• Geophys. J. Int. (IF 2.777) Pub Date : 2020-01-16
Furumura T, Kennett B.

SUMMARYSurface waves are usually dispersive with long wavetrains 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 wavespeed 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 e.g. 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, e.g. 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 three-dimensional finite-difference simulation of seismic wave propagation and analysis of dispersion curves.

更新日期：2020-01-16
• Geophys. J. Int. (IF 2.777) Pub Date : 2020-01-16
Montiel-Álvarez A, Romo J, Constable S, et al.

ABSTRACTThe magnetotelluric (MT) impedance tensor has a nil diagonal when one of the axes of the coordinate system coincides with the strike of a two-dimensional (2D) structure. In general, real data are full tensors either because of 3D effects or measurements not aligned to the geological strike. The usual practice to adapt the field tensor to the 2D assumption is to rotate to a new system of coordinates. In most cases, there is no single angle of rotation that warranties that the diagonal elements become zeros as in the ideal 2D case. Even maximizing the off-diagonal elements does not necessarily produce a nil diagonal. Consequently, the 2D inversions proceed by neglecting whatever there is left in the diagonals. In this work, we explore an alternative that places no constraints on direction but assures a nil diagonal. We use two rotational invariants that compact the four elements of the tensor into only two and reduce in 2D to the TE and TM impedances. These are obtained readily by solving a quadratic equation. We explore four different scenarios: 1) using the invariants, 2) rotating the tensor perpendicular to the profile, 3) rotating to the average maximum orientation for each station, and 4) maximizing the off-diagonal elements of the tensor for each site, frequency to frequency. These approaches were applied to 3D synthetic and field data. The field data correspond to two marine magnetotelluric surveys in the Gulf of California. In one of them, there is no information on the instrument orientation because the compasses failed. In this case, the rotational invariants come handy to overcome the problem. In the other survey, there was orientation information and the 2D inversions illustrate the better performance of the invariants relative to the traditional approaches.

更新日期：2020-01-16
• Geophys. J. Int. (IF 2.777) Pub Date : 2020-01-16
Marotta A, Restelli F, Bollino A, et al.

SUMMARYThe anomalous density structure at subduction zones, both in the wedge and in the upper mantle, is analysed to shed light on the processes that are responsible for the characteristic gravity fingerprints of two types of subduction: ocean-continent and ocean-ocean. Our modelling is then performed within the frame of the EIGEN-6C4 gravitational disturbance pattern of two subductions representative of the above two types, the Sumatra and Mariana complexes, finally enabling the different characteristics of the two patterns to be observed and understood on a physical basis, including some small-scale details. A 2D viscous modelling perpendicular to the trench accounts for the effects on the gravity pattern caused by a wide range of parameters in terms of convergence velocity, subduction dip angle and lateral variability of the crustal thickness of the overriding plate, as well as compositional differentiation, phase changes and hydration of the mantle. Plate coupling, modelled within a new scheme where the relative velocity at the plate contact results self-consistently from the thermo-mechanical evolution of the system, is shown to have an important impact on the gravity signature. Beyond the already understood general bipolar fingerprint of subduction, perpendicular to the trench, we obtain the density and gravity signatures of the processes occurring within the wedge and mantle that are responsible for the two different gravity patterns. To be compliant with the geodetic EIGEN-6C4 gravitational disturbance and to compare our predictions with the gravity at Sumatra and Mariana, we define a model normal Earth. Although the peak-to-peak gravitational disturbance is comparable for the two types of subductions, approximately 250 mGal, from both observations and modelling, encompassing the highest positive maximum on the overriding plates and the negative minimum on the trench, the trough is wider for the ocean-ocean subduction: approximately 300 km compared to approximately 180 km for the ocean-continent subduction. Furthermore, the gravitational disturbance pattern is more symmetric for the ocean-ocean subduction compared to the ocean-continent subduction in terms of the amplitudes of the two positive maxima over the overriding and subducting plates. Their difference is, for the ocean-ocean type, approximately one half of the ocean-continent one. These different characteristics of the two types of subductions are exploited herein in terms of the different crustal thicknesses of the overriding plate and of the different dynamics in the wedge and in the mantle for the two types of subduction, in close agreement with the gravity data.

更新日期：2020-01-16
• Geophys. J. Int. (IF 2.777) Pub Date : 2020-01-15
Hinderer J, Hector B, Riccardi U, et al.

We analyze a nearly 8-year 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, 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 3D 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：2020-01-15
• Geophys. J. Int. (IF 2.777) Pub Date : 2020-01-15
Birnie C, Chambers K, Angus D, et al.

SUMMARYTesting with synthetic datasets is a vital stage in an algorithm’s development for benchmarking the algorithm’s performance. A common addition to synthetic datasets is White, Gaussian Noise (WGN) which is used to mimic noise that would be present in recorded datasets. The first section of this paper focusses on comparing the effects of WGN and realistic modelled noise on standard microseismic event detection and imaging algorithms using synthetic datasets with recorded noise as a benchmark. The datasets with WGN under-perform on the trace-by-trace algorithm whilst over-performing on algorithms utilising the full array. Throughout, the datasets with realistic modelled noise perform near identically to the recorded noise datasets. The study concludes by testing an algorithm which 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 datasets 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 minimise the risk of misinterpretation of the results.

更新日期：2020-01-15
• Geophys. J. Int. (IF 2.777) Pub Date : 2020-01-15
Ernst T, Nowozynski K, Jozwiak W.

We have analyzed 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 one-minute 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. Analyzing the estimated external parts of vertical components 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：2020-01-15
• Geophys. J. Int. (IF 2.777) Pub Date : 2020-01-14
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：2020-01-14
• Geophys. J. Int. (IF 2.777) Pub Date : 2020-01-11
Elison P, Dukalski M, de Vos K, et al.

SummaryShort-period internal multiples, resulting from closely-spaced interfaces, may interfere with their generating (bandlimited) primaries, and hence they pose a long-standing challenge in their prediction and removal. A recently proposed method based on the Marchenko equation enables removal of the entire overburden-related scattering by means of calculating an inverse transmission response. However, the method relies on time windowing and can thus be inexact in the presence of short-period internal scattering. In this work, we present a detailed analysis of the impact of band-limitation on the Marchenko method. We show the influence of an incorrect first guess, and that adding multidimensional energy conservation and a minimum phase principle may be used to correctly account for both long- and short-period internal multiple scattering. The proposed method can currently only be solved for media with a laterally invariant overburden, since a multidimensional minimum phase condition is not well understood for truly 2-D and 3-D media. We demonstrate the virtue of the proposed scheme with a complex acoustic numerical model that is based on sonic log measurements in the Middle East. The results suggest not only that the conventional scheme can be robust in this setting, but that the ‘augmented’ Marchenko method is superior, as the latter produces a structural image identical to one where the finely layered overburden is missing. This is the first demonstration of a data-driven method to account for short-period internal multiples beyond 1-D.

更新日期：2020-01-13
• 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 two-dimensional 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 employed 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：2020-01-13
• 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 kilometers 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 non-linear 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：2020-01-13
• Geophys. J. Int. (IF 2.777) Pub Date : 2019-11-28
McCubbine J, Stagpoole V, Caratori Tontini F, et al.

SUMMARYQuasigeoid models can be determined from surface gravity anomalies, so are sensitive to changes in the shape of the topography as well as changes in gravity. Here we present results of forward modelling gravity/quasigeoid changes from synthetic aperture radar data following the 2016 Mw 7.8 Kaikōura earthquake with land uplift of up to 10 m. We assess the impact of the topographic deformation on the reference surface of the New Zealand vertical datum in lieu of costly field gravity field measurements. The most significant modelled gravity and quasigeoid changes are—2.9 mGal and 5–7 mm, respectively. We compare our forward modelled gravity signal to terrestrial gravity observation data and show that differences between the data sets have a standard deviation of ±0.1 mGal. The largest modelled change in the quasigeoid is an order of magnitude smaller than the 57.7 mm estimated precision of the most recently computed NZGeoid model over the Kaikōura region. Modelled quasigeoid changes implied by this particular deformation event are not statistically significant with respect to estimated precision of the New Zealand quasigeoid model.

