Geophysical Journal International

SummaryIn this work, the propagation and attenuation of vertically polarized surface waves when interacting with terraced slopes is studied experimentally and numerically. To validate the devised simulation, a laboratory-scale physical model is tested in order to examine the attenuation properties of this well-known artificial landform. The experiment involves formation of a terraced slope, in a laboratory setup, via use of an unconsolidated granular medium made of silica microbeads. This granular medium exhibits a gravity-induced power-law stiffness profile, resulting in a depth-dependent velocity profile. A piezoelectric actuator was used to excite vertically polarized surface acoustic modes localized near the surface of the medium. The three components of the particle velocity field of these modes were measured by means of a three-dimensional laser Doppler vibrometer. In accordance with the terraced slope, a simple inclined plane was further tested to investigate and highlight the differences in terms of wave propagation along these two different ground formations. The results of this research provide significant experimental evidence that the terraced slopes form mechanisms which attenuate low frequency surface waves, thus acting as metasurfaces. This work suggests the use of laboratory-scale physical models to investigate the wave propagation in different landforms, which extend beyond typical horizontal ground morphologies, and which could be linked to atypical wave propagation properties, possibly even influencing propagation of seismic waves.

SummaryShear wave velocity (Vs) is a fundamental property of elastic media whose estimation from PS converted waves is challenging and requires modeling the boundary where P to S conversion occurs. This paper presents a PS tomography where seismic wave conversion/reflection points correspond to reflectors modelled with the level-set function set to zero (φ(x, z) = 0). The proposed method aims for stable Vs inversion in a seismic acquisition setting using multicomponent receivers. Synthetic models simulating true Vs, Vp and the location of the geological reflector are used in the study. The inversion starts by locating a flat reflector, φ(x, z) = 0, which defines the zone Ω1 between the surface and the reflector, where the initial Vs and Vp fields are also set. To calculate the traveltimes of incident PT (P wave that propagates in Ω1 from source to the reflector) , converted PS, and reflected PP waves, for both observed and modelled data (forward problem), the methodology proposed by Rawlinson and Sambridge is adopted. This method uses the arrival times of the P-waves, Tpt, from the seismic source at each reflector point as secondary sources generating the times Tps and Tpp. These times are calculated as a solution to the Eikonal equation by using the Fast Marching method. The PS and PP residual times are minimized by updating Vs, Vp, and φ(x, z) = 0 through adjoint variables designed from a formulation using Lagrange Multipliers in a variational context. The performance of the algorithm is evaluated for models with synclinal, sinusoidal and monoclinal reflector geometries using numerical tests considering the inversion of: 1) φ, given the true values of Vs and Vp; 2) φ and Vs, given the true value of Vp; 3) φ and Vp, given the true value of Vs; and 4) the three parameters φ, Vs, and Vp, simultaneously. Good results are obtained by inverting Vs and φ, given the true value of Vp. The simultaneous inversion of the three parameters exhibits promising results, despite the illumination problems caused by the different distribution of the PS, PP, and PT time gradients due to the geometry of the reflectors and the acquisition setting (sources-receivers in the same plane). The proposed tomography estimates Vs and reflector positions which could help in statics corrections and improve the lithological characterization of near surface.

SummaryConstitutive laws are a necessary ingredient in calculations of glacial isostatic adjustment (GIA) or other surface loading problems (e.g., loading by ocean tides). An idealized constitutive law governed by the Maxwell viscoelastic model is widely used but increasing attention is being directed towards more intricate constitutive laws that, in particular, include transient rheology. In this context, transient rheology collectively refers to dissipative mechanisms activated in addition to creep modeled by the Maxwell viscoelastic model. Consideration of such viscoelastic models in GIA is in its infancy and to encourage their wider use, we present constitutive laws for several experimentally derived transient rheologies and outline a flexible method in which to incorporate them into geophysical problems, such as the viscoelastic deformation of the Earth induced by surface loading. To further motivate this need, we demonstrate, via the Love number collocation method, how predictions of crustal displacement depart significantly between Earth models that adopt only Maxwell viscoelasticity and those with transient rheology. Throughout this paper, we highlight the differences in terminology and emphases between the rock mechanics, seismology, and GIA communities, which have perhaps contributed towards the relative scarcity in integrating this broader and more realistic class of constitutive laws within GIA. We focus on transient rheology since the associated deformation has been demonstrated to operate on timescales that range from hours to decades. With ice mass loss enhanced at similar timescales as a consequence of anthropogenically caused climate change, the ability to model GIA with more accurate constitutive laws is an important tool to investigate such problems.

