthorhombic models which are often used in the seismic industry nowadays to describe azimuthal and polar anisotropy. We derive the effective frequency-dependent anisotropy parameters based on the Chapman model which accounts for the WIFF mechanism. First, we summarize two major problems to establish the link between frequency-dependent seismic anisotropy and the multiple sets of fractures with different scales and orientations. Then we specify the multiple mescoscale fractures to be vertical and orthogonal so as to simplify the rock physics model to be an ORT (orthorhombic) medium. We also give the explicit expressions for the effective stiffness and Thomsen-style parameters (vP0, vS0, ε1, ε2, γ1, γ2, δ1, δ2, δ3). Finally, we derive the effective frequency-dependent anisotropy parameters for ORT multiple layers using Backus averaging under the approximation of weak-contrast between layers. We also investigate the influence of frequency, fracture parameters (density and scale), effective porosity and volume fraction on the Thomsen-style parameters.

    更新日期:2020-02-14
  • Seismic inversion of soil damping and stiffness using multi-channel analysis of surface wave measurements in the marine environment
    Geophys. J. Int. (IF 2.777) Pub Date : 2020-02-14
    Armstrong M, Ravasio M, Versteijlen W, et al.

    SUMMARYDetermination of soil material damping is known to be difficult and uncertain, especially in the offshore environment. Using an advanced inversion methodology based on multi-channel spectral analysis, Scholte and Love wave measurements are used to characterize sub-sea soil from a North Sea site. After normalization, a determinant-based objective function is used in a genetic algorithm optimization to estimate the soil shear-modulus. The inverted shear-modulus profile is comparable to previously published results for the same data, although a higher degree of certainty is achieved in the near-surface layers. The half-power bandwidth method is used for extracting the attenuation curve from the measurements and efficient reference data points are chosen based on wavelet compression. The material-damping ratio inversion is performed using a modified stochastic optimization algorithm. Accounting for measurement errors, the material-damping ratio profile is retrieved from the fundamental-mode Scholte wave with a high degree of certainty. Furthermore, a method is proposed for identifying the frequency dependence of the material-damping ratio from in-situ measurements. No evidence for frequency dependence is found and the small-strain soil material-damping ratio at this site can be said to be frequency independent for the measured conditions.

    更新日期:2020-02-14
  • Probabilistic estimation of structure coefficients and their uncertainties, for inner-core sensitive modes, using matrix autoregression
    Geophys. J. Int. (IF 2.777) Pub Date : 2020-02-13
    Pachhai S, Masters G, Laske G.

    SUMMARYNormal-mode structure coefficients are crucial observations to infer the velocity, density and attenuation structure of the deep Earth interior, but estimating these coefficients from Earth's normal mode spectra is a non-linear inverse problem. Additionally, complete source information is typically unknown for large earthquakes, and there is a trade-off between the earthquake source and attenuation. Therefore, proper estimation of elastic and anelastic structure coefficients with their uncertainties becomes challenging. Here, we combine a matrix autoregression and a fully non-linear probabilistic sampling to address existing limitations. After successful feasibility experiments using synthetic data with noise, we apply this combined approach to the data for 19 inner-core sensitive spheroidal (S) modes measured for earthquakes from 1994 to 2016. We further implement a model selection criterion to assess whether anelastic structure is significant. Our model selection criterion indicates that anelastic structure coefficients are required only for modes with strong shear-wave energy in the inner core. Inversion results also show a strong correlation between elastic and anelastic splitting functions for these modes. This indicates that the seismic waves travel faster and strongly attenuate along polar paths such that the m = 0 singlet remains poorly observed for these modes.

    更新日期:2020-02-13
  • The exponentiated phase measurement, and objective-function hybridization for adjoint waveform tomography
    Geophys. J. Int. (IF 2.777) Pub Date : 2020-02-13
    Yuan Y, Bozdağ E, Ciardelli C, et al.

    SUMMARYSeismic tomography has arrived at the threshold of the era of big data. However, how to extract information optimally from every available time series remains a challenge; one that is directly related to the objective function chosen as a distance metric between observed and synthetic data. Time-domain cross-correlation and frequency-dependent multitaper traveltime measurements are generally tied to window selection algorithms in order to balance the amplitude differences between seismic phases. Even then, such measurements naturally favor the dominant signals within the chosen windows. Hence, it is difficult to select all usable portions of seismograms with any sort of optimality. As a consequence, information ends up being lost, in particular from scattered waves. In contrast, measurements based on instantaneous phase allow extracting information uniformly over the seismic records without requiring their segmentation. And yet, measuring instantaneous phase, like any other phase measurement, is impeded by phase wrapping. In this paper, we address this limitation by using a complex-valued phase representation that we call ‘exponentiated phase’. We demonstrate that the exponentiated phase is a good substitute for instantaneous-phase measurements. To assimilate as much information as possible from every seismogram while tackling the nonlinearity of inversion problems, we discuss a flexible hybrid approach to combine various objective functions in adjoint seismic tomography. We focus on those based on the exponentiated phase, to take into account relatively small-magnitude scattered waves; on multitaper measurements of selected surface waves; and on cross-correlation measurements on specific windows to select distinct body-wave arrivals. Guided by synthetic experiments, we discuss how exponentiated-phase, multitaper, and cross-correlation measurements, and their hybridization, affect tomographic results. Despite their use of multiple measurements, the computational cost to evaluate gradient kernels for the objective functions is scarcely affected, allowing for issues with data quality and measurement challenges to be simultaneously addressed efficiently.

    更新日期:2020-02-13
  • On Formulations of Compressible Mantle Convection
    Geophys. J. Int. (IF 2.777) Pub Date : 2020-02-13
    Gassmöller R, Dannberg J, Bangerth W, et al.

    SUMMARYMantle convection and long-term lithosphere dynamics in the Earth and other planets can be treated as the slow deformation of a highly viscous fluid, and as such can be described using the compressible Navier-Stokes equations. Since on Earth-sized planets the influence of compressibility is not a dominant effect, density deviations from a reference profile are at most on the order of a few percent, and using the full governing equations poses numerical challenges, most modelling studies have simplified the governing equations. Common approximations assume a temporally constant, but depth-dependent reference profile for the density (the Anelastic Liquid Approximation), or drop compressibility altogether and use a constant reference density (the Boussinesq Approximation). In most previous studies of mantle convection and crustal dynamics, one can assume that the error introduced by these approximations was small compared to the errors that resulted from poorly constrained material behavior and limited numerical accuracy. However, as model parameterizations have become more realistic, and model resolution has improved, this may no longer be the case and the error due to using simplified conservation equations might no longer be negligible: while such approximations may be reasonable for models of mantle plumes or slabs traversing the whole mantle, they may be unsatisfactory for layered materials experiencing phase transitions or materials undergoing significant heating or cooling. For example at boundary layers or close to dynamically changing density gradients, the error arising from the use of the aforementioned compressibility approximations can be the dominant error source, and common approximations may fail to capture the physical behaviour of interest. In this paper, we discuss new formulations of the continuity equation that include dynamic density variations due to temperature, pressure, and composition without using a reference profile for the density. We quantify the improvement in accuracy relative to existing formulations in a number of benchmark models, and evaluate for which practical applications these effects are important. Finally, we consider numerical aspects of the new formulations. We implement and test these formulations in the freely available community software aspect, and use this code for our numerical experiments.

    更新日期:2020-02-13
  • Eigenvector models for solving the seismic inverse problem for the Helmholtz equation
    Geophys. J. Int. (IF 2.777) Pub Date : 2020-01-08
    Faucher F, Scherzer O, Barucq H.