更新日期：2020-01-13
• Geophys. J. Int. (IF 2.777) Pub Date : 2019-12-02
Guo Z, Zhou Y.

SUMMARYWe report finite-frequency imaging of the global 410- and 660-km discontinuities using boundary sensitivity kernels for traveltime measurements made on SS precursors. The application of finite-frequency sensitivity kernels overcomes resolution limits in previous studies associated with large Fresnel zones of SS precursors and their interferences with other seismic phases. In this study, we calculate the finite-frequency sensitivities of SS waves and their precursors based on a single-scattering (Born) approximation in the framework of travelling-wave mode summation. The global discontinuity surface is parametrized using a set of triangular gridpoints with a lateral spacing of about 4°, and we solve the linear finite-frequency inverse problem (2-D tomography) based on singular value decomposition (SVD). The new global models start to show a number of features that were absent (or weak) in ray-theoretical back-projection models at spherical harmonic degree l > 6. The thickness of the mantle transition zone correlates well with wave speed perturbations at a global scale, suggesting dominantly thermal origins for the lateral variations in the mantle transition zone. However, an anticorrelation between the topography of the 410-km discontinuity and wave speed variations is not observed at a global scale. Overall, the mantle transition zone is about 2–3 km thicker beneath the continents than in oceanic regions. The new models of the 410- and 660-km discontinuities show better agreement with the finite-frequency study by Lawrence & Shearer than other global models obtained using SS precursors. However, significant discrepancies between the two models exist in the Pacific Ocean and major subduction zones at spherical harmonic degree >6. This indicates the importance of accounting for wave interactions in the calculations of sensitivity kernels as well as the use of finite-frequency sensitivities in data quality control.

更新日期：2020-01-13
• Geophys. J. Int. (IF 2.777) Pub Date : 2019-12-03
Shirzad T, Assumpcao M, Bianchi M.

SUMMARYSurface wave analysis provides important information on crustal structure, but it is challenging to obtain accurate/robust models in aseismic regions because of the lack of local earthquake records. In this paper, interstation empirical Green's functions retrieved by ambient seismic noise in 75 broad-band stations from 2016 January to 2018 September were used to study crustal structure in west-central Brazil. Fast marching method was applied to calculate the 2-D surface wave tomographic maps, and local dispersion curves were estimated in the period range of 4–80 s for each geographic cell. 1-D damped least squares inversion method was then conducted to obtained shear wave velocity model. Finally, the average ($\tilde{\rm V}$S) of the calculated VSV and VSH quasi 3-D models were used to characterize the crustal structure. Besides the checkerboard test resolution, a stochastic test with the effect of errors in the dispersion curves and choice of inversion parameters were carried out to better evaluate model uncertainties. Our results show a clear relation between the sedimentary thickness and geological units with the shorter period tomographic maps. Agreement has also been observed in longer periods such as the clear N–S anomaly along the Asuncion and Rio Grande Arches representing the boundary between the Chaco-Paraná and the Paraná basins. A 3-D composite velocity model shows a crustal structure consisting of three main layers. Some differences in lower crustal properties were found between the Paraná and Chaco-Paraná basins, consistent with a recently postulated, gravity-derived Western Paraná suture zone. However, no high velocities along the SW–NE axis of the Paraná basin were found to confirm proposed underplating. At the eastern edge of the Pantanal basin, the thin crust seems to be associated with a very thin (or lack of) lower crustal layer, consistent with a recently proposed crustal delamination hypothesis for the formation of the Pantanal basin.

更新日期：2020-01-13
• Geophys. J. Int. (IF 2.777) Pub Date : 2019-12-05
Stovas A, Roganov Y, Roganov V.

SUMMARYApplication of the Floquet theorem and the matrix propagator method reduces the problem of the plane wave propagation in a periodically layered anisotropic media, to analysis of the properties of stationary envelopes of different wave modes propagating up- and downwards. We analyse the interchanging of stop- and pass-bands and their structure at low frequencies for a periodically layered medium with monoclinic symmetry. The analysis shows the effect of interaction between P,S1 and S2 wave multipliers for stop- and pass-band structure and gives insight into the wave propagation in vertically heterogeneous anisotropic media which is important in modelling and interpretation of seismic data.

更新日期：2020-01-13
• Geophys. J. Int. (IF 2.777) Pub Date : 2019-12-05
Jordi C, Doetsch J, Günther T, et al.

SUMMARYStructural joint inversion of several data sets on an irregular mesh requires appropriate coupling operators. To date, joint inversion algorithms are primarily designed for the use on regular rectilinear grids and impose structural similarity in the direct neighbourhood of a cell only. We introduce a novel scheme for calculating cross-gradient operators based on a correlation model that allows to define the operator size by imposing physical length scales. We demonstrate that the proposed cross-gradient operators are largely decoupled from the discretization of the modelling domain, which is particularly important for irregular meshes where cell sizes vary. Our structural joint inversion algorithm is applied to a synthetic electrical resistivity tomography and ground penetrating radar 3-D cross-well experiment aiming at imaging two anomalous bodies and extracting the parameter distribution of the geostatistical background models. For both tasks, joint inversion produced superior results compared with individual inversions of the two data sets. Finally, we applied structural joint inversion to two field data sets recorded over a karstified limestone area. By including geological a priori information via the correlation-based operators into the joint inversion, we find P-wave velocity and electrical resistivity tomograms that are in accordance with the expected subsurface geology.

更新日期：2020-01-13
• Geophys. J. Int. (IF 2.777) Pub Date : 2019-12-06
Jing X, Li L.

SUMMARYAs seismic waves propagate in the Earth, the directions of particle motions are affected by the media that they encounter, and thus seismic wave polarization direction carries the information on the media. So far there remains unclear about what can be inferred from the P-wave polarization direction data. For clarifying it, we discuss the mapping relation between polarization direction and velocity distribution. It is found that the velocity model cannot be derived uniquely from the polarization direction data. By analysing the relation between slowness vectors of the seismic ray at the source and the receiver, we find that relative velocity gradient is the physical quantity that describes the capability to deflect seismic rays in a continuous medium. The equation describing the relation between polarization direction and relative velocity gradient is given. For imaging relative velocity gradients, we derive the calculation formula for the partial derivative of polarization direction with respect to velocity gradient parameters. Synthetic experiments are conducted. The test results demonstrate that the absolute velocity model cannot be recovered from P-wave polarization direction data, but the relative velocity gradient model can. Polarization direction tomography gives a way to build gradient maps for the geometric characteristic of the subsurface velocity structures.

更新日期：2020-01-13
• Geophys. J. Int. (IF 2.777) Pub Date : 2019-12-06
Tchawe F, Froment B, Campillo M, et al.

SUMMARYThe horizontal to vertical spectral ratio (HVSR) of seismic ambient noise has been proven to be a fast and efficient method for characterizing the 1-D resonance frequency of the local subsurface in a practical framework. Over the last decades, theories have been developed in order to extend the exploitation of HVSR beside the frequency of its first peak, notably the diffuse field assumption (DFA) which links the HVSR to the Green’s function of the local medium assuming the diffuseness of the seismic ambient noise wavefield. However, the underlying assumption of the seismic ambient noise being a diffuse, equipartitioned field may not be satisfied under certain circumstances. In order to exploit the contribution of scattering in forging diffuse wave fields, we leverage the advantages of coda waves and present a novel procedure for computing the HVSR, using the coda part of ambient noise correlations. We applied this technique to data gathered at the plio-quaternary sedimentary basin of Argostoli, Greece. Results on this data set show the potential of the method to improve the temporal stability of the HVSR measurements compared to the classical computation, and the fit with the theoretical HVSR curve derived from the DFA theory. These results suggest that this procedure could help in extracting physical information from the HVSR and thus could lead to an extended use of these measurements to characterize the mechanical properties of the medium.