SummaryThis study estimated the gravity change expected on the sea surface due to magma accumulation beneath submarine volcanoes. For calculation, a semi-analytical model describing the deformation of an infinite elastic cone was considered, in which a point spherical source was embedded on the axis. The expected gravity change exceeds 0.2 mGal in certain scenarios when δV = 108 m3 of source volume change occurs 2 km below the summit, and the summit is shallower than 300 m below the sea surface. The steeper the slope of a submarine volcano, the greater the expected gravity change. 0.2 mGal of gravity variation can be detected with recent marine gravimeters such as cold atom interferometers. The computation method for the gravity change based on the integral transform (Mellin type) is elaborated in this paper.

SummaryFull-waveform inversion (FWI) is an effective method for imaging subsurface properties using sparsely recorded data. It involves solving a wave propagation problem to estimate model parameters that accurately reproduce the data. Recent trends in FWI have seen a renewed interest in extended methodologies, among which source extension methods leveraging reconstructed wavefields to solve penalty or augmented Lagrangian (AL) formulations have emerged as robust algorithms, even for inaccurate initial models. Despite their demonstrated robustness on synthetic data, challenges remain, such as the lack of a clear physical interpretation and reliance on difficult-to-compute least-squares (LS) wavefields. Moreover, the literature lacks a general and through comparison of these source extension methods with each other and with the standard FWI. This paper is divided into three critical parts. In the first, a novel formulation of these methods is explored within a unified Lagrangian framework. This novel perspective permits the introduction of alternative algorithms that employ LS multipliers instead of wavefields. These multiplier-oriented variants appear as regularizations of the standard FWI, are suitable to the time domain, offer tangible physical interpretations, and foster enhanced convergence efficiency. The second part of the paper delves into understanding the underlying mechanisms of these techniques. This is achieved by solving the associated nonlinear equations using iterative linearization and inverse scattering methods. The paper provides insight into the role and significance of Lagrange multipliers in enhancing the linearization of the equations. It explains how different methods estimate multipliers or make approximations to increase computing efficiency. Additionally, it presents a new physical understanding of the Lagrange multiplier used in the AL method, highlighting how important it is for improving algorithm performance when compared to penalty methods. In the final section, the paper presents numerical examples that compare different methods within a unified iterative algorithm, utilizing benchmark Marmousi and 2004 BP salt models.

SummaryMeteorite impacts have proved to be a significant source of seismic signal on the Moon, and have now been recorded on Mars by InSight seismometers. Understanding how impacts produce seismic signal is key to the interpretation of this unique data, and to improve their identification in continuous seismic records. Here, we use the seismic Representation Theorem, and particularly the stress glut theory, to model the seismic motion resulting from impact cratering. The source is described by equivalent forces, some resulting from the impactor momentum transfer, and others from the stress glut, which represents the mechanical effect of plasticity and non linear processes in the source region. We condense these equivalent forces into a point-source with a time-varying single force and nine-component moment tensor. This analytical representation bridges the gap between the complex dynamics of crater formation, and the linear point-source representation classically used in seismology. Using the multi-physics modelling software HOSS, we develop a method to compute the stress glut of an impact, and the associated point-source from hypervelocity impact simulations. For a vertical and an oblique impact at 1000 m/s, we show that the moment tensor presents a significant deviatoric component. Hence, the source is not an ideal isotropic explosion contrary to previous assumptions, and draws closer to a double couple for the oblique impact. The contribution of the point force to the seismic signal appears negligible. We verify this model by comparing two signals: (1) HOSS is coupled to SPECFEM3D to propagate the near-source signal elastically to remote seismic stations; (2) The point-source model derived from the stress-glut theory is used to generate displacements at the same distance. The comparison shows that the point-source model is accurately simulating the low-frequency impact seismic waveform, and its seismic moment is in trend with Lunar and Martian impact data. High-frequencies discrepancies exist, which are partly related to finite-source effects, but might be further explained by the difference in mathematical framework between classical seismology and HOSS’ numerical modelling.

