Abstract In this study, we present the work done to review the existing historical earthquake information of the Dead Sea Transform Fault Zone (DSTFZ). Several studies from various sources have been collected and reassessed, with the ultimate goal of creating of new homogenized parametric earthquake catalog for the region. We analyze 244 earthquakes between 31 BC and 1900, which are associated with the geographical buffer extending from 27 N to 36 N and from 31 E to 39 E. Of these, 93 were considered real seismic events with moment magnitude (Mw) greater than 5 that indeed occurred within this zone. While we relied on past parametric data and did not assign new macroseismic intensities, magnitude values, or epicenters for several controversial events, we did however resort to the primary sources to obtain a more critical perspective for the various assigned macroseismic intensities. In order to validate the derived parametric information, we tried to associate the events present in the historical records, with any evidence coming from past field investigations, i.e., geological or archaeological studies. Acknowledging the uneven quality and quantity of data reporting each event, we provided each entry with an uncertainty range estimate. Our catalog lists 33 events of Mw ≥ 6 absent from the latest published compilation with compatible time span and areal coverage. The whole catalog is considered complete down to Mw 7 and in certain areas down to Mw 6 after the year 1000, with majority of the larger earthquakes located in the part of DSTFZ, which extends from the southeast part of Dead Sea lake till Antioch.

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

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

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

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

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

Abstract The long-term trend and seasonal variability of sea surface temperature (SST) over tropical Indo-Pacific region under the global warming projection simulated by FIO-ESM model are analyzed. At seasonal time scale, significant warming trend over Indo-Pacific area is well captured by FIO-ESM model over the warm pool region under RCP8.5 scenario. The La Niña-like warming pattern is dominant in the tropical Pacific and the negative Indian Ocean Dipole warming pattern takes place in the Indian Ocean in global warming projection separately. The strength of SST warming trend is found to be seasonal dependent over Indo-Pacific region. The spatial distribution of calendar month where fastest/slowest SST warming trend take place in tropical Indo-Pacific has been assessed in FIO-ESM simulation, which is corresponded with the distribution of SST climatology closely in Pacific but not in Indian Ocean.

Abstract A coupled ocean–atmosphere–wave model was used to assess the impact of model coupling on the simulations of air–sea fluxes, surface currents, waves, and temperature profile during the passage of a tropical cyclone (TC) Phailin in the Bay of Bengal. Four numerical experiments with different coupling configurations among the atmosphere, ocean, and wave models were carried out to identify differences in simulated atmospheric and oceanic parameters. The simulated track and intensity of Phailin agree well with the observations. The inter-comparison of model experiments with different coupling options highlights the importance of better air–sea fluxes in the coupled model as compared to the uncoupled model towards an improvement in the simulation of TC Phailin. The coupled model configurations overcome the cold bias (up to − 2 °C) in sea surface temperature simulated by the uncoupled ocean model. A higher magnitude of the surface drag coefficient in the uncoupled atmosphere model enhanced the bottom stress (> 2 N m−2). As a result of excess momentum transfer to the sea surface, the uncoupled ocean model produced stronger surface currents as compared to the coupled model. The inclusion of the wave model increases the sea surface roughness and, thereby, improves the wind speed and momentum flux at the air–sea interface. The maximum significant wave height in the coupled model was about 2 m lower than the uncoupled wave model. The model experiments demonstrate that the periodic feedback among the atmosphere, ocean, and wave models leads to a better representation of momentum and heat fluxes that improves the prediction of a tropical cyclone.

Abstract Quaternary Mount Nemrut stratovolcano, having a spectacular summit caldera and associated lakes, is located north of the Bitlis–Zagros suture zone, Eastern Turkey. Although much attention has been paid to its geology, morphology, history and biology, a detailed geophysical investigation has not been performed in this special region. Thus, we attempted to characterize the stratovolcano and the surroundings using total field aeromagnetic anomalies. Potential field data processing techniques helped us to interpret geologic sources causing magnetic signatures. Resulting image maps obtained from some linear transformations and a derivative-based technique revealed general compatibility between the aeromagnetic anomalies and the near-surface geology of the study area. Some high amplitude magnetic anomalies observed north of the Nemrut caldera rim are associated with the latest bimodal volcanic activity marked by lava fountains and comenditic-basaltic flows occurred along the rift zone. After minimizing the high-frequency effects, a pseudogravity-based three-dimensional inversion scheme revealed that the shallowest deep-seated sources are located about 3.0 km below the ground surface. Two-dimensional normalized full gradient solutions also exposed the depths of these anomaly sources, in good agreement with the inversion results. This first geophysical study performed through aeromagnetic anomalies clearly gave insights into some main magnetized structures of the Mount Nemrut stratovolcano.