    SUMMARYWe study the seismic inverse problem for the recovery of subsurface properties in acoustic media. In order to reduce the ill-posedness of the problem, the heterogeneous wave speed parameter is represented using a limited number of coefficients associated with a basis of eigenvectors of a diffusion equation, following the regularization by discretization approach. We compare several choices for the diffusion coefficient in the partial differential equations, which are extracted from the field of image processing. We first investigate their efficiency for image decomposition (accuracy of the representation with respect to the number of variables). Next, we implement the method in the quantitative reconstruction procedure for seismic imaging, following the full waveform inversion method, where the difficulty resides in that the basis is defined from an initial model where none of the actual structures is known. In particular, we demonstrate that the method may be relevant for the reconstruction of media with salt-domes. We use the method in 2-D and 3-D experiments, and show that the eigenvector representation compensates for the lack of low-frequency information, it eventually serves us to extract guidelines for the implementation of the method.

    更新日期:2020-02-13
  • Present-day interseismic deformation characteristics of the Beng Co-Dongqiao conjugate fault system in central Tibet: implications from InSAR observations
    Geophys. J. Int. (IF 2.777) Pub Date : 2020-01-08
    Yongsheng L, Yunfeng T, Chen Y, et al.

    SUMMARYNumerous V-shaped conjugate strike-slip fault systems distributed between the Lhasa block and the Qiangtang block serve as some of the main structures accommodating the eastward motion of the Tibetan Plateau. The Beng Co-Dongqiao conjugate fault system is a representative section, and determining its tectonic environment is a fundamental issue for understanding the dynamic mechanism of the V-shaped conjugate strike-slip fault systems throughout central Tibet. In this paper, we investigate the deformation rates of the Beng Co-Dongqiao conjugate faults using 3 yr of SAR data from both ascending and descending tracks of Sentinel-1 satellites. Only interferograms with a long temporal baseline were used to increase the proportion of the deformation signals. The external atmospheric delay product and the InSAR stacking strategy were employed to reduce various errors in the large-spatial-coverage Sentinel-1 data. The InSAR results revealed that the fault-parallel deformation velocities along the eastern and western segments of the Beng Co fault are 5 ± 1 mm/yr and 2.5 ± 1 mm/yr, respectively. The second invariant of the horizontal strain rates shows that the accumulated strain is centered on the eastern segment of the Beng Co Fault and Gulu rift. The velocity and strain rate fields show that the Anduo-Peng Co faults may be paired with the Beng Co fault to form a new conjugate system and the tectonic transformation between the Beng Co fault and Gulu rift. These results can better explain the tectonic deformation environment of the Beng Co-Dongqiao conjugate fault system and provide insights on the crustal dynamics throughout the entire plateau interior.

    更新日期:2020-02-13
  • Spatial variation in the present-day stress field and tectonic regime of Northeast Tibet from moment tensor solutions of local earthquake data
    Geophys. J. Int. (IF 2.777) Pub Date : 2020-01-08
    Pan Z, He J, Shao Z.

    SUMMARYFocal mechanism solutions and their predicted stress pattern can be used to investigate tectonic deformation in seismically active zones and contribute to understanding and constraining the kinematic patterns of the outward growth and uplift of the Tibetan Plateau. Herein, we determined the focal mechanisms of 398 earthquakes in Northeast Tibet recorded by the China National Seismic Network (CNSN) by using the cut-and-paste method. The results show that the earthquakes predominately exhibited thrust and strike-slip faulting mechanisms with very few normal events. We then combined the derived focal mechanisms with global centroid moment tensor (GCMT) catalogue solutions and previously published solutions to predict the regional distribution of the stress field through a damped linear inversion. The inversion results show that most of region is dominated by a thrust faulting regime. From the southern East Kunlun fault in the west to the northern Qilian Mountains along the Altyn Tagh fault (ATF), the maximum compression axis rotates slightly clockwise; farther to the south of the Haiyuan fault in the east, there is an evident clockwise rotation of the maximum compression axis, especially at the eastern end of the Haiyuan fault. In the Qilian Mountains, the axis of the compressive stress orientation approximately trends NE–SW, which does not markedly differ from the direction of India–Eurasia convergence, emphasizing the importance of the compressive stress in reflecting the remote effects of this continental collision. The overall spatial pattern of the principal stress axes is closely consistent with the GPS-derived horizontal surface velocity. A comparison of the stress and strain rate fields demonstrated that the orientations of the crustal stress axes and the surface strain axes were almost identical, which indicates that a diffuse model is more suitable for describing the tectonic characteristics of Northeast Tibet. Additionally, the compressive stress orientation rotated to ENE–WSW in the northern Qilian Mountains along the ATF and to ENE–WSW or E–W along the eastern part of the Haiyuan fault and its adjacent area to the south, highlighting the occurrence of strain partitioning along large left-lateral strike-slip faults or the lateral variation of crustal strength across these faults. Combining geodetic, geological and seismological results, we suggest that a hybrid model incorporating both the diffuse model associated with shortening and thickening of the upper crust and the asthenospheric flow model accounting for the low-velocity zone in the middle-lower crust may reflect the primary mode of crustal deformation in Northeast Tibet.

    更新日期:2020-02-13
  • A study of the monsoonal hydrology contribution using a 8-yr record (2010–2018) from superconducting gravimeter OSG-060 at Djougou (Benin, West Africa)
    Geophys. J. Int. (IF 2.777) Pub Date : 2020-01-15
    Hinderer J, Hector B, Riccardi U, et al.

    SUMMARYWe analyse a nearly 8-yr record (2010–2018) of the superconducting gravimeter OSG-060 located at Djougou (Benin, West Africa). After tidal analysis removing all solid Earth and ocean loading tidal contributions and correcting for the long-term instrumental drift and atmospheric loading, we obtain a gravity residual signal which is essentially a hydrological signal due to the monsoon. This signal is first compared to several global hydrology models (ERA, GLDAS and MERRA). Our superconducting gravimeter residual signal is also superimposed onto episodic absolute gravity measurements and to space gravimetry GRACE data. A further comparison is done using local hydrological data like soil moisture in the very superficial layer (0–1.2 m), water table depth and rainfall. The temporal evolution of the correlation coefficient between the gravity observation and both the soil moisture and the water table is well explained by the direct infiltration process of rain water together with the lateral transfer discharging the water table.Finally, we compute the water storage changes (WSC) using a simulation based on the physically based Parflow-CLM numerical model of the catchment, which solves the water and energy budget from the impermeable bedrock to the top of the canopy layer using the 3-D Richards equation for the water transfers in the ground, the kinematic wave equation for the surface runoff and a land surface model (CLM) for the energy budget and evapotranspiration calculation.This model forced by rain is in agreement with evapotranspiration and stream flow data and leads to simulated water storage changes that nicely fit to the observed gravity signal. This study points out the important role played by surface gravity changes in terms of a reliable proxy for water storage changes occurring in small catchments.

    更新日期:2020-02-13
  • On the importance of benchmarking algorithms under realistic noise conditions
    Geophys. J. Int. (IF 2.777) Pub Date : 2020-01-15
    Birnie C, Chambers K, Angus D, et al.

    SUMMARYTesting with synthetic data sets is a vital stage in an algorithm’s development for benchmarking the algorithm’s performance. A common addition to synthetic data sets is White, Gaussian Noise (WGN) which is used to mimic noise that would be present in recorded data sets. The first section of this paper focuses on comparing the effects of WGN and realistic modelled noise on standard microseismic event detection and imaging algorithms using synthetic data sets with recorded noise as a benchmark. The data sets with WGN underperform on the trace-by-trace algorithm while overperforming on algorithms utilizing the full array. Throughout, the data sets with realistic modelled noise perform near identically to the recorded noise data sets. The study concludes by testing an algorithm that simultaneously solves for the source location and moment tensor of a microseismic event. Not only does the algorithm fail to perform at the signal-to-noise ratios indicated by the WGN results but the results with realistic modelled noise highlight pitfalls of the algorithm not previously identified. The misleading results from the WGN data sets highlight the need to test algorithms under realistic noise conditions to gain an understanding of the conditions under which an algorithm can perform and to minimize the risk of misinterpretation of the results.

    更新日期:2020-02-13
  • Palaeoslab and plume signatures in the mantle transition zone beneath Eastern Himalaya and adjoining regions
    Geophys. J. Int. (IF 2.777) Pub Date : 2020-01-09
    Saikia D, Kumar M, Singh A.