SummaryImaging seismic velocity discontinuities within the Earth's crust and mantle offers important insight into our understanding of the tectonic plate, associated mantle dynamics, and the evolution of the planet. However, imaging velocity discontinuities in locations where station coverage is sparse, is sometimes challenging. Here we demonstrate the effectiveness of a new imaging approach using deconvolved SS precursor phases. We demonstrate its effectiveness by applying it to synthetic seismograms. We also apply it to ∼1.6M SS precursor waveforms from the global seismic database (1990 – 2018) for comparison with Crust1.0. We migrate to depth and stack the data in circular 6° bins. The tests demonstrate that we can recover Moho depths as shallow as 20 km. The Moho is imaged at 21 – 67 km depth beneath continental regions. The Moho increases in depth from 21 km ± 4 km beneath the continental shelf to 45 – 50 km beneath the continental interiors and is as deep as 67 ± 4 km beneath Tibet. We resolve the Moho in 77% of all continental bins, within 10 km of Crust 1.0, with all outliers located in coastal regions. We also demonstrate the feasibility of using this method to image discontinuities associated with the mantle transition zone with both synthetic and real data. Overall, the approach shows broad promise for imaging mantle discontinuities.

SummaryIn a large body of rock-physics research, seismic-wave velocity dispersion and attenuation in fluid-saturated porous rock are studied by constructing analytical or numerical models for time- or frequency-dependent dynamic (effective, or viscoelastic) moduli. A key and broadly used model of such kind is the Zener's, or the standard linear solid (SLS). This model is qualitatively successful in explaining many field and laboratory observations and serves as the key element of many generalizations such as the Burgers model for plastic deformations or the generalized SLS explaining band-limited or near-constant seismic attenuation. However, as a physical model of fluid-saturated porous rock, the SLS has several major limitations: disregard of inertial effects, absence of secondary wave modes, and lack of key physical parameters such as porosity and Skempton coefficients. Grainy and porous rock is an unconsolidated material in which the effective density is frequency-dependent, and its effects on wave velocities may exceed those of the dynamic modulus. To overcome these limitations of the empirical SLS, we propose a rigorous rheologic model based on classical continuum mechanics and called the extended SLS, or eSLS. This rheology explains the available attenuation and dispersion observations equally well, but it is also close to Biot's model, honors all poroelastic relations, includes inertial effects, and reveals several new physical properties of the material. Detailed comparison of the eSLS and Biot's models gives a physical-mechanism based classification of wave-induced fluid flow (WIFF) phenomena. In this classification, the so-called “global-scale” flows occur in Biot's type structures within the material, whereas the “local-scale” WIFF occurs in eSLS-type structures. Combining Biot's and eSLS models gives a broad class of rheologies for linear anelastic phenomena within rock with a single type of porosity. The model can be readily generalized to multiple porosities and different types of internal variables, such as describing squirt flows, wetting or thermoelastic effects. Modeling is conducted with relatively little effort, using a single matrix equation similar to a mechanical form of the standard SLS. By combining the eSLS and Biot's models, observations of dynamic-modulus dispersion and attenuation can be inverted for macroscopic mechanical properties of porous materials.