Abstract This paper presents a scenario for the spatial variation of the fundamental frequency of the sediment deposits above the basement and the corresponding amplification as well as the average spectral amplification in different frequency bandwidths for the National Capital Territory Delhi (the capital of India). The exposed central quartzite ridge and the Yamuna River channel are responsible for very large spatial variations of the fundamental frequency in the eastern part of the National Capital Territory Delhi. At 20% of the considered sites, a good match is obtained between the fundamental frequency computed numerically using available S-wave velocities to a certain depth and their extrapolation and that obtained experimentally. The computed fundamental and dominant frequencies reveal that both medium-rise (5–10 storey) and high-rise (> 10 storey) buildings in the western part and medium-rise buildings lying in the localities east of or very near to the Yamuna River may suffer heavy to very heavy damage due to the occurrence of the double resonance phenomenon. Furthermore, 1–2-storey buildings lying on the weathered exposed quartzite rock may also suffer heavy damage during local earthquakes because of the occurrence of double resonance. The possible reasons behind the lack of earthquake damage to the Qutab Minar, the tallest brick masonry minaret in the world, over the last 800 years may be the nonoccurrence of double resonance and almost no amplification in the low frequency range. There are two localities in the western part of the National Capital Territory Delhi, namely Kanganheri-Chhawla and Buradi, wherein all sorts of buildings are highly vulnerable to earthquake damage. For the closed Chhatarpur Basin and a semiclosed basin to its northeast, formed due to exposed quartzite rock, three-dimensional (3D) simulations are required to predict the characteristics of basin-generated surface waves and their focusing effects in the Chhatarpur Basin. The average spectral amplification map developed for the 0–10 Hz bandwidth depicts a range of 2.25–4.82 in the National Capital Territory Delhi and may be directly used to transfer the estimated seismic hazard at basement to the free surface.

Abstract We study damage induced by low temperature variations in granite samples given their role in shallow geological reservoirs. We consider two thermal treatments, slow cooling and thermal shock, and implement a multi-geophysical approach to characterize the induced micro-scale damage. The methodology consists in monitoring elastic wave velocity and thermal conductivity as well as describing the damage by the way of Hg-porosity measurements and microscopic observations. To discuss the reproducibility of the induced damage, the same thermal protocol is performed on five samples. Our first results indicate that the thermal shock leads to a more pronounced damage. This is interpreted to be due to a larger variety of nucleated intragranular and intergranular cracks as observed by SEM and optic microscope. Yet, this more significant damage does not appear reproducible from one sample to another compared to the damage introduced by slow cooling. According to this first result, thereby, we propose a timely monitoring of elastic wave velocity, conductivity and Hg-porosity. It appears that the damage introduced by the slow cooling, unlike the thermal shock, does not present a long persistence. Indeed, after 15 days, the different properties had returned to their initial state. A time-dependence mechanism is proposed to discuss this observed process.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Abstract The present work focuses on the seismic response and damage distribution studies on two Thessaloniki monuments with different structural systems. The first one is the Acheiropoietos basilica—a Byzantine monument included in the World Heritage List of UNESCO—with a flexible structural system consisted of internal colonnades and perimeter walls. The second one is the Bey hamam, an Ottoman bath with a stiff structural system. Its successive square or rectangular functional spaces are organized in such a way as to obtain both spatial and structural (bearing structure) autonomy; thick internal and external walls in two directions bear the domes directly or through single or double arches. Both monuments were subjected to the June 20, 1978 Thessaloniki earthquake ground motion (EQ) (M6.5) with significant damage which were accurately recorded and demonstrated in the Ephorate of Antiquities of Thessaloniki (EFAPOTH) archives. Sets of ambient vibration measurements were performed before any intervention work at the basilica. For the present study, similar measurements were performed at both monuments. Their eigen-properties were then calculated based on this data. Both monuments were modeled in details by finite element method; the analytical models were calibrated for the optimum approach of the eigen-properties resulted by the measurements. The main calibration parameter was the modulus of elasticity of the masonry walls. These calibrated models were also analytically subjected to the recorded acceleration time history of the Thessaloniki 1978 EQ. The findings of this work are related to the appropriately documented material properties and factors of these monuments that are unique. The result of this procedure was the calibration of reliable finite element models for response evaluation of the structures to potential ground motions in future earthquakes. These models were capable to quantitatively explain the damage patterns observed in the monuments. The monument with higher stiffness (Ottoman bath) exhibited damage concentrated to fewer elements than the more flexible monument (Byzantine basilica) where the damages were distributed to many structural elements.