    SUMMARYA comprehensive data set of 73 876 high quality receiver functions computed using waveforms recorded by 327 broad-band seismic stations is used to investigate the mantle transition zone (MTZ) structure beneath the eastern Himalaya, southern Tibet, Assam valley and the previously unexplored Burmese arc and Bengal basin regions. A highly variable and perturbed mantle transition zone, with depressed 410 and 660 km discontinuities, is observed beneath the Bengal basin and to the east of the eastern Himalayan syntaxis. The 410 is elevated by ∼10 km along the Himalayan collision front, while it deviates in the range of ±5 km beneath most parts of Tibet and the Himalayan Foredeep. In northern Tibet and along the Red River Fault, delayed conversions from the 410 reveal a deepening of more than 10 km. The 410 and 660 km discontinuities are uplifted by nearly 10 km beneath the Arunachal Himalaya, due to the presence of a subducting Indian lithosphere, as evident in the regional tomographic images. We observe a thick (>20 km) transition zone beneath the Burmese Arc and close to the Tengchong volcano. An uplifted 410 together with a depressed 660 km discontinuity requires presence of lithospheric slabs within the MTZ. Delayed P-to-s conversions from the 410 and 660 km discontinuities in the proximity of the Jinsha suture zone seem to be consistent with the earlier results that invoke flow of a hot Tibetan asthenosphere into the mantle transition zone, as an explanation. Interestingly, results from the Bengal basin reveal a deepening (∼10 km) of both the 410 and 660 km discontinuities. Similar results from other plume affected regions prompt us to interpret this as a signature of the Kergulean plume.

    更新日期:2020-02-13
  • Sequential modelling of the 2016 Central Italy earthquake cluster using multisource satellite observations and quantitative assessment of Coulomb stress change
    Geophys. J. Int. (IF 2.777) Pub Date : 2020-01-21
    Xu Q, Chen Q, Zhao J, et al.

    SUMMARYA sequence of earthquake events consisting of three large shocks occurred in Central Italy from August to October in 2016 with the duration of almost 2 months. The preliminary study on the seismic mechanism suggests that the sequence of events is the result from the activity of the SW dipping Mt Bove–Mt Vettore–Mt Gorzano normal fault system. For investigation and understanding of the coseismic faulting of the seismogenic fault alignment, we collect a set of comprehensive satellite observations including the Sentinel-1A, ALOS-2/PALSAR-2 and GPS data to map the coseismic surface deformation and estimate the source models in this study. The derived faulting model for the first Amatrice event is characterized by two distinct slip asperities suggesting that it is a predominantly normal dip-slip motion with slight strike-slip component. The second event, Visso earthquake is almost a purely normal rupture. The third Norcia event is dominated by the normal dip-slip rupture of the seismogenic fault, and has propagated up to the ground with significant slip. The three faulting models are then utilized to quantify the Coulomb failure stress (CFS) change over the seismic zone. First, the CFS change on the subsequent two seismogenic faults of the earthquake sequence is estimated, and the derived positive CFS change induced by the preceding earthquakes suggests that the early events have positive effects on triggering the subsequent seismicity. We then explore the response relation of the aftershocks including 961 events with magnitudes larger than M 3.0 to the CFS change over the seismic zone. It suggests that the rupture pattern of the aftershocks is similar to the major shocks with predominantly normal dip-slip. To assess the risk of the future seismic hazard, we analyse quantitatively the spatial distribution of aftershock occurrence and CFS transfer at the seismogenic depth, indicating that the ruptures of the three major shocks do partly release the accumulated strain on the associated fault alignment as well as the dense aftershock, but the CFS increase zone with few aftershocks in the southwest of the eastern Quaternary fault alignment of Central Italy poses the potential of further rupture. In particular, the distribution of aftershock migration also suggests that the north extension of the Mt Bove fault is the potential zone with rupture risk.

    更新日期:2020-02-13
  • A-optimal design method to determine the regularization parameter of coseismic slip distribution inversion
    Geophys. J. Int. (IF 2.777) Pub Date : 2020-01-17
    Wang L, Gu W.

    ABSTRACTThe key to the inversion of a coseismic slip distribution is to determine the regularization parameters. In view of the determination of regularization parameters in seismic slip distribution inversion, the A-optimal design method is proposed in this paper. The L-curve method and A-optimal design method are used to design simulation experiments, and the inversion results show that the A-optimal design method is superior to the L-curve method in determining the regularization parameters. These two methods are also used to determine the regularization parameters of the L'Aquila and Lushan earthquake slip distribution inversions, and the results are consistent with those of other research conducted at home and abroad. Compared with the L-curve method, the A-optimal design method has the advantages of a high accuracy that does not rely on the data fitting accuracy.

    更新日期:2020-02-13
  • The reduction of source effect for reliable estimation of geomagnetic transfer functions
    Geophys. J. Int. (IF 2.777) Pub Date : 2020-01-15
    Ernst T, Nowożyński K, Jóźwiak W.

    SUMMERYWe have analysed the literature suggestions regarding possible changes in vertical magnetic transfer function (VTF) over time. We have shown that for periods above 1500 s the observed changes in VTF are caused by the source effect and we proposed how to reduce this negative impact. For calculations we used 1-min recordings of geomagnetic variations registered between 2002 and 2017 in various geomagnetic observatories. In data processing we used frequency-domain Egbert's algorithm and our original algorithm based on the method of least squares in the time domain for some comparisons and tests. We have shown that for periods over 1500 s the VTFs calculated separately from summer and winter data are different. However, our analysis shows that the variability of the VTF values obtained is misleading and results from time-changing presence of magnetic field variations that do not fulfill the assumption of plane wave (there is a vertical component in the incident magnetic field). These variations are much more numerous in summer than in winter. More detailed analysis has shown also that they are usually small at night and big during the day. The vertical components of these variations constitute an error correlated with input signals (horizontal components), which alters the values of the determined VTF. Furthermore, error bars do not take this effect into account. It makes it impossible to improve the accuracy of calculations by increasing the amount of data. Analysing the estimated external parts of vertical components from the Central European observatories we noticed a great similarity of these signals even if the induction components were clearly different, which indicates that this is a regional effect. On this basis, we proposed a procedure to improve the accuracy of VTF determination by means of pre-selection of data.

    更新日期:2020-02-13
  • Erratum: Thermal softening induced subduction initiation at a passive margin
    Geophys. J. Int. (IF 2.777) Pub Date : 2020-02-13
    Kiss D, Candioti L, Duretz T, et al.

    In the original version of this manuscript, several erroneous proofing questions were retained in the text. These have now been removed and the publisher apologises for the error.

    更新日期:2020-02-13
  • Three-station interferometry and tomography: coda versus direct waves
    Geophys. J. Int. (IF 2.777) Pub Date : 2020-01-28
    Zhang S, Feng L, Ritzwoller M.

    SUMMARYTraditional two-station ambient noise interferometry estimates the Green’s function between a pair of synchronously deployed seismic stations. Three-station interferometry considers records observed three stations at a time, where two of the stations are considered receiver–stations and the third is a source–station. Cross-correlations between records at the source–station with each of the receiver–stations are correlated or convolved again to estimate the Green’s function between the receiver–stations, which may be deployed asynchronously. We use data from the EarthScope USArray in the western United States to compare Rayleigh wave dispersion obtained from two-station and three-station interferometry. Three three-station interferometric methods are distinguished by the data segment utilized (coda-wave or direct-wave) and whether the source–stations are constrained to lie in stationary phase zones approximately inline with the receiver–stations. The primary finding is that the three-station direct wave methods perform considerably better than the three-station coda-wave method and two-station ambient noise interferometry for obtaining surface wave dispersion measurements in terms of signal-to-noise ratio, bandwidth, and the number of measurements obtained, but possess small biases relative to two-station interferometry. We present a ray-theoretic correction method that largely removes the bias below 40 s period and reduces it at longer periods. Three-station direct-wave interferometry provides substantial value for imaging the crust and uppermost mantle, and its ability to bridge asynchronously deployed stations may impact the design of seismic networks in the future.