SummarySeismic full-waveform inversion (FWI) or waveform inversion (WI) has gained extensive attention as a cutting-edge imaging method, which is expected to reveal the high-resolution images of complex geological structures. In this paper, we regard each 1-D signal in the inversion system as a 1-D probability distribution, then use the Jensen-Shannon divergence (JSD) from information theory to measure the discrepancy between the predicted and observed signals, and finally implement a novel 2-D multiparameter shallow-seismic waveform inversion (MSWI). Essentially, the novel approach achieves an implicit weighting along the time-axis for each 1-D adjoint source defined by the classical waveform inversion (CWI), thus enhancing the extra illumination for a deeper medium compared with the CWI. By evaluating the inversion results of the two-layer model and fault model, the reconstruction accuracy for S-wave velocity and density of the new method is increased by about 30% and 20% compared with that of the CWI under the same conditions, respectively. The reconstruction performance for P-wave velocity of these two methods is almost equal. In addition, the new 2-D MSWI is also resilient to white Gaussian noise in the data. Numerically, the inversion system has almost the strongest sensitivities to the S-wave velocity and density, performing the poorest sensitivity to the P-wave velocity. Finally, we test the novel method with a detection case for a power tunnel.

SummaryWe apply the adjoint method to efficiently calculate sensitivity kernels for long-period seismic spectra with respect to structural and source parameters. Our approach is built around the solution of the frequency-domain equations of motion using the Direct Solution Method (DSM). The DSM is currently applied within large-scale mode coupling calculations and is also likely to be useful within finite-element type methods for modelling seismic spectra that are being actively developed. Using mode coupling theory as a framework for solving both the forward and adjoint equations, we present numerical examples that focus on the spectrum close to four eigenfrequencies (the low-frequency mode, 0S2, and higher frequency modes, namely 2S2, 0S7, and 0S10 for comparison). For each chosen observable, we plot sensitivity kernels with respect to 3D perturbations in density and seismic wave-speeds. We also use the adjoint method to calculate derivatives of observables with respect to the matrices occurring within mode coupling calculations. This latter approach points towards a generalisation of the two-stage splitting function method for structural inversions that does not rely on inaccurate self-coupling or group-coupling approximations. Finally, we verify through direct calculation that our sensitivity kernels correctly predict the linear dependence of the chosen observables on model perturbations. In doing this, we highlight the importance of non-linearity within inversions of long-period spectra.

SummaryRapid and accurate characterization of earthquake sources is crucial for mitigating seismic hazards. In this study, based on 18000 scenario ruptures ranging from Mw 6.4 to Mw 8.3 and corresponding synthetic high-rate Global Navigation Satellite System (HR-GNSS) waveforms, we developed a multi-branch neural network framework, the Continental Large Earthquake Agile Respond (CLEAR), to simultaneously determine the magnitude and slip distributions. We apply CLEAR to recent large strike-slip events, including the 2021 Mw 7.4 Maduo earthquake and the 2023 Mw 7.8 and Mw 7.6 Turkey doublet. The model generally estimates the magnitudes successfully at 32 s with errors of less than 0.15, and predicts the slip distributions acceptably at 64 s, requiring only approximately 30 ms on a single CPU. With optimal azimuthal coverage of stations, the system is relatively robust to the number of stations and the time length of the received data.

SummaryLaunched on December 16, 2022, the Surface Water and Ocean Topography (SWOT) satellite, using synthetic aperture radar interferometric techniques, measures sea surface heights (SSHs) across two 50-km-wide swaths, offering high-resolution and accurate two-dimensional SSH observations. This study explores the efficiency of SWOT in seamount detection employing the vertical gravity gradient (VGG) derived from simulated SWOT SSH data. Simulated circular and elliptical seamounts (height: 900-1500 m) are integrated within the South China Sea's 4000 m background depths. Geoid perturbations induced by these seamounts are extracted through the residual depth model principle, subsequently merged with the DTU21MSS model for simulating SWOT SSH observations. For comparative assessment, SSH data from Jason-2 and Cryosat-2 are included. An automatic algorithm (AIFS) is presented to identify seamount centers and base polygons using VGG derived from simulated altimeter SSH data. The analysis reveals SWOT-derived VGGs precisely locate all seamount centers, base polygons, and elliptical seamount azimuths. The merged Jason-2 and Cryosat-2 data face challenges with identifying small circular and elliptical seamounts. Detecting long narrow elliptical seamounts remains arduous; however, SWOT-derived VGGs successfully elucidate the approximate shapes and major axis azimuths of the elliptical seamounts. Validated against ‘true values’ of VGG, the root-mean-squared deviation (RMSD) of SWOT-derived VGG stands at 1.33 Eötvös, whereas the merged Jason-2 and Cryosat-2 data exhibit an RMSD of 1.93 Eötvös. This study shows the enhanced capability of SWOT from its high-resolution two-dimensional SSH observations in advancing seamount detection via satellite-derived VGG. We identify challenges and recommend improved detections using data integration and machine learning.