In this study we investigate detailed Moho variations beneath Northeast China by applying the arithmetic mean, back-projected and Fresnel-zone migration imaging methods to a total of 169,602 high-quality P-wave receiver functions from seismograms of 2903 teleseismic events recorded at 127 NECESSArray stations and 321 China Earthquake Administration stations. Our results show that the Moho depth variations are correlated with the surface geology in the study region. The Moho is deeper (~34.0–42.0 km) under the Great Xing'an range, the Lesser Xing'an range, the Zhangguangcai range, and the Changbaishan mountain, whereas it is shallower (~26.0–32.0 km) under the Songliao basin. Our results also reveal obvious Moho variations across the North-South Gravity Lineament. The Moho offsets up to ~5.0 km are clearly observed under the Nenjiang-Balihan, Yilan-Yitong, and Dunhua-Mishan faults, indicating that they are lithospheric-scale faults. A deeper Moho is revealed under the volcanoes, such as the Jingpohu, Wudalianchi, and Changbaishan volcanoes and Abaga and Halaha volcanic groups. In particular, the Moho under the Changbaishan volcano reaches ~40.0 km depth and the observation varies with the teleseismic back-azimuths, suggesting a complicated magma system in the crust. In addition, the Moho under the Songliao basin varies significantly from ~26.0 km depth in the east to ~32.0 km depth in the west, which could be related to the lithosphere extension and thinning. All these results suggest that there exists to a hot and wet mantle upwelling in the big mantle wedge formed by the deep dehydration of the long stagnant Pacific slab in the mantle transition zone under Northeast China.

Jurassic tectonism resulted in late Mesozoic intracontinental deformation of the North China Plate. A fold-and-thrust belt, related growth strata, and volcanic materials have been identified in the Jurassic Yungang Basin, which can be used to constrain the timing and scale of faulting and sedimentation. The age of volcanic materials in the growth strata and the sedimentary cycle of Jurassic strata indicate that the late Middle Jurassic Yungang Formation was the earliest sedimentary response to the onset of Jurassic deformation (i.e., the Yanshanian orogeny) from 170 to 160 Ma. Deformation involved detachment and fault propagation folding, which produced a marginal thrust belt and limb rotation of anticline. Restored cross-sections show that the northern basin experienced more intense shortening than the southern basin, consistent with field observations, possibly controlled by the Qifengshan strike-slip fault in the center of the fold-and-thrust belt. Fault striations, conjugate joints, and asymmetric folds show that the Yungang Basin underwent NW–SE-trending compression during the Middle–Late Jurassic. Therefore, we propose that the eastern Ordos Block was in a compressional setting during the Middle–Late Jurassic, due to flat subduction of the Paleo-Pacific Plate beneath the East Asian continent.

Abstract The run-up catalogs of two global tsunami databases maintained by the NCEI/WDC NOAA and NTL/ICMMG SD RAS are examined to compile the list of annual maximum runups observed or measured in the oceanic, marine and inland basins during the last 120 years (from 1900 to 2019). All the retrieved annual maximum runups were divided into four groups according to four main types of tsunami sources (seismogenic, landslide-generated, volcanic, and meteorological). Their distribution over the type of sources shows that of the 120 maximum runups only 78 (65%) resulted from seismogenic sources, while the remaining 42 runups were divided between landslide-generated (19%), volcanic (8%), and meteorological (7.5%) sources. The analysis of geographical distribution of source locations demonstrates that tsunamis are not exclusively a marine hazard—over 15% of all maximum runups were observed in coastal and inland water basins (narrow bays, fiords, lakes, and rivers). Temporal distribution of the collected runups shows that annual occurrence of large tsunamis was more or less stable throughout the twentieth century and only demonstrates some increase during the last 27 years (since 1992) when the practice of post-event surveys of all damaging tsunamis was implemented. This paper also outlines the existing problems with data compilation, cataloguing, and distribution, and discusses incompleteness of runup and wave-form data for a considerable number of non-damaging tsunamis, even those resulting from the strong (magnitude higher than 7.5) submarine earthquakes.

Some of the oldest surviving oceanic basins in the world, the Mozambique and West Somali basins, were created during the breakup of Gondwana, starting around 180 Ma. Between the two basins, relative movements of West Gondwana and East Gondwana, including Madagascar, created a shear zone, the Davie Fracture Zone (DFZ) with a topographic elevation (Davie Ridge - DR) marking its centre. The crustal composition of the DFZ and DR is a subject of speculation and debate. In this study, we present seismic refraction data across the prominent topography of the southern DR. Ray tracing of the wide-angle data as well as additional seismic amplitude modelling and 2.5D density modelling constrain its crustal structure and architecture. The data indicate that in the Mozambique Channel the DR consists of fragments of continental crust with a thickness of 10 to 12 km. An oceanic crust indenter extends northward from the Mozambique Basin into the area between the DR and the East African margin at 16.5° S. Northeast of the DR, at 41.8° W/14.5° S, the Somali Basin is probably floored by 6 km thick oceanic crust. Hence, the continental DR separates oceanic crust of the Somali and Mozambique basins. The transitional crustal area at the central Mozambican margin is underlain by high velocity lower crust (HVLC). The HVLC has velocities up to 7.3 km/s and extents along the margin, vanishing northward between 16.5° and 14.5° S. At the Madagascan side of the DR, at 16.5° S, the highly intruded stretched continental crust is 9 km thick and possibly underlain with a smaller HVLC of 2.9 km thickness and an E-W extent of 120 km. The oceanic crust at 14.5°S represents the oldest part the Somali Basin, which formed after the initial NW-SE rifting between East and West Gondwana.