    更新日期:2020-02-13
  • Impact of Uncertainties of GOCE Gravity Model on Crustal Thickness Estimates
    Geophys. J. Int. (IF 2.777) Pub Date : 2020-02-07
    van der Meijde M, Pail R.

    SummaryIn the last few years many studies have applied data of satellite gravity sensors for solid Earth applications. The use of different methodologies has been shown to result in large variations in crustal thickness even when using the same data as source. It is, however, difficult to estimate what is a significant difference between such models. Up to now the impact of the inherent uncertainty of GOCE data on solid Earth applications has never been quantified. With this study we will provide uncertainty boundaries for crustal modelling based on the GOCE TIM5 covariance matrix. Different noise realizations have been calculated using a Monte Carlo-like simulation and added to the TIM5 model coefficients. The resulting differences in crustal thickness amount to maximum ±0.2 km, which is less than 1 % of the total thickness, and much smaller than many other uncertainties involved in the inversion process.

    更新日期:2020-02-07
  • Outlier-insensitive Bayesian inference for linear inverse problems (OutIBI) with applications to space geodetic data
    Geophys. J. Int. (IF 2.777) Pub Date : 2019-12-12
    Hang Y, Barbot S, Dauwels J, et al.

    SUMMARYInverse problems play a central role in data analysis across the fields of science. Many techniques and algorithms provide parameter estimation including the best-fitting model and the parameters statistics. Here, we concern ourselves with the robustness of parameter estimation under constraints, with the focus on assimilation of noisy data with potential outliers, a situation all too familiar in Earth science, particularly in analysis of remote-sensing data. We assume a linear, or linearized, forward model relating the model parameters to multiple data sets with a priori unknown uncertainties that are left to be characterized. This is relevant for global navigation satellite system and synthetic aperture radar data that involve intricate processing for which uncertainty estimation is not available. The model is constrained by additional equalities and inequalities resulting from the physics of the problem, but the weights of equalities are unknown. We formulate the problem from a Bayesian perspective with non-informative priors. The posterior distribution of the model parameters, weights and outliers conditioned on the observations are then inferred via Gibbs sampling. We demonstrate the practical utility of the method based on a set of challenging inverse problems with both synthetic and real space-geodetic data associated with earthquakes and nuclear explosions. We provide the associated computer codes and expect the approach to be of practical interest for a wide range of applications.

    更新日期:2020-02-07
  • Geostatistical characterization of internal structure of mass-transport deposits from seismic reflection images and borehole logs
    Geophys. J. Int. (IF 2.777) Pub Date : 2019-12-19
    Ford J, Camerlenghi A.

    SUMMARYSeismic reflection images of mass-transport deposits often show apparently chaotic, disorded or low-reflectivity internal seismic facies. The lack of laterally coherent reflections can prevent horizon-based interpretation of internal structure. This study instead inverts for geostatistical parameters which characterize the internal heterogeneity of mass-transport deposits from depth-domain seismic reflection images. A Bayesian Markov Chain Monte Carlo inversion is performed to estimate posterior probability distributions for each geostatistical parameter. If the internal heterogeneity approximates an anisotropic von Kármán random medium these parameters can describe the structural fabric of the imaged mass-transport deposit in terms of lateral and vertical dominant scale lengths and the Hurst number (roughness). To improve the discrimination between vertical and lateral dominant scale lengths an estimate of the vertical dominant scale length from a borehole is used as a prior in the inversion. The method is first demonstrated on a synthetic multichannel seismic reflection image. The vertical and lateral dominant scale lengths are estimated with lower uncertainty when data from a synthetic borehole data are included. We then apply the method to a real data example from Nankai Trough, offshore Japan, where a large mass-transport deposit is imaged in a seismic profile and penetrated by a borehole. The results of the inversion show a downslope shortening in lateral scale length, consistent with progressive down-slope disaggregation of the mass-flow during transport. The dominant scale lengths can be used as a proxy for strain history, which can improve understanding of post-failure dynamics and emplacement of subacqueous mass-movements, important for constraining the geohazard potential from future slope failure.

    更新日期:2020-02-07
  • Establishment of a seismic topographic effect prediction model in the Lushan Ms 7.0 earthquake area
    Geophys. J. Int. (IF 2.777) Pub Date : 2020-01-06
    Zhou H, Li J, Chen X.

    SUMMARYThe seismic topographic effect is one of the debated research topics in seismology and earthquake engineering. This debate is due to the discrepancy between the observed amplification and the amplification underestimation in numerical simulations. Although the numerical simulation of ground motion, which began in the 1970s, has been an important and effective way to study topographic effects, the quantitative mathematical model of topographic amplification is urgent. The actual influences on ground motion due to the topography depends on multiple topographic features, such as the topographic slope, topographic geometrical scale. To date, no definite conclusions regarding the main influencing factors and how to express the influencing factors have been made. In this paper, by introducing the back-propagation (BP) neural network technique, a set of mathematical parameters are determined to establish a quantitative topographic effect prediction model. These parameters are the elevation, the first gradient of the elevation and the higher order gradient in two orthogonal directions. Theoretically, the set of mathematical parameters is directly related to the simple topographic features, such as the elevation, topographic slope and height-to-width ratio. Furthermore, their combinations indirectly denote the complex topographic geometrical features, such as the different topographic geometrical scales, designated by the elevation (large-scale variable), the first gradient (middle-scale variable), the second-order gradient (small-scale variable) and so on (smaller scale variable), and the hill ridges that correspond to the sites with the first gradient of the elevation equal to zero and an elevation larger than its surrounding. In 2013, an earthquake of Ms 7.0 occurred in the Lushan area of Sichuan Province in Western China, where the topography sharply fluctuates. At station 51BXD, an acceleration was recorded close to 1.0 g, while at station 51BXM (14 km away from station 51BXD), the acceleration was recorded at only 0.2–0.3 g. In this paper, the spectral element method (SEM) is used to simulate the ground motion in the Lushan Ms 7.0 earthquake area. Then, the topographic amplification ratio of the simulated ground motion is calculated. Furthermore, a BP topographic amplification prediction model is established and compared based on different parameters. A rms of less than or close to 10 per cent between the BP model prediction results and topographic amplification ratio calculated using the simulated ground motion suggests that the parameters of the topographic elevation, the first gradient of the elevation and the second-order gradient in two orthogonal directions are enough to provide the acceptable topographic effect model in the Lushan area. Finally, using the prediction model, the topographic spectral ratio at stations 51BXD and 51BXM is predicted, and the topography amplification due to the scattering of seismic waves by the irregular topography at 51BXD is found to be 1.5–2 times that of 51BXM. The most important highlights of this paper identify the main factors of the topographic effect for the first time and provide an effective method for establishing a quantitative topographic effect prediction model.

    更新日期:2020-02-07
  • A fast algorithm for multiple elimination and transmission compensation in primary reflections
    Geophys. J. Int. (IF 2.777) Pub Date : 2020-01-07
    Zhang L, Slob E.

    SUMMARYThe transmission compensated primary reflections can be obtained from the single-sided acoustic reflection response in the two-way traveltime domain. This is achieved by eliminating free-surface and internal multiple reflections and compensating for transmission losses in primary reflections without model information. The substantial computational cost of the proposed scheme can be reduced by an order of magnitude with a fast implementation version. This is achieved by using the previously computed filter functions as initial estimate for every new truncation time value. We evaluate the success of the scheme with simple and complex 2-D numerical examples. We find that the scheme has excellent performance in most cases, except for the case where strong reflectors are present. In such case, the current scheme suffers from lack of convergence.

    更新日期:2020-02-07
  • Application of temporal reweighting to ambient noise cross-correlation for improved seismic Green's function
    Geophys. J. Int. (IF 2.777) Pub Date : 2020-01-06
    Li J, Weaver R, Yoritomo J, et al.