SummaryBayesian methods provide a valuable framework for rigorously quantifying the model uncertainty arising from the inherent non-uniqueness in the magnetotelluric (MT) inversion. However, widely-used Markov chain Monte Carlo (MCMC) sampling approaches usually require a significant number of model samples for accurate uncertainty estimates, making their applications computationally challenging for 2D or 3D MT problems. In this study, we explore the applicability of the Hamiltonian Monte Carlo (HMC) method for 2D probabilistic MT inversion. The HMC provides a mechanism for efficient exploration in high-dimensional model space by making use of gradient information of the posterior probability distribution, resulting in a substantial reduction in the number of samples needed for reliable uncertainty quantification compared to the conventional MCMC methods. Numerical examples with synthetic data demonstrate that the HMC method achieves rapid convergence to the posterior probability distribution of model parameters with a limited number of model samples, indicating the computational advantages of the HMC in high-dimensional model space. Finally, we applied the developed approach to the COPROD2 field dataset. The statistical models derived from the HMC approach agree well with previous results obtained by 2D deterministic inversions. Most importantly, the probabilistic inversion provides valuable quantitative model uncertainty information associated with the resistivity structures derived from the observed data, which facilitates model interpretation.

SummaryThe Rogaland Igneous Complex (RIC) in southern Norway intruded into Sveconorwegian granulite crust beginning ∼930 Ma. Three massif anorthosite bodies, Egersund-Ogna, Helleren and Åna-Sira, were intruded some 10 myr later by the Bjerkreim-Sokndal layered intrusion. The Garsaknatt leuconorite and the ilmenite-rich Tellnes norite, one of the youngest rock in the complex at ∼920 Ma, intrude the anorthosite or nearby country rock. Magnetic mineralogy and paleomagnetic studies carried out on the Tellnes norite, the Garsaknatt leuconorite, and the surrounding Åna-Sira anorthosite, indicate the magnetization of all three bodies are dominated by hemo-ilmenite carrying the remanence as a thermo-chemical remanent magnetization (TCRM), although magnetite is present in some samples. The three bodies yield steep negative inclinations with northwesterly declinations (Tellnes, I = -71.9°, D = 305.0°, α95 = 10.6°; Garsaknatt, I = -73.1°, D = 312.7°, α95 = 4.7°; Åna-Sira, I = -81.2°, D = 326.3°, α95 = 6.7°). When combined with data from other bodies in the RIC, the older anorthosites have steeper inclinations, and higher paleolatitudes, while the younger units have less steep inclinations and shallower paleolatitudes by nearly 10°, indicating northward plate motion during cooling of the intrusions. Age of the remanence is difficult to determine precisely, however, best estimates are ∼910 Ma for the older anorthosites and ∼900 Ma for the younger intrusions. Although these differences are significant, a unified pole position (35.6°N, 215.1°E), combining all the 111 sites from the RIC, strongly supports the assumed position of southern Baltica in Rodinia at ∼900 MA.