Abstract We obtain the reflection and transmission coefficients for inhomogeneous plane waves incident on a flat interface separating two double-porosity media described by the Biot–Rayleigh model, which takes into account the effect of local fluid flow (LFF). Three longitudinal and one transverse waves are reflected and transmitted, represented by potential functions specified by the propagation and attenuation directions. The continuity of the energy at the interface for sealed and open-boundary conditions yields a system of equations for the coefficients, and the expressions of the energy ratios for the reflected and refracted waves are derived in closed form. Numerical examples showing the magnitude, phase and energy ratio as a function of frequency and incidence angle are carried out to investigate the influence of the inhomogeneity angle, boundary condition, type of incidence wave and LFF effect. The results confirm that the LFF affects the reflection and transmission behaviors for the incident P1 and SV waves, irrespective of whether the interface is open or sealed. The effect causes interference fluxes between different waves, a consequence of energy conservation at the interface. We also perform full-waveform simulations to validate the results.

Core freezing and resultant compositional convection are likely important drivers for dynamo activity in large terrestrial bodies like the Earth. The solidification of compositional mixtures, such as iron and sulfur, generates mush zones of partial melt at the freezing front, which can eject chemically buoyant or heavy liquid that then drives convection. For smaller bodies such as planetesimals in the asteroid belt, conditions for generating a dynamo are harder to achieve. Nevertheless, evidence for magnetization of achondrite meteorites is abundant, suggesting that many planetesimal cores were somehow magnetized. As such small bodies cool rapidly under low gravity they likely spend much of their evolution with a large poorly compacted partial melt mushy zone. The magneto-hydrodynamic behavior of a deformable partial melt zone can induce magnetism via separation of solid and liquid phases, and conversely magnetism can impose extra forces on the phase separation in the mush zone. To this end, we have developed a new two-phase magneto-hydrodynamic theory for deformable mushes and slurries. The model includes the standard effects of Lorentz forces, and the competition between magnetic field stretching and diffusion. There are additional effects at the liquid pore or solid grain scale, which involve the interaction between phases, akin to Darcy or Stokes drag; these include Lorentz drag, as well as pore/grain-scale diffusive exchange of magnetism between solid and fluid phases, and field stretching due to relative motion between phases. Magnetic induction by gravitational phase separation is most significant after extensive mixing of liquid and solid phases, such as induced by vigorous mechanical stirring due to, for example, tidal and elliptic instabilities, or impacts with other bodies, all of which are conceivably common in the asteroid belt, especially in the early solar system. Gravitational phase separation following such events can induce significant magnetism in the liquid and solid phases, and much more rapidly than can magnetization by large-scale circulation. Magnetic field variances can be at first orders of magnitude larger than an imposed background field, during initial gravitational phase separation of the well-mixed slurry. As the phases separate toward the top or bottom, the solid phase compacts, the separation velocity decreases, and the magnetic field variance likewise diminishes. However, solitary waves in the compacting region can cause an additional large magnetic induction in the liquid, taking the form of strongly magnetized wave packets that can be trapped in the solid. Thus, phase separation, segregation and compaction potentially induce large magnetic field anomalies. A linear stability analysis for convection in a porous medium (with a rigid matrix) is also explored. Pore and grain scale effects, such as field stretching due to phase separation, are found to enhance the influence of the magnetic field on convective instability.

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

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

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

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

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

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

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

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

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

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

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

Abstract Stick–slips are spontaneous, unstable slip events during which a natural or man-made system transitions from a strong, sticking stage to a weaker, slipping stage. Stick–slips were proposed by Brace and Byerlee (Science 153:990–992, 1966) as the experimental analogue of natural earthquakes. We analyze here the mechanics of stick–slips along brittle faults by conducting laboratory experiments and by modeling the instability mechanics. We performed tens of shear tests along experimental faults made of granite and gabbro that were subjected to normal stresses up to 14.3 MPa and loading velocities of 0.26–617 µm/s. We observed hundreds of spontaneous stick–slips that displayed shear stress drops up to 0.66 MPa and slip-velocities up to 14.1 mm/s. The pre-shear and post-shear fault surface topography were mapped with atomic force microscopy at pixel sizes as low as 0.003 µm2. We attribute the sticking phase to the locking of touching asperities and the slipping phase to the brittle failure of these asperities, and found that the fault asperities are as strong as the inherent strength of the host rock. Based on the experimental observations and analysis, we derived a mechanical model that predicts the relationships between the measured stick–slip properties (stress-drop, duration, and slip-distance) and asperity strength.