    SUMMARYDue to the partly diffuse character of ambient noise, the retrieval of amplitude information and attenuation from noise cross-correlations has been difficult. Here, we apply the temporal reweighting method proposed by Weaver & Yoritomo to seismic data from the USArray in the central-midwest US. The results show considerable improvements in retrieved Green's functions in both symmetry and causality. The reweighting is able to make the effective incident noise field more isotropic (though not yet truly isotropic). It produces more robust amplitude measurements and also makes both the causal and anticausal parts usable. This suggests that it could be widely applicable for retrieval of Green's functions from ambient noise for attenuation study. The results also suggest an alternative measure of signal-to-noise ratio that complements the conventional one.

    更新日期:2020-02-07
  • Earthquake source arrays: optimal configuration and applications in crustal structure studies
    Geophys. J. Int. (IF 2.777) Pub Date : 2020-01-06
    Karamzadeh N, Heimann S, Dahm T, et al.

    SUMMARYA collection of earthquake sources recorded at a single station, under specific conditions, are considered as a source array (SA), that is interpreted as if earthquake sources originate at the station location and are recorded at the source location. Then, array processing methods, that is array beamforming, are applicable to analyse the recorded signals. A possible application is to use source array multiple event techniques to locate and characterize near-source scatterers and structural interfaces. In this work the aim is to facilitate the use of earthquake source arrays by presenting an automatic search algorithm to configure the source array elements. We developed a procedure to search for an optimal source array element distribution given an earthquake catalogue including accurate origin time and hypocentre locations. The objective function of the optimization process can be flexibly defined for each application to ensure the prerequisites (criteria) of making a source array. We formulated four quantitative criteria as subfunctions and used the weighted sum technique to combine them in one single scalar function. The criteria are: (1) to control the accuracy of the slowness vector estimation using the time domain beamforming method, (2) to measure the waveform coherency of the array elements, (3) to select events with lower location error and (4) to select traces with high energy of specific phases, that is, sp- or ps-phases. The proposed procedure is verified using synthetic data as well as real examples for the Vogtland region in Northwest Bohemia. We discussed the possible application of the optimized source arrays to identify the location of scatterers in the velocity model by presenting a synthetic test and an example using real waveforms.

    更新日期:2020-02-07
  • Erratum to: How to be fooled searching for significant variations of the b-value
    Geophys. J. Int. (IF 2.777) Pub Date : 2020-02-07
    Marzocchi W, Spassiani I, Stallone A, et al.

    In the original version of this manuscript the acknowledgment section was missing. It has now been added to the published manuscript:

    更新日期:2020-02-07
  • Inelastic strain rate and stress fields in and around an aseismic zone of Kyushu Island, Japan, inferred from seismic and GNSS data
    Geophys. J. Int. (IF 2.777) Pub Date : 2020-01-09
    Yuasa Y, Matsumoto S, Nakao S, et al.

    SUMMARYUnderstanding earthquake processes and crustal deformation requires knowledge of the stress concentration process in the crust. With the enhancement of observation networks, it has become possible to consider in detail the relationships between localized deformation and seismic activity in island arcs and the process of stress concentration. According to previous studies, inelastic deformation in localized weak zones in the crust is considered to play an important role in the stress concentration process. Kyushu, located in southwest Japan, has a 20–30 km band-like active seismic activity and an enclosed aseismic zone. In particular, a part of the seismic active region called the Beppu-Simahara Graben, which is dominated by north–south extensional deformation, is characterized by high seismic activity and a remarkable aseismic zone. We identified the relationship between inelastic deformation and stress concentration processes in this area by using analyses of geodetic and seismic data. The results inverted from both the strain rate field obtained by the geodetic observations and the deviatoric stress field estimated from focal mechanism data reveal a large inelastic deformation zone ($\sim {10^{ - 7}} \,\mathrm{ yr}^{-1}$) beneath the area of active seismicity. From comparison with previous works, the inelastic deformation zone in the lower crust may correspond to an area with high temperature and/or fluid. This may suggest that inelastic deformation is in progress in the area where the strength of lower crustal rocks has reduced due to the presence of geothermics and/or fluids. Furthermore, we confirmed that this inelastic deformation causes stress concentrations of up to $10\,\,{\rm{kPa}}\,\,{\rm{yr}}^{-1}$ in the upper crust. These results show that stress concentration occurs locally in the upper crust, above the inelastic deformation zone in the weakened lower crust, owing to the presence of geothermal and/or fluid; this stress concentration induces seismic activity and crustal deformation.

    更新日期:2020-02-07
  • CLEAN beamforming for the enhanced detection of multiple infrasonic sources
    Geophys. J. Int. (IF 2.777) Pub Date : 2020-01-08
    den Ouden O, Assink J, Smets P, et al.

    SUMMARYThe detection and characterization of signals of interest in the presence of (in)coherent ambient noise is central to the analysis of infrasound array data. Microbaroms have an extended source region and a dynamical character. From the perspective of an infrasound array, these coherent noise sources appear as interfering signals that conventional beamform methods may not correctly resolve. This limits the ability of an infrasound array to dissect the incoming wavefield into individual components. In this paper, this problem will be addressed by proposing a high-resolution beamform technique in combination with the CLEAN algorithm. CLEAN iteratively selects the maximum of the f/k spectrum (i.e. following the Bartlett or the Capon method) and removes a percentage of the corresponding signal from the cross-spectral density matrix. In this procedure, the array response is deconvolved from the f/k spectral density function. The spectral peaks are retained in a ‘clean’ spectrum. A data-driven stopping criterion for CLEAN is proposed, which relies on the framework of Fisher statistics. This allows the construction of an automated algorithm that continuously extracts coherent energy until the point is reached that only incoherent noise is left in the data. CLEAN is tested on a synthetic data set and is applied to data from multiple International Monitoring System infrasound arrays. The results show that the proposed method allows for the identification of multiple microbarom source regions in the Northern Atlantic that would have remained unidentified if conventional methods had been applied.

    更新日期:2020-02-07
  • Recent geomagnetic variations and the force balance in Earth’s core
    Geophys. J. Int. (IF 2.777) Pub Date : 2020-01-07
    Aubert J.

    SUMMARYThe nature of the force balance that governs the geodynamo is debated. Recent theoretical analyses and numerical simulations support a quasigeotrophic (QG), magneto-Archimedes-Coriolis (MAC) balance in Earth’s core, where the Coriolis and pressure forces equilibrate at leading order in amplitude, and where the buoyancy, Lorentz and ageostrophic Coriolis forces equilibrate at the next order. In contrast, earlier theoretical expectations have favoured a magnetostrophic regime where the Lorentz force would reach leading order at the system scale. The dominant driver (buoyant or magnetic) for the general circulation in Earth’s core is equally debated. In this study, these questions are explored in the light of the high-quality geomagnetic data recently acquired by satellites and at magnetic ground observatories. The analysis involves inverse geodynamo modelling, a method that uses multivariate statistics extracted from a numerical geodynamo model to infer the state of Earth’s core from a geomagnetic field model interpretation of the main field and secular variation data. To test the QG-MAC dynamic hypothesis against the data, the framework is extended in order to explicitly prescribe this force balance into the inverse problem solved at the core surface. The resulting inverse solutions achieve a quantitatively adequate fit to the data while ensuring deviations from the QG-MAC balance (which amount to an inertial driving of the flow) lower than each of the leading forces. The general circulation imaged within the core over the past two decades confirms the existence of a planetary-scale, eccentric, axially columnar gyre that comprises an intense, equatorially symmetric jet at high latitudes in the Pacific hemisphere. The dominant driver of this circulation is shown to be of buoyant nature, through a thermal wind balance with a longitudinally hemispheric buoyancy anomaly distribution. Geomagnetic forecasts initiated with the inverted core states are systematically more accurate against the true interannual geomagnetic field evolution when enforcing the QG-MAC constraint. This force balance is therefore consistent with the geomagnetic data at the large scales of Earth’s core that can be imaged by the method.

    更新日期:2020-02-07
  • Finite-frequency traveltime tomography using the Generalized Rytov approximation
    Geophys. J. Int. (IF 2.777) Pub Date : 2020-02-06
    Feng B, Xu W, Wu R, et al.