SummarySeismic travel time anomalies of waves that traverse the uppermost 100-200 km of the outer core have been interpreted as evidence of reduced seismic velocities (relative to radial reference models) just below the core-mantle boundary. These studies typically investigate differential travel times of SmKS waves, which propagate as P-waves through the shallowest outer core and reflect from the underside of the core-mantle boundary m times. The use of SmKS and S(m-1)KS differential travel times for core imaging are often assumed to suppress contributions from earthquake location errors and unknown and unmodelled seismic velocity heterogeneity in the mantle. The goal of this study is to understand the extent to which differential SmKS travel times are, in fact, affected by anomalous mantle structure, potentially including both velocity heterogeneity and anisotropy. Velocity variations affect not only a wave's travel time, but also the path of a wave, which can be observed in deviations of the wave's incoming direction. Since radial velocity variations in the outer core will only minimally affect the wave path, in contrast to other potential effects, measuring the incoming direction of SmKS waves provides an additional diagnostic as to the origin of travel time anomalies. Here we use arrays of seismometers to measure travel time and direction anomalies of SmKS waves that sample the uppermost outer core. We form subarrays of EarthScope's regional Transportable Array stations, thus measuring local variations in travel time and direction. We observe systematic lateral variations in both travel time and incoming wave direction, which cannot be explained by changes to the radial seismic velocity profile of the outer core. Moreover, we find a correlation between incoming wave direction and travel time anomaly, suggesting that observed travel time anomalies may be caused, at least in part, by changes to the wave path and not solely by perturbations in outer core velocity. Modelling of 1-D ray and 3-D wave propagation in global 3-D tomographic models of mantle velocity anomalies match the trend of the observed travel time anomalies. Overall, we demonstrate that observed SmKS travel time anomalies may have a significant contribution from 3-D mantle structure, and not solely from outer core structure.

SummaryIncreasing evidence from seismic methods shows that anisotropy within subduction zones should consist of multiple layers. To test this, we calculate and model shear wave splitting across the Alaska-Aleutians Subduction Zone (AASZ), where previous studies have argued for separate layers of anisotropy in the subslab, slab, and mantle wedge. We present an updated teleseismic splitting catalog along the span of the AASZ, which has many broadband seismometers recently upgraded to three components. Splitting observations are sparse in the Western Aleutians, and fast directions are oriented generally trench parallel. There are significantly more splitting measurements further east along the AASZ. We identify six regions in the Central and Eastern Aleutians, Alaskan Peninsula, and Cook Inlet with a high density of splits suitable for multilayered anisotropy analyses. These regions were tested for multilayer anisotropy, and for five of the six regions we favor multiple layers over a single layer of anisotropy. We find that the optimal setup for our models is one with a dipping middle layer oriented parallel to paleospreading. A prominent feature of our modeling is that fast directions above and below the dipping layer are generally oriented parallel to the strike of the slab. Additionally, we lay out a framework for robust and statistically reliable multilayer shear wave splitting modeling.

SummaryAmbient noise tomography on the basis of Distributed Acoustic Sensing (DAS) deployed on existing telecommunication networks provides an opportunity to image the urban subsurface at regional scales and high-resolution. This capability has important implications in the assessment of the urban subsurface’s potential for sustainable and safe utilization, such as geothermal development. However, extracting coherent seismic signals from the DAS ambient wavefield in urban environments at low cost remains a challenge. One obstacle is the presence of complex sources of noise in urban environments, which may not be homogeneously distributed. Consequently, long recordings are required for the calculation of high-quality virtual shot gathers, which necessitates significant time and computational cost. In this paper, we present the analysis of 15 days of DAS data recorded on a pre-existing fiber optic cable (dark fibers), running along an 11 km long major road in urban Berlin (Germany), hosting heavy traffic including vehicles and trains. To retrieve virtual shot gathers, we apply interferometric analysis based on the cross-correlation approach where we exclude low-quality virtual shot gathers to increase the signal-to-noise ratio of the stacked gathers. Moreover, we modify the conventional ambient noise interferometry workflow by incorporating a coherence-based enhancement approach designed for wavefield data recorded with large-N arrays. We then conduct Multichannel Analysis of Surface Waves (MASW) to retrieve 1D velocity models for two exemplary fiber subsegments, and compare the results of the conventional and modified workflows. The resulting 1D velocity models correspond well with available lithology information. The modified workflow yields improved dispersion spectra, particularly in the low-frequency band (< 1 Hz) of the signal. This leads to an increased investigation depth along with lower uncertainties in the inversion result. Additionally, these improved results were achieved using significantly less data than required using conventional approaches, thus opening the opportunity for shortening required acquisition times and accordingly lowering costs.