更新日期：2020-01-13
• Geophys. J. Int. (IF 2.777) Pub Date : 2019-12-06
Deng W, Morozov I.

SUMMARYWave-induced fluid flows (WIFF) can be viewed as cases of broader local-deformation (LD) phenomena and represent the principal causes of seismic-wave attenuation in fluid-saturated porous rock. Most existing WIFF models refer to greatly simplified microstructures and specific flow patterns, such as planar divergent flows within thin cracks (squirt flows, SF) or flows within patchy-saturation zones. However, such microstructures represent only idealized mathematical models that may be impossible to consistently identify within a given rock. At the same time, most details of such microstructures are insignificant for seismic waves, which are only sensitive to averaged properties of the medium. To perform microstructure-independent modelling of LD effects, we develop a simple yet general approach based entirely on a macroscopic local-deformation variable. This variable is broadly analogous to Biot's fluid content and is illustrated for two specific microstructural models. The macroscopic model is Biot-consistent and uses only time- and frequency-independent material properties. Both local and global (Biot's) pore flows and all types of waves and deformations are explained in a rigorous and consistent manner. The model allows constraining a minimal set of material properties responsible for all observed elastic and anelastic effects in porous rock. Because of making no assumptions about the microstructures and their spatial scales, this approach should comprise at least some of the existing WIFF models. In particular, this model accurately reproduces all attenuation and velocity dispersion spectra predicted by a broadly used SF model. The model also contains effects not considered previously, such as bulk viscosity of pore fluid and viscous coupling between the rock frame and fluid-filled pores. The model offers straightforward extensions to multiple porosities and cases of viscous fluids in primary pores. Based on the resulting differential equations, physically consistent schemes for numerical modelling of seismic wavefields can be developed for porous rock with arbitrary LD effects.

更新日期：2020-01-13
• Geophys. J. Int. (IF 2.777) Pub Date : 2019-12-13
Saito T, Noda A.

SUMMARYThis study investigates the strain energy change caused by earthquake faulting. While conventional theories often assumed uniform stress change on the fault plane, this study supposed the slip fluctuation and non-uniform stress change on the fault. By using a stochastic modelling of the slip distribution, we represent the ensemble average of the strain energy change by using the power spectral density function of the slip fluctuation. This yields the following results. (1) When the initial stress is uniform and the earthquake contains a fluctuating slip distribution, the released strain energy is less than the one by an earthquake with the uniform stress change on the fault with the same seismic moment. (2) On the other hand, when the initial stress is fluctuating, the earthquake contains a fluctuating slip distribution, and the final stress is uniform, the released strain energy is more than the one by an earthquake with the uniform stress change on the fault. (3) The stress drop becomes large due to the fluctuating slip distribution from the viewpoint of the strain energy release. We derived the analytical solution of the stress change by using the power spectral density function of the random slip fluctuation. (4) The strain energy change is proportional to the seismic moment when ${\epsilon ^2}/a \propto {( {{M_0}} )^{ - 1/3}}$ (${\epsilon ^2}$ is the variance of the fractional slip fluctuation and $a$ is the correlation distance). (5) The energy balance gives the value of initial stress that is required for the earthquake generation. In order to generate an earthquake, the initial stress needs to be larger than the sum of half of the stress drop and the apparent stress. In other words, earthquakes having rich short-wavelength components in the slip distribution are not generated under a low initial stress level.

更新日期：2020-01-13
• Geophys. J. Int. (IF 2.777) Pub Date : 2019-12-13
Kamm J, Becken M, Abreu R.

SUMMARYMaxwell’s equations are valid regardless of the choice of the coordinate system. By this property a change of coordinates can be equivalently expressed as a change of the material parameters. This idea opens a new approach to the problem of accurate electromagnetic modelling in the vicinity of steep topography or bathymetry. Via a change of coordinates, any earth model with complicated layer interfaces can be represented by an equivalent model where those interfaces are flat, but with its materials correspondingly altered. This new model could then be discretized on a regular mesh and fields could be computed by an appropriate finite difference or integral equation code. Unfortunately, this is not straightforward because both the new electric and magnetic materials are fully anisotropic. By instead applying a finite element secondary field approach to the equivalent model, we can completely account for the topography interface in the planar layered background model. The only modification required to existing finite element formulations is a slightly more complicated right-hand side of the linear system of equations, whereas the system matrix is unchanged in any coordinate system. In a numerical modelling experiment we confirm that our technique gives increased accuracy when compared with a recently published technique for dealing with topography in a secondary field formulation for the case of a magnetotelluric source field. In turn, in the vicinity of conductivity anomalies, accuracy can also be negatively affected.

更新日期：2020-01-13
• Geophys. J. Int. (IF 2.777) Pub Date : 2019-12-13
Shcherbakova V, Bakhmutov V, Thallner D, et al.

SUMMARYThe time-averaged geomagnetic field is generally purported to be uniformitarian across Earth history—close to a geocentric axial dipole, with average strength within one order of magnitude of that at present. Nevertheless, recent studies have reported that the field was approximately ten times weaker than present in the mid-Palaeozoic (∼410–360 Ma) and late Ediacaran (∼565 Ma). Here we present the first whole-rock palaeointensity determinations of Ediacaran age outside of Laurentia. These were obtained by the Thellier-Coe, Wilson and microwave methods for basaltic rocks of 560–580 Ma age of the Ediacaran traps, southwestern margin of the East European Craton, Ukraine. All four studied sites showed extremely low instantaneous field values of (3–7) μT with corresponding VDMs of (0.4–1) × 1022 Am2. Summarizing all available data, the Ediacaran field appears to be anomalously characterized by ultra-low dipole moment and ultra-high reversal frequency. According to some geodynamo models, this state could indicate a weak dipole field regime prior to the nucleation of the solid inner core. However, given that ultra-low field intensities have also been detected in the Devonian, and that virtually no palaeointensity data exist for the intervening ∼150 Ma, the date of inner core nucleation remains extremely uncertain. Our new evidence of persistent ultra-weak magnetospheric shielding in the Ediacaran may be considered consistent with the recently hypothesized link between enhanced UV-B radiation in this interval and the subsequent Cambrian evolutionary radiation.

更新日期：2020-01-13
• Geophys. J. Int. (IF 2.777) Pub Date : 2019-12-16
Mariniere J, Nocquet J, Beauval C, et al.

SUMMARYQuito, the capital city of Ecuador hosting ∼2 million inhabitants, lies on the hanging wall of a ∼60-km-long reverse fault offsetting the Inter-Andean Valley in the northern Andes. Such an active fault poses a significant risk, enhanced by the high density of population and overall poor building construction quality. Here, we constrain the present-day strain accumulation associated with the Quito fault with new Global Positioning System (GPS) data and Persistent Scatterer Interferometric Synthetic Aperture Radar (PS-InSAR) analysis. Far field GPS data indicate 3–5 mm yr–1 of horizontal shortening accommodated across the fault system. In the central segment of the fault, both GPS and PS-InSAR results highlight a sharp velocity gradient, which attests for creep taking place along the shallowest portion of the fault. Smoother velocity gradients observed along the other segments indicate that the amount of shallow creep decreases north and south of the central segment. 2-D elastic models using GPS horizontal velocity indicate very shallow (<1 km) locking depth for the central segment, increasing to a few kilometres south and north of it. Including InSAR results in the inversion requires locking to vary both along dip and along strike. 3-D spatially variable locking models show that shallow creep occurs along the central 20-km-long segment. North and south of the central segment, the interseismic coupling is less resolved, and data still allows significant slip deficit to accumulate. Using the interseismic moment deficit buildup resulting from our inversions and the seismicity rate, we estimate recurrence time for magnitude 6.5 + earthquake to be between 200 and 1200 yr. Finally, PS-InSAR time-series identify a 2 cm transient deformation that occurred on a secondary thrust, east of the main Quito fault between 1995 and 1997.