Abstract We infer the elastic and petrophysical properties from pre-stack seismic data through a transdimensional Bayesian inversion. In this approach the number of model parameters (i.e. the number of layers) is treated as an unknown, and a reversible jump Markov Chain Monte Carlo (rjMCMC) algorithm is used to sample the variable-dimension model space. This inversion scheme provides a parsimonious solution, and reliably quantifies the uncertainties affecting the estimated model parameters. Parallel tempering, which employs a sequence of interacting Markov chains in which the likelihood function is successively relaxed, is used to improve the efficiency of the probabilistic sampling. In addition, the delayed rejection updating scheme is employed to speed up the convergence of the rjMCMC algorithm to the stationary regime. Both elastic and petrophysical inversions invert the amplitude versus angle responses and employ a convolutional forward modelling based on the exact Zoeppritz equations. First, synthetic tests are used to assess the reliability of the implemented rjMCMC algorithms, then their applicability is demonstrated by inverting field seismic data acquired onshore. In this case the inversion was aimed at inferring the elastic and petrophysical properties around a gas-saturated reservoir hosted in a shale-sand sequence. In this case, the final outcomes provided by the rjMCMC algorithms are also compared with the predictions of linear Bayesian elastic and petrophysical inversions. The synthetic and field data examples demonstrate that the implemented algorithms can successfully estimate model uncertainty, model dimensionality and subsurface parameters with an affordable computational cost.

Abstract In the southeastern part of Tibet, an earthquake with a local magnitude of 6.9 occurred in the prefecture of Mainling on 18 November 2017. The mainshock and more than 900 aftershocks were recorded by a local seismic network comprising seven three-component seismic stations. In this study, both HypoDD location of aftershocks and focal mechanism inversion of moderate events were performed in order to accurately identify the pattern of active faults. The result reveals that the mainshock has a thrust source mechanism located at a depth of 14 km beneath the NE flank of the Namcha Barwa–Gyala Peri (NB-GP) massif. The aftershock sequences are caused mainly by two determined faults, one of which is the seismogenic fault stretching with a SE–NW trend parallel to the GP ridge and with a high NE-oriented dipping angle, and the other is activated by the mainshock and displays features of a SSE-NNW trend and SW-dipping, inferring the adjustment of stress in the focal area. The source parameters of the mainshock and the selected aftershocks show the reverse property of the seismogenic fault and its adjunct fault, thus inferring the backlash and uplift of the NB-GP massif, especially GP, for adjusting the uneven extrusion from the eastern Himalayan syntaxis to the adjacent Lhasa block. Furthermore, it is deduced that the rupture energy of the mainshock and aftershocks was limited by the surrounding rigid rock mass with high seismic velocity, such as the Lhasa block in the north and east, and Namcha Barwa complex in the south, and other aftershocks appearing at the NW top of GP and the SE side of Yarlung Tsangpo Big Bend reflect the strong squeezing effect of the NB-GP massif to its northeastern geological mass.

Abstract Mineral exploration is often associated with the generation of environmental liabilities, whose potential damages might imperil local water quality. An example of these environmental impacts is the acid mine drainage—AMD, caused by sulfides oxidation and production of acid and saline effluents. The analysis of critical areas with generation and spread of contamination plumes becomes more feasible due to the possibility to obtain geophysical models of water systems, especially to identify regions with accumulation of reactive minerals and preferential water flows. The rock-waste pile named BF-04 fits in this context of contamination, and it was studied based on the Electrical Resistivity Tomography technique, inversion models and isosurface models, providing conditions to recognize sulfide zones (> 10.1 mV/V), whereas chaotic high salt content underground flows, along several depths, were identified by low resistivity zones (< 75.8 Ω m). The complex behavior of groundwater flow in this kind of artificial granular aquifer is caused by its granulometric and lithologic heterogeneities, and compacted material. In addition, the results reveled a substantial water infiltration from Consulta creek, however the most critic zones for AMD generation are located at shallow levels where the waste rock material is more exposed to atmospheric O2 and meteoric water infiltration. The bedrock was not associated with significant low resistivity anomalies, which means that its contribution to AMD generation was considered relatively less important. The results will contribute to the environmental remediation management and also to demonstrate the potential applicability of geophysical methods in mining wastes.