    SummaryWave-equation-based traveltime tomography has been extensively applied in both global tomography and seismic exploration. Typically, the traveltime Fréchet derivative is obtained using the first-order Born-approximation, which is only satisfied for weak velocity perturbations and small phase shifts (i.e. the weak-scattering assumption). Although the small phase-shift restriction can be handled with the Rytov approximation, the weak velocity-perturbation assumption is still a major limitation. The recently-developed generalized Rytov approximation (GRA) method can achieve an improved phase accuracy of the forward-scattered wavefield, in the presence of large-scale and strong velocity perturbations. In this paper, we combine GRA with the classical finite-frequency theory and propose a GRA-based traveltime sensitivity kernel (GRA-TSK), which overcomes the weak-scattering limitation of the conventional finite-frequency methods. Numerical examples demonstrate that the accumulated time-delay of forward-scattered waves caused by large-scale smooth perturbations can be correctly handled by the GRA-based traveltime sensitivity kernel, regardless of the magnitude of the velocity perturbations. Then, we apply the new sensitivity kernel to solve the traveltime inverse problem, and we propose a matrix-free Gauss-Newton method which has a faster convergence rate compared with the gradient-based method. Numerical tests show that, compared with the conventional adjoint traveltime tomography, the proposed GRA-based traveltime tomography can obtain a more accurate model with a faster convergence rate, making it more suited for recovering the large-intermediate scale of the velocity model, even for strong-perturbation and complex subsurface structures.

    更新日期:2020-02-07
  • Hydrogeophysical data integration through Bayesian Sequential Simulation with log-linear pooling
    Geophys. J. Int. (IF 2.777) Pub Date : 2020-02-06
    Nussbaumer R, Mariethoz G, Gloaguen E, et al.

    SummaryBayesian sequential simulation (BSS) is a geostastistical technique, which uses a secondary variable to guide the stochastic simulation of a primary variable. As such, BSS has proven significant promise for the integration of disparate hydrogeophysical datasets characterized by vastly differing spatial coverage and resolution of the primary and secondary variables. An inherent limitation of BSS is its tendency to underestimate the variance of the simulated fields due to the smooth nature of the secondary variable. Indeed, in its classical form, the method is unable to account for this smoothness because it inherently assumes independence of the secondary variable with regard to neighbouring values of the primary variable. To overcome this limitation, we have modified the Bayesian updating with a log-linear pooling approach, which allows to account for the inherent interdependence between the primary and the secondary variable by adding exponential weights to the corresponding probabilities. The proposed method is tested on a pertinent synthetic hydrogeophysical dataset consisting of surface-based electrical resistivity tomography (ERT) data and local borehole measurements of the hydraulic conductivity. Our results show that, compared to classical BSS, the proposed log-linear pooling method using equal constant weights for the primary and secondary variable enhances the reproduction of spatial statistics of the stochastic realizations, while maintaining a faithful correspondence with the geophysical data. Significant additional improvements can be achieved by optimizing the choice of these constant weights. We also explore a dynamic adaptation of the weights during the course of the simulation process, which provides valuable insights into the optimal parameterization of the proposed log-linear pooling approach. The results corroborate the strategy of selectively emphasizing the probabilities of the secondary and primary variables, at the very beginning and for the remainder of the simulation process, respectively.

    更新日期:2020-02-07
  • Consistent quantification of the impact of key mission design parameters on the performance of next-generation gravity missions
    Geophys. J. Int. (IF 2.777) Pub Date : 2020-02-05
    Purkhauser A, Siemes C, Pail R.

    SummaryThe GRACE and GRACE-FO missions have been observing time variations of the Earth's gravity field for more than 15 years. For a possible successor mission, the need to continue mass change observations have to be balanced with the ambition for monitoring capabilities with an enhanced spatial and temporal resolution that will enable improved scientific results and will serve operational services and applications. Various study groups performed individual simulations to analyse different aspects of possible NGGMs from a scientific and technical point of view. As these studies are not directly comparable due to different assumptions regarding mission design and instrumentation, the goal of this paper is to systematically analyse and quantify the key mission parameters (number of satellite pairs, orbit altitude, sensors) and the impact of various error sources (AO, OT models, post-processing) in a consistent simulation environment. Our study demonstrates that a single-pair mission with laser interferometry in a low orbit with a drag compensation system would be the only possibility within the single-pair options to increase the performance compared to the GRACE/GRACE-FO. Tailored post-processing is not able to achieve the same performance as a double-pair mission without post-processing. Also, such a mission concept does not solve the problems of temporal aliasing due to observation geometry. In contrast, double-pair concepts have the potential to retrieve the full AOHIS signal and in some cases even double the performance to the comparable single-pair scenario. When combining a double-pair with laser interferometry and an improved accelerometer, the sensor noise is, apart from the ocean tide modeling errors, one of the limiting factors. Therefore, the next big step for observing the gravity field globally with a satellite mission can only be taken by launching a double pair mission. With this quantification of key architecture features of a future satellite gravity mission, the study aims to improve the available information to allow for an informed decision making and give an indication of priority for the different mission concepts.

    更新日期:2020-02-06
  • Susceptibility and remanent magnetization inversion of magnetic data with a priori information of the Köenigsberger ratio
    Geophys. J. Int. (IF 2.777) Pub Date : 2020-02-03
    Liu S, Hu X, Zuo B, et al.

    SUMMARYMagnetic susceptibility and natural remanent magnetization of rocks are useful parameters to study geological structures and geodynamical processes. Traditional widely-used algorithms for the inversion of magnetic data can recover the distribution of the apparent susceptibility or total magnetization intensity, but do not provide information on the remanent magnetization. In this paper, we propose a framework to directly invert for the magnetic susceptibility and the natural remanent magnetization vector using surface or airborne magnetic data, assuming that the Köenigsberger ratio of the rock is known or approximately deducible. The susceptibility and remanence are computed using two different approaches: 1) the susceptibility, intensity, and direction of the remanent magnetization are continuously recovered for each discretized cell; and 2) the remanence direction is assumed to be uniform in each sub-zone and is iteratively computed as discrete values. Both processes are implemented using the preconditioned conjugate gradient algorithm. The method is tested on three synthetic models and one field dataset from the Zaohuohexi iron-ore deposit, Qinghai Province, NW China. The results of the continuous inversion show the trend of the remanent magnetization directions, while the discrete inversion yields more specific values. This inversion framework can determine the source bodies’ geometry and position, and also provide superposed and comprehensive information on the natural remanent magnetization, which may be useful to investigate geological bodies bearing stable primary remanent magnetization.

    更新日期:2020-02-04
  • Marchenko multiple elimination of a Laboratory example
    Geophys. J. Int. (IF 2.777) Pub Date : 2020-02-03
    Zhang L, Slob E.

    SUMMARYThe Marchenko multiple elimination scheme is derived from the coupled Marchenko equations. It is proposed for filtering primary reflections with two-way travel time from the measured acoustic data. The measured acoustic reflection data is used as its own filter and no model information or adaptive subtraction is required to apply the method. The data obtained after Marchenko multiple elimination is better suited for velocity model construction and artefact-free migration than the measured data. We apply the Marchenko multiple elimination scheme to a measured laboratory dataset to evaluate the success of the method. The results suggest that Marchenko multiple elimination scheme can be the appropriate choice, when high-quality pre-processing is performed successfully.

    更新日期:2020-02-04
  • Electrical conductivity in the mantle transition zone beneath Eastern China derived from L1-Norm C-responses
    Geophys. J. Int. (IF 2.777) Pub Date : 2020-02-03
    Zhang Y, Weng A, Li S, et al.