更新日期：2020-01-13
• Geophys. J. Int. (IF 2.777) Pub Date : 2019-12-17
Lynner C, Koch C, Beck S, et al.

SUMMARYThe Ecuadorian convergent margin has experienced many large mega-thrust earthquakes in the past century, beginning with a 1906 event that propagated along as much as 500 km of the plate interface. Many subsections of the 1906 rupture area have subsequently produced Mw ≥ 7.7 events, culminating in the 16 April 2016, Mw 7.8 Pedernales earthquake. Interestingly, no large historic events Mw ≥ 7.7 appear to have propagated southward of ∼1°S, which coincides with the subduction of the Carnegie Ridge. We combine data from temporary seismic stations deployed following the Pedernales earthquake with data recorded by the permanent stations of the Ecuadorian national seismic network to discern the velocity structure of the Ecuadorian forearc and Cordillera using ambient noise tomography. Ambient noise tomography extracts Vsv information from the ambient noise wavefield and provides detailed constraints on velocity structures in the crust and upper mantle. In the upper 10 km of the Ecuadorian forearc, we see evidence of the deepest portions of the sedimentary basins in the region, the Progreso and Manabí basins. At depths below 30 km, we observe a sharp delineation between accreted fast forearc terranes and the thick crust of the Ecuadorian Andes. At depths ∼20 km, we see a strong fast velocity anomaly that coincides with the subducting Carnegie Ridge as well as the southern boundary of large mega-thrust earthquakes. Our observations raise the possibility that upper-plate structure, in addition to the subducting Carnegie Ridge, plays a role in the large event segmentation seen along the Ecuadorian margin.

更新日期：2020-01-13
• Geophys. J. Int. (IF 2.777) Pub Date : 2019-12-18
Agudo Ò, da Silva N, Stronge G, et al.

SUMMARYThe potential of full-waveform inversion (FWI) to recover high-resolution velocity models of the subsurface has been demonstrated in the last decades with its application to field data. But in certain geological scenarios, conventional FWI using the acoustic wave equation fails in recovering accurate models due to the presence of strong elastic effects, as the acoustic wave equation only accounts for compressional waves. This becomes more critical when dealing with land data sets, in which elastic effects are generated at the source and recorded directly by the receivers. In marine settings, in which sources and receivers are typically within the water layer, elastic effects are weaker but can be observed most easily as double mode conversions and through their effect on P-wave amplitudes. Ignoring these elastic effects can have a detrimental impact on the accuracy of the recovered velocity models, even in marine data sets. Ideally, the elastic wave equation should be used to model wave propagation, and FWI should aim to recover anisotropic models of velocity for P waves (vp) and S waves (vs). However, routine three-dimensional elastic FWI is still commercially impractical due to the elevated computational cost of modelling elastic wave propagation in regions with low S-wave velocity near the seabed. Moreover, elastic FWI using local optimization methods suffers from cross-talk between different inverted parameters. This generally leads to incorrect estimation of subsurface models, requiring an estimate of vp/vs that is rarely known beforehand. Here we illustrate how neglecting elasticity during FWI for a marine field data set that contains especially strong elastic heterogeneities can lead to an incorrect estimation of the P-wave velocity model. We then demonstrate a practical approach to mitigate elastic effects in 3-D yielding improved estimates, consisting of using a global inversion algorithm to estimate a model of vp/vs, employing matching filters to remove elastic effects from the field data, and performing acoustic FWI of the resulting data set. The quality of the recovered models is assessed by exploring the continuity of the events in the migrated sections and the fit of the latter with the recovered velocity model.

更新日期：2020-01-13
• Geophys. J. Int. (IF 2.777) Pub Date : 2019-12-20
Kiss D, Candioti L, Duretz T, et al.

SUMMARYWe present 2-D numerical simulations of convergence at a hyperextended passive margin with exhumed subcontinental mantle. We consider viscoelasto-plastic deformation, heat transfer and thermomechanical coupling by shear heating and associated thermal softening due to temperature dependent viscosity. The simulations show subduction initiation for convergence velocities of 2 cm yr−1, initial Moho temperatures of 525 °C and maximal deviatoric stresses of ca. 800 MPa, around the Moho, prior to localization. Subduction initiates in the region with thinned continental crust and is controlled by a thermally activated ductile shear zone in the mantle lithosphere. The shear zone temperature can be predicted with a recently published analytical expression. The criterion for subduction initiation is a temperature difference of at least 225 °C between predicted temperature and initial Moho temperature. The modelled forced subduction broadly agrees with geological data and reconstructions of subduction during closure of the Piemont-Liguria basin, caused by convergence of the European and Adriatic plates during the Alpine orogeny.

更新日期：2020-01-13
• Geophys. J. Int. (IF 2.777) Pub Date : 2020-01-13
Cheng Y, Renner J.

AddendaErrataEuropeFourier analysisFracture and flowHydrothermal systemsMechanicsand ModellingNumerical modellingTheory

更新日期：2020-01-13
• Geophys. J. Int. (IF 2.777) Pub Date : 2020-01-13
Petrescu L, Stuart G, Houseman G, et al.

SUMMARYSince the Mesozoic, central and eastern European tectonics have been dominated by the closure of the Tethyan Ocean as the African and European plates collided. In the Miocene, the edge of the East European Craton and Moesian Platform were reworked in collision during the Carpathian orogeny and lithospheric extension formed the Pannonian Basin. To investigate the mantle deformation signatures associated with this complex collisional-extensional system, we carry out SKS splitting analysis at 123 broad-band seismic stations in the region. We compare our measurements with estimates of lithospheric thickness and recent seismic tomography models to test for correlation with mantle heterogeneities. Reviewing splitting delay times in light of xenolith measurements of anisotropy yields estimates of anisotropic layer thickness. Fast polarization directions are mostly NW–SE oriented across the seismically slow West Carpathians and Pannonian Basin and are independent of geological boundaries, absolute plate motion direction or an expected palaeo-slab roll-back path. Instead, they are systematically orthogonal to maximum stress directions, implying that the indenting Adria Plate, the leading deformational force in Central Europe, reset the upper-mantle mineral fabric in the past 5 Ma beneath the Pannonian Basin, overprinting the anisotropic signature of earlier tectonic events. Towards the east, fast polarization directions are perpendicular to steep gradients of lithospheric thickness and align along the edges of fast seismic anomalies beneath the Precambrian-aged Moesian Platform in the South Carpathians and the East European Craton, supporting the idea that craton roots exert a strong influence on the surrounding mantle flow. Within the Moesian Platform, SKS measurements become more variable with Fresnel zone arguments indicating a shallow fossil lithospheric source of anisotropy likely caused by older tectonic deformation frozen in the Precambrian. In the Southeast Carpathian corner, in the Vrancea Seismic Zone, a lithospheric fragment that sinks into the mantle is sandwiched between two slow anomalies, but smaller SKS delay times reveal weaker anisotropy occurs mainly to the NW side, consistent with asymmetric upwelling adjacent to a slab, slower mantle velocities and recent volcanism.

更新日期：2020-01-13
• 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 modeling 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：2020-01-10
• Geophys. J. Int. (IF 2.777) Pub Date : 2020-01-10
Liu B, Pang Y, Mao D, et al.

SummaryFour-dimensional (4D) 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 4D inversion. To address this issue, a scheme is proposed in this paper for restructuring continuously measured datasets 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 4D inversion. In addition, the response of the 4D 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 dataset and historical 4D inversion model. Numerical simulations and experiments show that this new 4D inversion method is able to locate and depict rapid changes in medium resistivity with a high level of accuracy.

更新日期：2020-01-10
• Geophys. J. Int. (IF 2.777) Pub Date : 2020-01-09
Saikia D, Kumar M, Singh A.