Abstract There have been many studies to improve visibility forecast skills using numerical models, but their performance still remains behind the forecast skills for other meteorological phenomena. This study attempted to improve visibility forecasts using a newly established automatic visibility observation network composed of 291 forward-scattering sensors in South Korea. In the analysis of recent 3-year visibility observations, clear days (visibility above 20 km) were reported for 46% of the days, and fog cases (visibility less than 1 km) accounted for 1.58% of the total observations. The Very short-range Data Assimilation and Prediction System (VDAPS) of the Korea Meteorological Administration (KMA) assimilated the visibility observations based on the Met Office Unified Model with visibility data assimilation of Clark et al. (Q J R Meteorol Soc 134:1801–1816, 2008). Prior to the data assimilation, a precipitation check eliminated visibility data with precipitation (9.4% in total, 23% for visibility less than 1 km), and a consistency check removed visibility observations that were inappropriate to relative humidity, temperature, and pressure. In a case study on two consecutive fog days, visibility forecast skills were improved by applying visibility data assimilation, mostly through modifications of aerosol concentrations. A 3-month model run in the winter of 2016 showed a positive bias in visibility predictions, especially for the low-visibility cases. Visibility data assimilation improved the prediction skills, but the positive effects were limited within 9 forecast hours and were smaller for extremely low-visibility events. Sensitivity experiments were performed using local aerosol observations with a larger number of smaller aerosol particles. Modifications in aerosol properties made better results in frequency bias for the whole forecast ranges and also improved the equitable threat score (ETS) for relatively longer forecast hours (more than 4 h).

Abstract In the numerical simulation of induced seismicity, much attention is generally paid to the calibration of the frictional resistance of the causative fault to obtain a seismic moment consistent with that of the actual event, whereas sufficient investigation is not made in the estimation of the slip-weakening distance Dc as well as in the calibration of seismically radiated energy. The present study addresses this problem by numerically and analytically investigating the relation between Dc and seismic source parameters. First, this study performs the dynamic simulation of an induced seismic event caused by a decrease in the effective normal stress. The analysis demonstrated that seismic efficiency η can be used to improve the accuracy of estimating the critical slip-weakening distance and the coefficient of kinetic friction µd whilst considering not only seismic moment but also radiated energy in the calibration. This gave insight into the development of the new calibration method for induced seismicity that considers energy-related seismic source parameters. Furthermore, a new scaling law of the slip-weakening distance was derived from the theoretical expression of seismic efficiency η, considering seismic moment Mo and scaled energy \( \hat{e} \). The proposed scaling law can yield the relation between Dc and Mo, which is shown to be similar to that obtained from a previous study, but additionally considers the relation between seismically radiated energy and Dc. The dependency of Dc on seismically radiated energy implied from the proposed scaling law has been verified from the dynamic analyses where η = 0.06 was used to place a constraint on Dc for seismic events with different magnitudes. The developed numerical simulation methodology of induced seismicity as well as the scaling law considering the energy indices significantly contributes to improving the accuracy of back-analysis, thus leading to a more accurate estimation of the mechanical properties of faults and/or shear zones in seismically active regions of deep underground mines or reservoirs composed of discontinuous hard rock masses.

Abstract Pure and Applied Geophysics (PAGEOPH) is one of the leading journals in the field of geophysics. The first issue was published in 1939; thus, the journal is celebrating its 80th anniversary in 2018. The aim of this paper is to provide a complete lifetime overview of the academic structure of the journal using bibliometric indicators. This analysis includes key factors such as the most cited articles, leading authors, originating institutions and countries, publication and citation structures, and the most commonly used keywords. The bibliometric data used to conduct this analysis comes from the Scopus database. Additionally, the visualization of similarities (VOS)viewer software is used to create a graphic map of some of the bibliometric results. The graphical analysis uses co-citation, bibliographic coupling and co-occurrence of keywords. The results indicate that PAGEOPH is a leading journal in the areas in which it is indexed, with publications from a wide range of authors, institutions, and countries around the world.

Abstract On the basis of the continuous and dense GPS observations covering the northern segment of the Xiaojiang fault zone (n-XJFZ) from March 2012 to March 2016, we present the velocity field, spatiotemporal deformation, slip rate and locking depth of the n-XJFZ. The results provide strong support for achieving a better understanding of the deformation behavior of this fault. The heterogeneity of the GPS velocity field and relatively nonuniform distribution of seismicity suggest that the observational area is fragmented. Shear strain has been accumulating with an almost constant azimuth, which is consistent with the trends of the mapped major faults. The 2014 Ms 6.5 Ludian earthquake produced a sudden change in the dilatational strain, which was almost constant prior to the event, and an increase in the shear strain rate. The near-field deformation of the n-XJFZ estimated with the near-field data was larger than expected, revealing that the n-XJFZ is becoming more locked. These results imply that the seismic risk in the study area is currently rising and that, similar to the 2014 Ms 6.5 Ludian earthquake, future earthquakes will possibly occur away from mapped faults.