SummaryThis study attempts to interrogate the upper mantle deformation pattern beneath the Kumaon-Garhwal region, located in the western Himalaya, using shear wave splitting (SWS) analysis of core-refracted (XK(K)S) phases recorded at 53 broadband stations. The fast polarisation azimuths (FPAs) revealed by 338 well constrained measurements are dominantly clustered around ENE-WSW, with a few along the NE and E-W directions. The delay times vary from 0.2 to 1.4 s, with an average of 0.6 s that is smaller than that for the Indian shield (∼0.8 s), central and eastern Himalayas. The northern part of the lesser Himalaya shows a slightly smaller delay time compared to the southern part, which is attributed to the weakening of azimuthal anisotropy caused by the dipping of the Indian lithosphere. In order to understand the crustal contribution, its anisotropy is measured by analysing the splitting of Ps conversions from the Moho (Pms), akin to that of the XK(K)S phases. However, reliable results for crustal anisotropy could be obtained only at 10 stations. The average delay time due to crustal anisotropy is 0.47 s, with a variation from 0.2 to 0.9 s. Although the dominant period of Pms is smaller than that of SK(K)S, crustal anisotropy contributing to splitting of the latter phases cannot be ruled out. The orientation of FPAs obtained from Pms phases is found to be parallel or sub-parallel to those from XK(K)S phases, suggesting a similar deformation mechanism in the mid- to lower-crust and upper mantle. On the basis of FPAs derived from XK(K)S measurements, the Kumaon-Garhwal Himalaya (KGH) region can be divided into four sub-regions. In the western and eastern parts, the FPAs are mostly aligned along NE and ENE-WSW, and NE, respectively. In the central and south-eastern parts, their orientation is along ENE-WSW and NW, respectively. The strong ENE-WSW orientation in the central part could result from a slightly variable anisotropy in the crust to the upper part of the lithosphere or basal topography causing deflection of mantle flow. Also, the NW orientation in the south-eastern part of KGH is associated with a shallow source within the lithosphere. Application of the spatial coherency technique to single-layered anisotropic parameters results in a depth of 220-240 km, implying that the dominant source of anisotropy could lie in the upper mantle.

SummaryModeling seismic wavefields in complex 3D elastic media is the key in many fields of Earth Science: seismology, seismic imaging, seismic hazard assessment, earthquake source mechanism reconstruction. This modeling operation can incur significant computational cost, and its accuracy depends on the ability to take into account the scales of the subsurface heterogeneities varying. The theory of homogenization describes how the small-scale heterogeneities interact with the seismic waves and allows to upscale elastic media consistently with the wave equation. In this study, an efficient and scalable numerical homogenization tool is developed, relying on the similarity between the equations describing the propagation of elastic waves and the homogenization process. By exploiting the optimized implementation of an elastic modeling kernel based on a spectral element discretization and domain decomposition, a fully scalable homogenization process, working directly on the spectral-element mesh, is presented. Numerical experiments on the entire SEAM II foothill model and a 3D version of the Marmousi II model illustrate the efficiency and flexibility of this approach. A reduction of two orders of magnitude in terms of absolute computational cost is observed on the elastic wave modeling of the entire SEAM II model at a controlled accuracy.

SummaryThe increasing need for petrophysical parameter inversion has spurred the advancement of full-waveform inversion (FWI). This study introduces a new inversion method that can directly estimate petrophysical parameters from seismic waveform data. Specifically, we modified the regular elastic wave equation by including Biot poroelastic parameters and designed an inversion workflow based on the framework of elastic FWI. The newly introduced inversion algorithms have demonstrated their effectiveness and accuracy using the Marmousi model and a synthetic model related to the gas hydrate deposits based on the Shenhu area of the South China Sea. Our work demonstrates the feasibility of directly inverting petrophysical parameters and assessing if a deposit contains gas hydrate.