SummaryA comprehensive data set of 73876 high quality receiver functions computed using waveforms recorded by 327 broadband 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：2020-01-09
• Geophys. J. Int. (IF 2.777) Pub Date : 2020-01-09
Sato T, No T, Arai R, et al.

SummaryWe obtain the crustal structure from active-source seismic surveys using ocean bottom seismographs and seismic shots to elucidate the evolutionary process from continental rifting to the back-arc basin opening in the Yamato Basin and Oki Trough in the southern Japan Sea. Results show that the crust changes from approximately 14–15 km thick in the basin (the southern Yamato Basin) to 16.5–17 km in the margin of the basin (the southwestern edge of the Yamato Basin). The P-wave velocity distribution in the crust of the southern Yamato Basin is missing a typical continental upper crust with P-wave velocities of 5.4–6.0 km/s, and is thought be a thicker oceanic crust formed by a back-arc basin opening. By contrast, the crust of the southwestern edge of the Yamato Basin might have been formed by continental rifting because there is an unit with P-wave velocities of 5.4–6.0 km/s and with a gentle velocity gradients, corresponding to the continental upper crust in this area. This variation might reflect differences in mantle properties from continental rifting to back-arc basin opening of the Yamato Basin. Because the Oki Trough has a crustal thickness of 17–19 km and having a unit with P-wave velocities of 5.4–6.0 km/s, corresponding to the continental upper crust with a high-velocity lower crust, we infer that this trough was formed by continental rifting with magmatic intrusion or underplating. These crustal variations might reflect transitional stages from continental rifting to back-arc basin opening in the southern Japan Sea.

更新日期：2020-01-09
• 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 ($\sim {10^{ - 7}}/yr$) 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 $10$ ${\rm{kPa}}/{\rm{yr}}$ 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：2020-01-09
• Geophys. J. Int. (IF 2.777) Pub Date : 2020-01-09
Hagen C, Reilly B, Stoner J, et al.

SummaryWe present and make publicly available a dynamic programming algorithm to simultaneously align the inclination and declination vector directions of sedimentary paleomagnetic secular variation data.This algorithmgenerates a library of possible alignments through the systematic variation of assumptions about the relative accumulation rate and shared temporal overlap of two or more time-series. The paleomagnetist can then evaluate this library of reproducible and objective alignments using available geologic constraints, statistical methods, and expert knowledge.We apply the algorithm to align previously (visually) correlated medium to high accumulation rate northern North Atlantic Holocene deposits (101– 102 cm/ka) with strong radiocarbon control. The algorithm generates plausible alignments that largely conform with radiocarbon and magnetic acquisition process uncertainty. These alignments illustrate the strengths and limitations of this numerical approach.

更新日期：2020-01-09
• 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 employ the method in two and three-dimensional 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：2020-01-08
• 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 three years 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：2020-01-08
• Geophys. J. Int. (IF 2.777) Pub Date : 2020-01-08
den Ouden O, Assink J, Smets P, et al.

The 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 which 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 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 that 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 IMS 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：2020-01-08
• Geophys. J. Int. (IF 2.777) Pub Date : 2020-01-08
Kassaras I, Kapetanidis V, Karakonstantis A, et al.

SummaryThis research provides new constraints on the intermediate depth upper-mantle structure of the Hellenic lithosphere using a three-step Rayleigh-wave tomography. Broadband waveforms of about 1000 teleseismic events, recorded by ∼200 permanent broadband stations between 2010 and 2018 were acquired and processed. Through a multichannel cross-correlation technique, the fundamental mode Rayleigh-wave phase-velocity dispersion curves in the period range 30 to 90 s were derived. The phase-velocities were inverted and a 3-D shear velocity model was obtained down to the depth of 140 km. The applied method has provided 3-D constraints on large-scale characteristics of the lithosphere and the upper mantle of the Hellenic region. Highlighted resolved features include the continental and oceanic subducting slabs in the region, the result of convergence between Adria and Africa plates with the Aegean. The boundary between the oceanic and continental subduction is suggested to exist along a trench-perpendicular line that connects NW Peloponnese with N. Euboea, bridging the Hellenic Trench with the North Aegean Trough. No clear evidence for trench-perpendicular vertical slab tearing was resolved along the western part of Hellenic Subduction Zone; however, subcrustal seismicity observed along the inferred continental-oceanic subduction boundary indicates that such an implication should not be excluded. The 3-D shear velocity model supports an N-S vertical slab tear beneath SW Anatolia that justifies deepening, increase of dip and change of dip direction of the Wadati-Benioff Zone. Low velocities found at depths < 50 km beneath the island and the back-arc, interrelated with recent/remnant volcanism in the Aegean and W. Anatolia, are explained by convection from a shallow asthenosphere.

更新日期：2020-01-08
• Geophys. J. Int. (IF 2.777) Pub Date : 2020-01-08
Howe M, Ekström G, Richards P.

SummaryWe have re-analyzed observations of body waves and surface waves for 71 well-recorded underground nuclear explosions (UNEs) that were conducted between 1977 and 1989 at the Balapan sub-region of the Semipalatinsk Test Site in Kazakhstan. To reconcile differences between body-wave and surface-wave amplitudes we solve for a scaling factor between vertical and horizontal forces in the explosion model. We find that the estimated scaling factor is anti-correlated with the scaled depth of burial for the subset of UNEs at Balapan that have published depths. The observed anti-correlation and the inferred variations in force scaling suggest that recorded surface-wave amplitudes are significantly influenced by UNE burial depth as well as by previously recognized tectonic release. As part of our analysis, we revisit the relationship between teleseismic mb(P) and yield for UNEs at Balapan, and discuss the physical basis for effectiveness of the mb–MS discriminant.

更新日期：2020-01-08
• Geophys. J. Int. (IF 2.777) Pub Date : 2020-01-08
Pan Z, He J, Shao Z.

Focal 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：2020-01-08
• 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 travel time 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 2D 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：2020-01-07
• 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 dynamical 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：2020-01-07
• Geophys. J. Int. (IF 2.777) Pub Date : 2020-01-06
Celli N, Lebedev S, Schaeffer A, et al.

SummaryWe present a tomographic model of the crust, upper mantle and transition zone beneath the South Atlantic, South America and Africa. Taking advantage of the recent growth in broadband data sampling, we compute the model using waveform fits of over 1.2 million vertical-component seismograms, obtained with the Automated Multimode Inversion of surface, S and multiple S waves. Each waveform provides a set of linear equations constraining perturbations with respect to a three-dimensional (3D) reference model within an approximate sensitivity volume. We then combine all equations into a large linear system and solve it for a 3D model of S- and P-wave speeds and azimuthal anisotropy within the crust, upper mantle and uppermost lower mantle. In South America and Africa, our new model SA2019 reveals detailed structure of the lithosphere, with structure of the cratons within the continents much more complex than seen previously. In South America, lower seismic velocities underneath the Transbrasilian Lineament (TBL) separate the high-velocity anomalies beneath the Amazon Craton from those beneath the São Francisco and Paraná Cratons. We image the buried portions of the Amazon Craton, the thick cratonic lithosphere of the Paraná and Parnaíba Basins, and an apparently cratonic block wedged between western Guyana and the slab to the west of it, unexposed at the surface. Thick cratonic lithosphere is absent under the Archean crust of the São Luis, Luis Álves and Rio de La Plata Cratons, next to the continental margin. The Guyana Highlands are underlain by low velocities, indicating hot asthenosphere. In the transition zone, we map the subduction of the Nazca Plate and the Chile Rise under Patagonia. Cratonic lithosphere beneath Africa is more fragmented than seen previously, with separate cratonic units observed within the West African and Congo Cratons, and with cratonic lithosphere absent beneath large portions of Archean crust. We image the lateral extent of the Niassa Craton, hypothesized previously, and identify a new unit, the Cubango Craton, near the southeast boundary of the grater Congo Craton, with both of these smaller cratons unexposed at the surface. In the South Atlantic, the model reveals the patterns of interaction between the Mid-Atlantic Ridge (MAR) and the nearby hotspots. Low-velocity anomalies beneath major hotspots extend substantially deeper than those beneath the MAR. The Vema Hotspot, in particular, displays a pronounced low-velocity anomaly under the thick, high-velocity lithosphere of the Cape Basin. A strong low velocity anomaly also underlies the Cameroon Volcanic Line and its offshore extension, between Africa and the MAR. Subtracting the global, age-dependent Vs averages from those in the South Atlantic Basins, we observe areas where the cooling lithosphere is locally hotter than average, corresponding to the location of the Tristan da Cunha, Vema and Trindade hotspots. Beneath the anomalously deep Argentine Basin, we image unusually thick, high-velocity lithosphere, which suggests that its anomalously great depth can be explained, at least to a large extent, by isostatic, negative lithospheric buoyancy.