Abstract The subsurface geometry of five representative late Archaean ‘Chilimanzi and Razi’ suite plutons in the Zimbabwe craton (ZC) has been investigated by gravity modelling constrained in part by surface geology, density measurements and seismic information, to determine their 3D configuration and infer tectonic context of emplacement. The generally K-rich, massive, homogeneous monzogranites are characterised by large Bouguer gravity lows (up to − 30 mGal amplitude) whose gradients outline their spatial extent well. The southernmost plutons and their anomalies have general trends paralleling the North Marginal Zone (NMZ) of the Limpopo orogenic belt (LB). Predictive gravity models indicate that the density contrast of the Chivi batholith (CB) adjacent to the ‘volcanic arc-like’ Belingwe greenstone belt extends to a depth of about 13 km. The nearby Razi pluton (RP) which intrudes the ZC-LB boundary appears to have been emplaced at shallower depths/levels. The gravity model suggests a thickness of about 5–6 km, and a moderate to shallow dip to the southeast under the NMZ, compatible with syn-kinematic intrusion during overthrust of the LB over the ZC. The smallest Nalatale granite (Ng) is on average 2.5 km thick under the Fort Rixon greenstone belt but includes a root up to 4.5 km thick under the anomaly peak, and a steep contact with the tonalite/gneiss to the east. These granites follow the general power-law for pluton dimension and are similar in this respect to the classical wedge-shaped plutons, extending largely in one direction, with large aspect ratios (Length (L)/Thickness (T) > 7). However, the overall shape of the RP is typical of a diapir (Width (W) < T), although it may have been affected by the LB deformation. Gravity modelling along a NS traverse crossing the Chilimanzi batholith (ChB), the Masvingo greenstone belt (MGB) and the Zimbabwe granite (ZG) indicate a thickness of around 6 km for the dense greenstone belt with a thickness of about 8.5 km for the adjacent ZG. The ‘complex’ shaped ChB shows a 2 km thick tabular body with a root zone extending to ~ 4.5 km depth on the south end, adjacent to the greenstone belt; typical of the so-called flat-floored plutons with a gently dipping floor towards the root zone. These two plutons roughly follow the power-law for laccolith/batholith dimensions (W/T > 5; L/T > 15). Overall, the CB and the ZG are interpreted as massive, deep-rooted batholithic intrusions (L/T ≅ 10), contrary to some geological interpretations of these late, post-kinematic intrusions as sheet-like bodies emplaced at relatively shallow levels in the crust. On the other hand, the ChB appears to be a tabular intrusion, probably fed by dykes; it exhibits a lateral extent much greater than the vertical one, outlining a sheeted geometry (W/T > 7; L/T > 18). The geophysical evidence, together with geological and fabric data, support and/or confirm the two main granite configurations: sheets and batholith; and thus also confirm the two main modes of emplacement: dyke and diapirism or ballooning plutonism. This is consistent with other known batholiths on the ZC but considered unusual for plutons of the same age and spatially close when compared to other Archaean cratons.

Abstract Deep longwall mining of coal seams is made in the Upper Silesian Coal Basin (USCB) under complicated and mostly unfavourable geological and mining conditions. Usually, it is correlated with rockburst hazard mostly at a high level. One of the geological factors affecting the state of rockburst hazard is the presence of competent rocks in the roof of extracted coal seams, so rock falling behind the longwall face does not occur, and hanging-up of roof rocks remains. The long-lasting absence of caving may lead to an occurrence of high-energy tremor in the vicinity of the longwall face. Roof caving behind the longwall face may be forced by blasting. The column of explosives is then located in blastholes drilled in layers of roof rocks, e.g. sandstones behind the longwall face. In this article, a characterization of tremors initiated by blasts for roof caving during underground extraction of coal seam no. 507 in one of the collieries in the USCB has been made using three independent methods. By the basic seismic effect method, the effectiveness of blasting is evaluated according to the seismic energy of incited tremors and mass of explosives used. According to this method, selected blasts gave extremely good or excellent effect. An inversion of the seismic moment tensor enables determining the processes happening in the source of tremors. In the foci of provoked tremors the slip mechanism dominated or was clearly distinguished. The expected explosion had lesser significance or was not present. By the seismic source parameters analysis, among other things, an estimation of the stress drop in the focus or its size may be determined. The stress drop in the foci of provoked tremors was in the order of 105 Pa and the source radius, according to the Brune’s model, varied from 44.3 to 64.5 m. The results of the three mentioned methods were compared with each other and observations in situ. In all cases the roof falling was forced.

Abstract A retrospective analysis of 17 years of NCMRWF Global Forecast System (NGFS) data was conducted to understand the state and variability of cold wave episodes over northwest India. During the 2000–2016 period, a total of 21 cold wave episodes (202 cold nights) were detected, out of which 5 severe cold episodes (63 cold nights) were registered. The 10 (6) episodes occurred during La Niña (El Niño) years suggesting that both phases of El Niño-Southern Oscillation provide a favourable background for the occurrence of cold waves. The average duration of a cold wave episode was ~ 9.6 days, with the longest (shortest) episode, seen in the year 2008 (2006), lasting for 26 (6) consecutive days. The average duration of a severe cold wave episode is ≈ 4 days longer than that of a normal cold wave. In the year 2005, both the earliest (11 December) and latest (16 February) onsets of cold waves were seen. The omnipresence of intense Siberian anticyclone and the presence of western disturbance brought cold winds to the study region. Also, temperature advection and geo-potential height anomalies play vital roles in the maintenance of cold waves. The cold waves exhibit a significant intra-annual variability over northwest India. The intensity of cold waves has shown an increase of 0.11 °C per cold episode.