    SUMMARYConstraining the distribution of water in different regions of the mantle remains one of the significant challenges to comprehend the global deep water cycle. Geomagnetic depth soundings can provide such constraint through the electrical conductivity structure. Hence, this study aims to propose a regularization technique that can estimate previously unavailable C-response. In the method, the objective function comprised an L1-norm measured data prediction error and a spectral smoothness constraint term. We used the data error of C-response to weight the predicted error. The L-BFGS method was introduced to determine the minimum point of the objective function, and the regularization parameter decreased adaptively during inversion. Thus, the geomagnetic data processed yielded high-quality C-responses in 31 stations in Eastern China. In addition, we obtained 1-D electrical conductivity profiles in the mantle transition zone (MTZ) beneath Eastern China from C-responses using the L-BFGS method. Compared with the global 1-D model, the conductivity–depth profiles revealed that the MTZ beneath Eastern China is more conductive in the east but more resistive in the west. The conversion of these conductivities to water content based on the mineral physics suggested that the MTZ beneath Eastern China is characterized by a high water concentration, approximately 0.2 and 1 wt per cent in the upper and lower MTZ, respectively. Owing to the inclusion of more stations, the water-rich region could be constrained roughly to the east of the North–South Gravity Lineament (NSGL). Further considering seismic images in the same area, this water content distribution pattern suggested that the front of the stagnant Pacific plate in the lower MTZ might have reached the NSGL. However, the dehydration reactions in the stagnant slab were more active in the eastern part. Perhaps, some of these fluids migrated into the upper MTZ and could be the source of the trapped water found in the xenoliths from the deep upper mantle beneath Eastern China.

    更新日期:2020-02-04
  • Seismic characterization of a clay-block rupture in Harmalière landslide, French Western Alps
    Geophys. J. Int. (IF 2.777) Pub Date : 2020-02-03
    Fiolleau S, Jongmans D, Bièvre G, et al.

    In late June 2016, the Harmalière clayey landslide (located 30 km south of the city of Grenoble, French Alps) was dramatically reactivated at the headscarp after a 35-year long period of continuous but limited activity. The total involved volume, which moved as sliding blocks of various sizes, was estimated to be about 1 × 106 m3. Two seismometers were installed at the rear of the main headscarp in August 2016, on both sides of a developing fracture delineating a block with a volume of a few hundred cubic meters. For 4 months, they continuously recorded seismic ambient vibrations and microearthquakes until the block broke. Five seismic parameters were derived from the monitoring: the cumulative number of microearthquakes (CNe), the seismic energy (SE), the block resonance frequency (fB), the relative variation in Rayleigh wave velocity (dV/V) deduced from noise cross-correlations between the two sensors and the associated correlation coefficient (CC). All parameters showed a significant precursory signal before the rupture, but at very different times, which indicates the complexity of the rupture mechanism in this clay material.

    更新日期:2020-02-04
  • A mushy Earth's mantle for more than 500 Myr after the magma ocean solidification
    Geophys. J. Int. (IF 2.777) Pub Date : 2020-02-03
    Monteux J, Andrault D, Guitreau M, et al.

    In its early evolution, the Earth mantle likely experienced several episodes of complete melting enhanced by giant impact heating, short-lived radionuclides heating and viscous dissipation during the metal/silicate separation. After a first stage of rapid and significant crystallization (Magma Ocean stage), the mantle cooling is slowed down due to the rheological transition, which occurs at a critical melt fraction of 40–50 per cent. This transition first occurs in the lowermost mantle, before the mushy zone migrates toward the Earth's surface with further mantle cooling. Thick thermal boundary layers form above and below this reservoir. We have developed numerical models to monitor the thermal evolution of a cooling and crystallizing deep mushy mantle. For this purpose, we use a 1-Dimension approach in spherical geometry accounting for turbulent convective heat transfer and integrating recent and solid experimental constraints from mineral physics.. Our results show that the last stages of the mushy mantle solidification occur in two separate mantle layers. The lifetime and depth of each layer are strongly dependent on the considered viscosity model and in particular on the viscosity contrast between the solid upper and lower mantle. In any case, the full solidification should occur at the Hadean-Eoarchean boundary 500–800 Myr after Earth's formation. The persistence of molten reservoirs during the Hadean may favor the absence of early reliefs at that time and maintain isolation of the early crust from the underlying mantle dynamics.

    更新日期:2020-02-04
  • Dynamic permeability functions for partially saturated porous media
    Geophys. J. Int. (IF 2.777) Pub Date : 2020-02-04
    Solazzi S, Rubino J, Jougnot D, et al.

    SUMMARYWhile the frequency-dependence of permeability under fully saturated conditions has been studied for decades, the corresponding characteristics of partially saturated porous media remain unexplored. Notably, it is not clear whether the use of effective pore fluid approaches under such conditions is valid. To address this issue, we propose a method that allows us to obtain dynamic permeability functions for partially saturated porous media. To this end, we conceptualise the considered pore space as a bundle of capillary tubes of different radii saturated by two immiscible fluid phases. We then solve the Navier-Stokes equations within the pore space and define a capillary pressure-saturation relationship, which permits to obtain saturation- and frequency-dependent effective permeability estimates. The application of this method to a realistic model of an unconsolidated granular sediment demonstrates that dynamic effective permeability functions for wetting and non-wetting fluid phases exhibit distinct characteristics, thus rendering effective pore fluid approaches inadequate. Finally, we explore the capability of the seminal dynamic permeability model developed by Johnson et al. (1987) to account for the effects of partial saturation. We find that the frequency scaling proposed by Johnson et al. (1987) prevails in partially saturated scenarios. However, the parameters associated with this model need to be redefined to account for saturation-dependent effects.

    更新日期:2020-02-04
  • Mapping of Eastern North Atlantic Ocean seismicity from Po/So observations at a mid-aperture seismological broadband deep sea array
    Geophys. J. Int. (IF 2.777) Pub Date : 2020-02-01
    Krüger F, Dahm T, Hannemann K.

    SUMMARYA mid aperture broadband test array (OBS array DOCTAR) was deployed from June 2011 to April 2012 about 100 km north of the Gloria fault in the Eastern North Atlantic in about 5000 m water depth. In addition arrays were installed on Madeira Island and in western Portugal mainland. For the first time in the Eastern North Atlantic, we recorded a large number of high frequency Po and So waves from local and regional small- and moderate earthquakes (ML < 4). An incoherent beamforming method was adapted to scan continuous data for such Po and So arrivals applying a sliding window waveform migration and frequency-wavenumber technique. We identify about 320 Po and 1550 So arrivals and compare the phase onsets with the ISC catalog (ISC 2015) for the same timespan. Up to a distance of 6○ to the DOCTAR stations all events listed in the ISC catalog could be associated to Po and So phases. Arrivals from events in more than 10○ distance could be identified only in some cases. Only few Po and/or So arrivals were detected for earthquakes from the European and African continental area, the continental shelf regions, and for earthquakes within or northwest of the Azores plateau. Unexpectedly, earthquake clusters are detected within the oceanic plates north and south of the Gloria fault and far from plate boundaries, indicating active intraplate structures. We also observe and locate numerous small magnitude earthquakes on the segment of the Gloria fault directly south of DOCTAR, which likely coincides with the rupture of the 25 November 1941 event. Local small magnitude earthquakes located beneath DOCTAR show hypocenters up to 30 km depth and strike-slip focal mechanisms. A comparison with detections at temporary mid-aperture arrays on Madeira and in western Portugal shows that the deep ocean array performs much better than the island and the continental array regarding the detection threshold for events in the oceanic plates. We conclude that sparsely distributed mid-aperture seismic arrays in the deep ocean could decrease the detection and location threshold for seismicity with ML < 4 in the oceanic plate and might constitute a valuable tool to monitor oceanic plate seismicity.

    更新日期:2020-02-03
  • Flow plane orientation in the upper mantle under the Western/Central United States from SKS shear-wave splitting observations
    Geophys. J. Int. (IF 2.777) Pub Date : 2020-02-03
    Löberich E, Bokelmann G.

    SummaryThe causes of seismic anisotropy are still under debate. In particular it is important to understand the extent to which seismic anisotropy is due to more recent geodynamic activities in the asthenosphere, or to frozen-in deformation in the lithosphere. We show that these two endmember cases can in principle be distinguished using shear-wave splitting observations from SKS waves. This is illustrated by the simple example of pure olivine with horizontal a-axis, and differing orientations of the other two axes, namely vertical b and vertical c. The azimuthal dependence of shear-wave splitting measurements is described by two parameters, which can provide additional information about subsurface deformation. In particular the oscillation parameter d1 constrains the orientation of foliation. We demonstrate that shear-wave splitting in the Western and Central United States indeed shows the predicted azimuthal dependence, related to a mainly subhorizontally-oriented flow plane of deformation in the upper mantle. This has important implications for asthenospheric flow.