更新日期：2020-01-06
• 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 1970 s, 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 root mean square (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：2020-01-06
• 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, retrieval of amplitude information and attenuation from noise cross-correlations has been difficult. Here we apply the temporal re-weighting method proposed by Weaver & Yoritomo (2018) 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 re-weighting 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 anti-causal 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：2020-01-06
• Geophys. J. Int. (IF 2.777) Pub Date : 2020-01-06
Ford S, Kraft G, Ichinose G.

SummaryEvent screening is an explosion monitoring practice that aims to identify an event as an explosion (“screened in”) or not (“screened out”). Confidence in event screening can be increased if multiple independent approaches are employed. We describe a new approach to event screening using the seismic moment tensor and its representation on the hypersphere, specifically the 5-sphere of 6-degree unit vectors representing the normalized symmetric moment tensor. The sample of moment tensors from an explosion dataset is unimodal on the 5-sphere and can be parameterized by the Langevin distribution, which is sometimes referred to as the Normal distribution on the hypersphere. Screening is then accomplished by finding the angle from the explosion population mean to any newly measured moment tensor and testing if that angle is in the tail of the Langevin distribution (conservatively quantified as greater than 99.9% of the cumulative density). We apply the screen to a sample of earthquakes from the Western USA and the September 2017 explosion and subsequent collapse at the Pungyye-Ri Test Site in North Korea. All the earthquakes and the collapse screen out, but the explosion does not.

更新日期：2020-01-06
• 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, i.e array beamforming, are applicable to analyze 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 catalog including accurate origin time and hypocenter 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 sub-functions 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, i.g, 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：2020-01-06
• Geophys. J. Int. (IF 2.777) Pub Date : 2019-12-31
Sly M, Thind A, Mishra R, et al.

SUMMARYLow-temperature plastic rheology of calcite plays a significant role in the dynamics of Earth's crust. However, it is technically challenging to study plastic rheology at low temperatures because of the high confining pressures required to inhibit fracturing. Micromechanical tests, such as nanoindentation and micropillar compression, can provide insight into plastic rheology under these conditions because, due to the small scale, plastic deformation can be achieved at low temperatures without the need for secondary confinement. In this study, nanoindentation and micropillar compression experiments were performed on oriented grains within a polycrystalline sample of Carrara marble at temperatures ranging from 23 to 175° C, using a nanoindenter. Indentation hardness is acquired directly from nanoindentation experiments. These data are then used to calculate yield stress as a function of temperature using numerical approaches that model the stress state under the indenter. Indentation data are complemented by uniaxial micropillar compression experiments. Cylindrical micropillars ∼1 μm and ∼3 μm in diameter were fabricated using a focused ion beam-based micromachining technique. Yield stress in micropillar experiments is determined directly from the applied load and micropillar dimensions. Mechanical data are fit to constitutive flow laws for low-temperature plasticity and compared to extrapolations of similar flow laws from high-temperature experiments. This study also considered the effects of crystallographic orientation on yield stress in calcite. Although there is a clear orientation dependence to plastic yielding, this effect is relatively small in comparison to the influence of temperature.

更新日期：2020-01-04
• Geophys. J. Int. (IF 2.777) Pub Date : 2020-01-03
So B, Capitanio F.

SummaryOur understanding of the seismicity of continental interiors, far from plate margins, relies on the ability to account for behaviors across a broad range of time and spatial scales. Deformation rates around seismic faults range from the slip-on-fault during earthquakes to the long-term viscous deformation of surrounding lithosphere, thereby presenting a challenge to modelling techniques. The aim of this study was to test a new method to simulate seismic faults using a continuum approach, reconciling the deformation of viscoelastoplastic lithospheres over geological timescales. A von Mises yield criterion is adopted as a proxy for the frictional shear strength of a fault. In the elastoplastic fault models a rapid change in strength occurs after plastic yielding, to achieve stress-strain equilibrium, when the coseismic slip and slip velocity from the strain-rate response and size of the fault are calculated. The cumulative step-function shape of the slip and temporally partitioned slip velocity of the fault demonstrated self-consistent discrete fault motion. The implementation of elastoplastic faults successfully reproduced the conceptual models of seismic recurrence, i.e. strictly periodic and time- and slip-predictable. Elastoplastic faults that include a slip velocity strengthening and weakening with reduction of the time-step size during the slip stage generated yield patterns of coseismic stress changes in surrounding areas, which were similar to those calculated from actual earthquakes. A test of fault interaction captured the migration of stress between two faults under different spatial arrangements, reproducing realistic behaviors across time and spatial scales of faults in continental interiors.

更新日期：2020-01-04
• Geophys. J. Int. (IF 2.777) Pub Date : 2020-01-03
Li W, Schmitt D, Chen X.

SummaryThe intrinsic anisotropy of rock influences the paths of propagating seismic waves and indicates mineralogical texture and strains; and as such it is important that laboratory measurements of such properties be fully understood. Usually, when studying anisotropy, ultrasonic wave speeds are measured in a variety of strategic directions and, subsequently transformed to the dynamic elastic moduli using symmetry-appropriate formula. For transversely isotropic rocks the moduli are ideally found by measuring wave speeds in directions vertical, parallel, and oblique to the foliation or bedding using finite-width ultrasonic transducers. An important, but ignored, complication is that at oblique angles the ultrasonic beam unavoidably deviates, or skews, away from the transmitter's normal axis making proper wave speed determinations difficult. The pressure dependence of the wave speeds further confounds finding a solution as skew angles, too, vary with confining pressure. We develop a new technique that incorporates dual ultrasonic receivers to account for and mitigate the effects of the pressure-dependent beam skew problem. Anisotropy measurements to 200 MPa hydrostatic confining pressure combined with recent beam modeling algorithms illustrate the errors obtained in the determined wave speeds that are subsequently magnified in calculating the full set of elastic stiffnesses. In materials with P-wave anisotropies near 30 per cent the error introduced by ignoring beam skew exceeds the transit time picking errors by more than a factor of three, these propagate to much larger errors in the stiffnesses particularly for C13 and the dynamic elastic moduli referred to C13. Meanwhile, shortening the sample or enlarging the transmitter size is not suggested to counter the beam skew issue because it reduces the beam skew effect but increases the diffraction effect.

更新日期：2020-01-04
• Geophys. J. Int. (IF 2.777) Pub Date : 2019-06-12
Agrusta R, Morison A, Labrosse S, et al.