Abstract Changes in overall observed precipitation have been recognized in many parts of the world in recent decades, leading to the argument on climate change and its impact on extreme precipitation. However, the concept of natural variations and the complex physical mechanisms hidden in the observed data sets must also be taken into consideration. This study aims to examine the matter further with reference to inter-decadal variability in extreme precipitation quantiles appropriate for risk analysis. Temporal changes in extreme precipitation are assessed using a parametric approach incorporating a regional method in region-of-influence form. The index-flood method with the application of generalized extreme value distribution is used to estimate the decadal extreme precipitation. The study also performs a significance test to determine whether the decadal extremes are significant. A case study is performed on the Yangtze River Basin, where annual maximum 1-day precipitation data for 180 stations were analyzed over a 50-year period from 1961 to 2010. Extreme quantiles estimated from the 1990s data emerged as the significant values on several occasions. The immediate drop in the quantile values in the following decade, however, suggested that it is not practical to assign more weight to recent data for the quantile estimation process. The temporal patterns identified are in line with the previous studies conducted in the region and thus make it an alternative way to perform decadal analysis with an advantage that the scheme can be transferred to ungauged conditions.

Abstract One of the main ionization sources of the F2 region of the Earth’s ionosphere is the solar EUV irradiance accounting for ~ 90% of its variability during quiet time. Consequently, prior to long-term trend estimations solar activity must be filtered out. The last two solar activity cycles present low activity levels, and particularly solar cycle 24 is the lowest in the last ten solar cycles. The effect of the inclusion of this last solar cycle on foF2 trend estimation is analyzed for two mid-latitude ionospheric stations: Kokubunji (35.7°N, 139.5°E) and Wakkanai (45.4°N, 141.7°E). Filtering is done considering the residuals of different regressions between foF2 and Rz and also between foF2 and F10.7. In both cases, foF2 trends become less negative when solar cycle 24 is included in trend estimations since foF2 residuals systematically exceeds the values predicted by a linear, quadratic or cubic fit between foF2 and F10.7 or Rz from 2008 onwards. In addition, the Earth’s magnetic field secular variation at both stations would induce a positive foF2 trend during daytime that could counteract the greenhouse gases decreasing trend. It is interesting to think that including the latest solar cycles does not necessarily imply incorrect results in the statistical analysis of the data, but simply that solar activity is decreasing on average and also the trend.

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

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

Abstract Our goal was to develop a robust algorithm for numerical simulation of one-dimensional shallow water flow in a complex multiply-connected channel network with arbitrary geometry and variable topography. We apply a central-upwind scheme with a novel reconstruction of the open water surface in partially flooded cells that does not require additional correction. The proposed reconstruction and an exact integration of source terms for the momentum conservation equation provide positivity preserving and well-balanced features of the scheme for various wet/dry states. We use two models based on the continuity equation and mass and momentum conservation equations integrated for a control volume around the channel junction to its treatment. These junction models permit to simulate subcritical and supercritical flows in a channel network. Numerous numerical experiments demonstrate the robustness of the proposed numerical algorithm and a good agreement of numerical results with exact solutions, experimental data, and results of the previous numerical studies. The proposed new specialized test on inundation and drying of an initially dry channel network shows the merits of the new numerical algorithm to simulate the subcritical/supercritical open water flows in the networks.

Abstract Some geophysical or geodynamic applications require the use of true vertical gradient of gravity (VGG). This demand may be associated with reductions of or corrections to observed gravity or its spatiotemporal changes. In the absence of in situ measured VGG values, the constant value of the theoretical (normal) free air gradient (FAG) is commonly used. We propose an alternative to this practice which may significantly reduce systematic errors associated with the use of constant FAG. The true VGG appears to be better approximated, in areas with prominent and rugged topography, such as alpine or some volcanic regions, by a value based on the modelled contribution of the topographic masses to the gradient. Such prediction can be carried out with a digital elevation model (DEM) of sufficient resolution and accuracy. Here we present the VGG field computed for Mt. Etna (Italy), one of the most active and best monitored volcanoes worldwide, to illustrate how strongly the VGG deviates spatially from constant FAG. The predicted (modelled) VGG field is verified by in situ observations. We also take a look at the sensitivity of the VGG prediction to the resolution and quality of used DEMs. We conclude with discussing the applicability of the topo-predicted VGG field in near surface structural and volcanological micro-gravimetric studies.