    更新日期:2020-02-03
  • The 2018 Mw 6.8 zakynthos (Ionian Sea, Greece) Earthquake: Seismic source and local tsunami characterization
    Geophys. J. Int. (IF 2.777) Pub Date : 2020-02-03
    Cirella A, Romano F, Avallone A, et al.

    SummaryWe investigated the kinematic rupture model of the 2018 Mw 6.8 Zakynthos, Ionian Sea (Greece), earthquake by using a non-linear joint inversion of strong motion data, high-rate GPS time series, and static co-seismic GPS displacements. We also tested inversion results against tide-gauge recordings of the small tsunami generated in the Ionian Sea. In order to constrain the fault geometry, we performed several preliminary kinematic inversions by assuming the parameter values resulting from different published moment tensor solutions. The lowest cost function values were obtained by using the geometry derived from the United States Geological Survey (USGS) focal solution. Between the two conjugate USGS planes, the rupture model which better fits the data is the one with the N9° E-striking 39°-ESE-dipping plane. The rupture history of this model is characterized by a bi-lateral propagation, featuring two asperities; a main slip patch extending between 14 and 28 km in depth, 9 km northeast from the nucleation and a slightly shallower small patch located 27 km southwest from the nucleation. The maximum energy release occurs between 8 s and 12 s, when both patches are breaking simultaneously. The maximum slip is 1.8 m and the total seismic moment is 2.4 × 1019 Nm, corresponding to a Mw value of 6.8. The slip angle shows a dominant right-lateral strike-slip mechanism, with a minor reverse component that increases on the deeper region of the fault. This result, in addition to the observed possibility of similar mechanisms for previous earthquakes occurred in 1959 and 1997, suggests that the tectonic deformation between the Cephalonia Transform Fault Zone and the northern tip of the Hellenic Arc Subduction zone may be accommodated by prevailing right lateral low-dipping faults, occurring on re-activated structures previously experiencing (until Pliocene) compressional regime. Comparison of predicted and observed tsunami data suggests the need of a better characterisation of local harbour response for this type of relatively short-wavelength events, which is important in the context of tsunami early warning. However, the suggested dominantly strike-slip character would in turn imply a reduced tsunami hazard as compared to a dominant thrust faulting regime from this source region.

    更新日期:2020-02-03
  • Seismic asperity size evolution during fluid injection: case study of the 1993 Soultz-sous-Forêts injection (revised)
    Geophys. J. Int. (IF 2.777) Pub Date : 2020-02-03
    Cauchie L, Lengliné O, Schmittbuhl J.

    SummaryThe injection of fluid in the upper crust, notably for the development or exploitation of geothermal reservoirs, is often associated with the onset of induced seismicity. Although this process has been largely studied, it is not clear how the injected fluid influences the rupture size of the induced events. Here we re-investigate the induced earthquakes that occurred during an injection at Soultz-sous-Forêts, France in 1993 and studied the link between the injected fluid and the source properties of the numerous induced earthquakes. We take advantage that deep borehole accelerometers were running in the vicinity of the injection site. We estimate the moment and radius of all recorded events based on a spectral analysis and classify them into 663 repeating sequences. We show that the events globally obey the typical scaling law between radius and moment. However, at the scale of the asperity, fluctuations of the moment are important while the radii remain similar suggesting a variable stress drop or a mechanism that prevents the growth of the rupture. This is confirmed by linking the event source size to the geomechanical history of the reservoir. In areas where aseismic slip on pre-existing faults has been evidenced, we observed only small rupture sizes whereas in part of the reservoir where seismicity is related to the creation of new fractures, a wider distribution and larger rupture sizes are promoted. Implications for detecting the transition between events related to pre-existing faults and the onset of fresh fractures are discussed.

    更新日期:2020-02-03
  • Accelerating numerical wave propagation by wavefield adapted meshes, Part II: Full-waveform inversion
    Geophys. J. Int. (IF 2.777) Pub Date : 2020-02-03
    Thrastarson S, van Driel M, Krischer L, et al.

    SummaryWe present a novel full-waveform inversion approach which can reduce the computational cost by up to an order of magnitude compared to conventional approaches, provided that variations in medium properties are sufficiently smooth. Our method is based on the usage of wavefield adapted meshes which accelerate the forward and adjoint wavefield simulations. By adapting the mesh to the expected complexity and smoothness of the wavefield, the number of elements needed to discretize the wave equation can be greatly reduced. This leads to spectral-element meshes which are optimally tailored to source locations and medium complexity. We demonstrate a workflow which opens up the possibility to use these meshes in full-waveform inversion and show the computational advantages of the approach. We provide examples in 2-D and 3-D to illustrate the concept, describe how the new workflow deviates from the standard full-waveform inversion workflow, and explain the additional steps in detail.

    更新日期:2020-02-03
  • Surface rupture in stochastic slip models
    Geophys. J. Int. (IF 2.777) Pub Date : 2020-02-03
    Murphy S, Herrero A.

    SummaryAs an alternative to spectral methods, stochastic self-similar slip can be produced through a composite source model by placing a power-law scaling size-frequency distribution of circular slip dislocations on a fault surface. However these model do not accurately account for observed surface rupture behaviour. We propose a modification to the composite source model that corrects this issue. The advantage of this technique is that it accommodates the use of fractal slip distributions on non-planar fault surfaces. However to mimic a surface rupture using this technique, releasing the boundary condition at the top of the fault, we observed a systematic decrease in slip at shallow depths. We propose a new strategy whereby the surface is treated like a reflector with the slip being folded back onto the fault. Two different techniques based on this principal are presented: the first is the method of images. It requires a small change to pre-existing codes and works for planar faults. The second involves the use of a multi-stage trilateration technique. It is applied to non-planar faults described by an unstructured mesh. The reflected slip calculated using the two techniques is near identical on a planar fault, suggesting they are equivalent. Applying this correction, where reflected slip is accounted for in the composite source model, the lack of slip at shallow depths is not observed any more and there is no systematic trend with depth. However, there are other parameters which may affect the spatial distribution of slip across the fault plane. For example, the type of probability density function used in the placement of the subevent is also important. In the case where the location of maximum slip is known to a first order, a Gaussian may be appropriate to describe the probability function. For hazard assessment studies a uniform probability density function is more suitable as it provides no underlying systematic spatial trend.

    更新日期:2020-02-03
  • Tangential stress at the core-mantle interface
    Geophys. J. Int. (IF 2.777) Pub Date : 2020-02-03
    Jault D.

    SummaryFluctuations in the rotation rate of the solid Earth over periods from 5 to 100 years result from exchanges of angular momentum between the fluid outer core and the solid mantle. The coupling mechanism mediating angular momentum transfer is not clear yet. Here, I revisit local Cartesian models for the pressure stress on a bumpy core-mantle interface. One common approach consists in analyzing forced magnetohydrodynamic modes arising from the interaction between a steady flow along the core-mantle interface and boundary topography. The wave amplitude scales as the height ζ of corrugations and the pressure stress as ζ2. As expected from Newton’s third law, the tangential stress on the fluid is opposite to the tangential stress on the solid. It is exactly compensated by non zero mean electromagnetic and Coriolis forces, which both result from interactions at infinity and not with the electrically insulating solid. Requiring zero net flux of mass and electrical current at infinity in order to better model closed systems necessitates to restore mean flow acceleration. This makes possible to investigate whether there is momentum transfer into the fluid interior or instead dissipation next to the boundary. Fluid stratification enhances the horizontal stress exerted by the pressure field on the core-mantle boundary but we have yet to describe the mechanism to transport momentum from the boundary into the fluid.

    更新日期:2020-02-03
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