SUMMARYThe presence of a magma ocean may have characterized the beginning of terrestrial planets and, depending on how the solidification has proceeded, the solid mantle may have been in contact with a magma ocean at its upper boundary, its lower boundary, or both, for some period of time. At the interface where the solid is in contact with the liquid the matter can flow through by changing phase, and this affects convection in the solid during magma ocean crystallization. Linear and weakly non-linear analyses have shown that Rayleigh–Bénard flow subject to two liquid–solid phase change boundary conditions is characterized by a non-deforming translation or weakly deforming long wavelength mode at relatively low Rayleigh number. Both modes are expected to transfer heat very efficiently, at least in the range of applicability of weakly non-linear results for the deforming mode. When only one boundary is a phase change, the critical Rayleigh number is also reduced, by a factor of about 4, and the heat transfer is also greatly increased. In this study we use direct numerical simulations in 2-D Cartesian geometry to explore how the solid convection may be affected by these boundary conditions for values of the Rayleigh number extending beyond the range of validity of the weakly non-linear results, up to 103 times the critical value. Our results suggest that solid-state convection during magma ocean crystallization may have been characterized by a very efficient mass and heat transfer, with a heat flow and velocity at the least twice the value previously thought when only one magma ocean is present, above or below. In the situation with a magma ocean above and below, we show that the convective heat flow through the solid layer could reach values of the same order as that of the black-body radiation at the surface of the magma ocean.

更新日期：2020-01-04
• Geophys. J. Int. (IF 2.777) Pub Date : 2019-10-07
Morozov I, Deng W, Cao D.

SUMMARYLinear and non-linear viscoelastic (VE) models such as the standard linear solid (SLS) and the generalized SLS (GSLS) are broadly used to represent the anelasticity of materials and Earth's media. However, although the VE approach is often satisfactory for any given observation, the inferred physical causes of anelasticity may be significantly misrepresented by this paradigm, and its predictions may be wrong or inaccurate in other cases. This problem is particularly important in heterogeneous media, including most cases of interest for seismology. For example, in homogenous media, VE and mechanics-based models predict identical quality-factor Q(f) and phase velocity c(f) spectra, but in heterogenous media, these models yield different time-stepping equations and interactions with material–property boundaries. The commonly used VE algorithms for modelling seismic waves rely on postulated convolutional integrals in time, whereas physically, models of rock rheologies should still be based on spatial interactions. To understand how VE models relate to mechanics, it is instructive to consider which physical properties of the medium are constrained reliably and which of them remain unconstrained by a pair of Q(f) and c(f) spectra, that is by VE properties. Despite its popular association with ‘attenuation,’ the peak value of Q−1(f) is actually a purely elastic property representing the existence of two (for SLS) or multiple (for GSLS) elastic moduli. These moduli are analogous to the drained and undrained moduli in poroelasticity or isothermal and adiabatic moduli in thermodynamics. By virtue of the Kramers–Krönig relations, the peak Q−1 is related to the total velocity dispersion, which is also caused by the difference between elastic moduli. By contrast, true anelasticity-related physical properties like viscosity are represented not by Q−1 values but by the frequencies of Q−1(f) peaks in the data. However, these frequencies also depend on multiple material properties that are not recognized or arbitrarily selected in the SLS and GSLS models. Inertial, body-force friction and the corresponding boundary effects are also ignored in VE models, which may again be improper for layered media. Thus, for physically accurate interpretation of laboratory experiments and numerical modelling of seismic waves, first-principle equations of mechanics should be used instead of VE models.

更新日期：2020-01-04
• Geophys. J. Int. (IF 2.777) Pub Date : 2019-10-15
Cho I.

SUMMARYWe build a model of discretization errors, known as directional aliasing, to theoretically evaluate how biases in the microtremor spatial autocorrelation (SPAC) coefficient, or the real part of the SPAC spectrum of microtremor analysis, are related to the magnitudes of the imaginary part when a seismic array of only two sensors is used. By using this model, we investigate the potential utility of the imaginary spectrum component as an indicator of applicability of the two-sensor SPAC method to the field of microtremors generated at an observation site. Field data of microtremors from compact seismic arrays (1–15 m) are used to test the model. It is found that, when the imaginary components are very large in magnitude (where the threshold depends on the rk, the array radius times the wavenumber), the field of microtremors is dominated by waves arriving from a single direction parallel to the array axis and the SPAC coefficients tend to be underestimated in small rk ranges (i.e. rk < 3.8; the range considered throughout this study). In this study, which is based on the observations of 400 microtremor arrays, the underestimates seldom exceeded 30 per cent. The SPAC coefficient estimates could be corrected in that case by using information on the imaginary part. When the imaginary components are very modest in magnitude, by contrast, there are two possible scenarios: either (i) the waves are arriving predominantly from a single direction perpendicular to the array axis and the SPAC coefficients are wildly overestimated (i.e. there was a small percentage of low-quality data, with relative errors exceeding +50 per cent, based on the observed data analyses) or (ii) the wavefield is close to isotropic and the SPAC coefficients are unbiased (i.e. 70–90 per cent of all observed data fell within the relative error range of ±20 per cent). It is difficult in that case to have SPAC coefficient estimates corrected by using information on the imaginary part alone.

更新日期：2020-01-04
• Geophys. J. Int. (IF 2.777) Pub Date : 2019-11-19
Valentine A, Sambridge M.

SUMMARYBy starting from a general framework for probabilistic continuous inversion (developed in Part I) and introducing discrete basis functions, we obtain the well-known algorithms for probabilistic least-squares inversion set out by Tarantola & Valette. In doing so, we establish a direct equivalence between the spatial covariance function that must be specified in continuous inversion, and the combination of basis functions and prior covariance matrix that must be chosen for discretized inversion. We show that the common choice of Tikhonov regularization ($\mathbf {C_m^{-1}} = \sigma ^2\mathbf {I}$) arises from a delta-function spatial covariance, and that this lies behind many of the artefacts commonly associated with discretized inversion. We show that other choices of spatial covariance function can be used to generate regularization matrices yielding substantially better results, and permitting localization of features even if global basis functions are used. We are also able to offer a straightforward explanation for the spectral leakage problem identified by Trampert & Snieder.

更新日期：2020-01-04
• Geophys. J. Int. (IF 2.777) Pub Date : 2019-11-19
Trabattoni A, Barruol G, Dreo R, et al.

SUMMARYBreakthroughs in understanding the structure and dynamics of our planet will strongly depend upon instrumenting deep oceans. Progress has been made these last decades in ocean-bottom seismic observations, but ocean-bottom seismometer (OBS) temporary deployments are still challenging and face set-up limitations. Launched from oceanographic vessels, OBSs fall freely and may slightly drift laterally, dragged by currents. Therefore, their actual orientation and location on the landing sites are hard to assess precisely. Numerous techniques have been developed to retrieve this key information, but most of them are costly, time-consuming or inaccurate. In this work, we show how ship noise can be used as an acoustic source of opportunity to retrieve both the orientation and the location of OBSs on the ocean floor. To retrieve the OBS orientation, we developed a first method based on a combination of seismic and pressure data through the use of the acoustic intensity. This latter can be used to quantify the OBS orientation from the ship noise direction of arrival (DOA), which can then be compared with known ship trajectories obtained from the automatic identification system (AIS). To accurately relocate OBSs, we also developed a second method based on the hydrophone data which computes distances of acoustical sources by measuring time differences of arrival (TDOA) between direct and reverberated phases. The OBS location is then retrieved by fitting measured ship distances with known ship trajectories. In this study, a full network of OBSs deployed in the SW Indian Ocean was reoriented and a test station was relocated. We demonstrate that our new methods may quantify the OBS orientation with an accuracy of about one degree, and its location with an accuracy of a few tens of metres, depending on the number of ships used in the analysis.

更新日期：2020-01-04
• Geophys. J. Int. (IF 2.777) Pub Date : 2019-11-19
Valentine A, Sambridge M.

SUMMARYWe develop a theoretical framework for framing and solving probabilistic linear(ized) inverse problems in function spaces. This is built on the statistical theory of Gaussian Processes, and allows results to be obtained independent of any basis, avoiding any difficulties associated with the fidelity of representation that can be achieved. We show that the results of Backus–Gilbert theory can be fully understood within our framework, although there is not an exact equivalence due to fundamental differences of philosophy between the two approaches. Nevertheless, our work can be seen to unify several strands of linear inverse theory, and connects it to a large body of work in machine learning. We illustrate the application of our theory using a simple example, involving determination of Earth’s radial density structure.

更新日期：2020-01-04
• Geophys. J. Int. (IF 2.777) Pub Date : 2019-11-20
Ba K, Gao S, Liu K, et al.

SUMMARYTo provide constraints on a number of significant controversial issues related to the structure and dynamics of the Australian continent, we utilize P-to-S receiver functions (RFs) recorded by 182 stations to map the 410 and 660 km discontinuities (d410 and d660, respectively) bordering the mantle transition zone (MTZ). The RFs are stacked in successive circular bins with a radius of 1° under a non-plane wave front assumption. The d410 and d660 depths obtained using the 1-D IASP91 earth model show a systematic apparent uplifting of about 15 km for both discontinuities in central and western Australia relative to eastern Australia, as the result of higher seismic wave speeds in the upper mantle beneath the former area. After correcting the apparent depths using the Australian Seismological Reference Model, the d410 depths beneath the West Australia Craton are depressed by ∼10 km on average relative to the normal depth of 410 km, indicating a positive thermal anomaly of 100 K at the top of the MTZ which could represent a transition from a thinner than normal MTZ beneath the Indian ocean and the normal MTZ beneath central Australia. The abnormally thick MTZ beneath eastern Australia can be adequately explained by subducted cold slabs in the MTZ. A localized normal thickness of the MTZ beneath the Newer Volcanics Province provides supporting evidence of non-mantle-plume mechanism for intraplate volcanic activities in the Australian continent.

更新日期：2020-01-04
• Geophys. J. Int. (IF 2.777) Pub Date : 2019-11-22
Li Z, Zhang J.

SUMMARYAccurate identification of the locations and orientations of small-scale faults plays an important role in seismic interpretation. We have developed a 3-D migration scheme that can image small-scale faults using diffractions in time. This provides a resolution beyond the classical Rayleigh limit of half a wavelength in detecting faults. The scheme images weak diffractions by building a modified dip-angle gather, which is obtained by replacing the two dip angles dimensions of the conventional 2-D dip-angle gather with tangents of the dip angles. We build the modified 2-D dip-angle gathers by calculating the tangents of dip angles following 3-D prestack time migration (PSTM). In the resulting modified 2-D dip-angle gathers, the Fresnel zone related to the specular reflection exhibits an ellipse. Comparing with the conventional 2-D dip-angle gather, diffraction event related a fault exhibits a straight cylinder shape with phase-reversal across a line related the orientation of the fault. As a result, we can not only mute the Fresnel zones related to reflections, correct phase for edge diffractions and obtain the image of faults, but also detect the orientations of 3-D faults using the modified dip-angle gathers. Like the conventional dip-angle gathers, the modified dip-angle gathers can also be used to image diffractions resulting from other sources. 3-D Field data tests demonstrate the validity of the proposed diffraction imaging scheme.

更新日期：2020-01-04
• Geophys. J. Int. (IF 2.777) Pub Date : 2019-11-26
Zhu J, Yin C, Liu Y, et al.

SUMMARYIn this paper, we propose a spectral element method (SEM) based on unstructured tetrahedral grids for direct current (dc) resistivity modelling. Unlike the tensor-product of 1-D Gauss–Lobatto–Legendre (GLL) quadrature in conventional SEM, we use Proriol–Koornwinder–Dubiner (PKD) polynomials to form the high-order basis polynomials on tetrahedral grids. The final basis functions are established by using Vandermonde matrix. Compared to traditional SEM, our method truly takes into account the high precision of spectral method and the flexibility of finite element method with unstructured grids for modelling the complex underground structures. After addressing the theory on the construction of basis functions and interpolation and integration nodes, we validate our algorithm using the analytical solutions for a layered earth model and the results from other methods for multiple geoelectrical models. We further investigate a dual-track scheme for improving the accuracy of our SEM by increasing the order of interpolation polynomials or by refining the grids.

更新日期：2020-01-04
• Geophys. J. Int. (IF 2.777) Pub Date : 2019-11-26
Vozar J, Jones A, Campanya J, et al.

SUMMARYWe present modelling of the geophysical data from the Newcastle area, west of Dublin, Ireland within the framework of the IRETHERM project. IRETHERM's overarching objective was to facilitate a more thorough strategic understanding of Ireland's geothermal energy potential through integrated modelling of new and existing geophysical, geochemical and geological data. The Newcastle area, one of the target localities, is situated at the southern margin of the Dublin Basin, close to the largest conurbation on the island of Ireland in the City of Dublin and surrounds. As part of IRETHERM, magnetotelluric (MT) soundings were carried out in the highly urbanized Dublin suburb in 2011 and 2012, and a description of MT data acquisition, processing methods, multidimensional geoelectrical models and porosity modelling with other geophysical data are presented. The MT time-series were heavily noise-contaminated and distorted due to electromagnetic noise from nearby industry and Dublin City tram/railway systems. Time-series processing was performed using several modern robust codes to obtain reasonably reliable and interpretable MT impedance and geomagnetic transfer function ‘tipper’ estimates at most of the survey locations. The most ‘quiet’ 3-hr subsets of data during the night time, when the DC ‘LUAS’ tram system was not operating, were used in multisite and multivariate processing. The final 2-D models underwent examination using a stability technique, and the final two 2-D profiles, with reliability estimations expressed through conductance and resistivity, were derived. In the final stage of this study, 3-D modelling of all MT data in the Newcastle area was also undertaken. Comparison of the MT models and their interpretation with existing seismic profiles in the area reveals that the Blackrock–Newcastle Fault (BNF) zone is visible in the models as a conductive feature down to depths of 4 km. The investigated area below Newcastle can be divided into two domains of different depths, formed as depth zones. The first zone, from the surface down to 1–2 km, is dominated by NE–SW oriented conductors connected with shallow faults or folds probably filled with less saline waters. The conductors are also crossing the surface trace of the BNF. The second depth domain can be identified from depths of 2–4 km, where structures are oriented along the BNF and the observed conductivity is lower. The deeper conductive layers are interpreted as geothermal-fluid-bearing rocks. Porosity and permeability estimations from the lithological borehole logs indicate the geothermal potential of the bedrock, to deliver warm water to the surface. The fluid permeability estimation, based on Archie's law for porous structures and synthetic studies of fractured zones, suggests a permeability in the range 100 mD–100 D in the study area, which is prospective for geothermal energy exploitation.

更新日期：2020-01-04
• Geophys. J. Int. (IF 2.777) Pub Date : 2019-11-26
Sánchez-Moreno E, Calvo-Rathert M, Goguitchaichvili A, et al.

SUMMARYA palaeointensity study has been carried out on a Pliocene sequence of 20 consecutive lava flows where previous directional results seem to reflect anomalous behaviour of the Earth's magnetic field (EMF), which can be explained by a polarity transition record or non-averaged palaeosecular variation or both. Here, we perform a total of 55 palaeointensity determinations using the original Thellier–Thellier (TT) method and 100 with the IZZI method. We assess the performance of our selection criteria using a set of strict threshold values applied to a set of test data whose TRMs were acquired in known fields. Absolute palaeointensity determinations that passed our selection criteria were obtained on four specimens with the TT method and on 41 specimens with the IZZI method. Application of reliability criteria at a site level yielded palaeointensity results in 8 of 20 studied lava flows. We obtained median values of VADM between 28.9 and 45.6 ZAm2 for the reverse polarity lower Apnia section, while the normal polarity upper section displayed a single value of 54.6 ZAm2. The low palaeointensity values before a transitional direction lava flow and the higher value after it, suggest the common behaviour at the start of a polarity reversal and the recovery after it. However, an isolated record of a stable EMF, where the intensity is lower than the current for the same location (83.7 ZAm2), cannot be discarded. Consequently, this interpretation would support a weak time-averaged field.

更新日期：2020-01-04
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