Geophysical Journal International

SummaryConstraining the knowledge of deep aquifer structure in the Angad Basin (Northeastern Morocco) remains one of the major challenges for successful borehole drilling projects. This study demonstrates how integrating gravity and electrical data can enhance subsurface geological imaging and provide valuable insights into geological structures relevant to groundwater exploration. Various gravity enhancement techniques, including vertical gradient, horizontal gradient, upward continuation, logistic filter, tilt angle, and Euler Deconvolution, have been applied to gravity data in the Angad Basin to identify gravity anomalies and tectonic discontinuities. The resulting gravity maps reveal two elongated depressions trending in the NE-SW direction: the Beni Drar depression in the north and the Oujda depression in the south. Both depressions are delineated by strong gravity gradients, indicating the presence of faults. Multiscale analysis of gravity lineaments highlighted sharper features and identified two principal orientations: N070°–085° and N040°–050°, with the latter being more predominant. Faults striking in the N120°–140° are less prominent. These fault systems play a crucial role in dividing the bedrock into distinct horst and graben structures, aligning with the regional tectonic phases observed in northeastern Morocco. The faults are generally sub-vertical, with estimated depth values ranging from 1 000 to 2 680 meters for 42 per cent of the lineaments. Additionally, the reinterpretation of Vertical Electrical Soundings (VES) using 2D inversion methodology provides insight into subsurface resistivity variations. The 2D resistivity sections generated from this process illustrate vertical and lateral variations in electrical resistivity over distances of up to 30 km and depths of up to 2 km, revealing the geological layers that form the aquifer. These resistivity sections confirm the presence of multiple faults that significantly influence the structural configuration of the study area, validating the geometry of the two depressions previously identified through gravity data. The 2D gravity modeling further corroborates and reinforces the findings from the resistivity section inversion, demonstrating that the shape of the residual gravity anomaly curve closely aligns with the morphology of the Jurassic roof structure. The results of this study highlight the complementarity and effectiveness of these two geophysical methods in advancing our understanding of the deep geological structure of the Angad Basin, particularly within the Jurassic limestone, which serves as the region's primary deep aquifer. These findings provide valuable insights for future hydrogeological research in the area.

SummaryUnderstanding the Earth's magnetic field through regional records of secular variation is essential for deciphering its short-term behaviour. This study presents an archaeomagnetic and rock-magnetic investigation of archaeological artefacts from Vadnagar, Gujarat, and introduces India's first continuous palaeosecular variation (PSV) curve for the last four millennia. Detailed rock magnetic analyses were applied to investigate the suitability of the artefacts for intensity measurements. The geomagnetic field intensity was calculated using the Thellier-Thellier method modified by Coe, with cooling rate and anisotropy of the thermoremanent magnetization (ATRM) corrections. A total of 80 independent fragments were analyzed, from which 66 gave positive responses, resulting in a success rate of 83 per cent after the application of corrections and quality selection criteria. Seven new archaeointensities were calculated, with values ranging from 33.58 ± 2.0 to 43.37 ± 1.9 µT. The new intensities were integrated with previously published data in order to construct India's first PSV curve from 2250 BCE to 2000 CE at the geographical Centre of India (20.5937° N, 78.9629° E) using two different modelling approaches. The first approach employs a bootstrap algorithm, yielding relatively smooth intensity variations, while the second utilizes a transdimensional Bayesian framework, producing sharper variations with occasionally greater amplitudes. The PSV curve developed using the bootstrap algorithm was compared with global models, showing precise temporal alignment only from 400 BCE to 200 CE. This indicates the necessity of new reliable archaeointensity data from dated artefacts in order to acquire a rigorous explanation of geomagnetic field intensity change during the past and gain a deeper understanding of local geomagnetic field variations in India.

SummaryIn this study, we illustrate the application of a 3D reflection oriented workflow for full waveform inversion (FWI) to the offset data from the Sleipner field. The data set is having maximum offset of less than 2000 m, and has been pre-processed with a low-cut filter below 6 Hz, imposing strong challenges for the FWI application. To tackle these challenges, a reflection oriented full waveform inversion is applied to the data set, which utilize joint full waveform inversion (JFWI) to constrain the low wavenumber updates at the deepest part of the reconstructed model. It consists of two steps, an impedance model building serving as a prior reflector information followed by a velocity model building. In this case, JFWI workflow is taking advantage of the pseudo-time formulation to honor the zero offset travel time, fast and robust asymptotic preconditioner for impedance model building, and graph space optimal transport misfit function to mitigate cycle skipping. To show the effects of limited offset, conventional FWI is performed. In this case, it is clear that the meaningful updates coming from the diving waves are restricted to the shallow part no deeper than 500 m of depth, while no meaningful perturbation are observed beyond the diving waves penetration. Taking advantage of the meaningful shallow updates, diving wave only inversion is performed prior to the impedance model building, and then followed by the JFWI workflow. The results of the field data application show that JFWI is able to produces meaningful velocity updates both in shallow part and the deeper part. The result is supported by satisfactory fit of the calculated data based on the JFWI model compared to the observed data. In addition, the velocity model fits the low wavenumber trend of the well log data. A subsequent run of conventional FWI is performed starting from JFWI model, in order to improve the resolution of the velocity model. The results is able to introduce higher wavenumber content to the velocity model, producing satisfactory fit with the observed data, and matching the well log data.

SummaryIt is well known that nonlinear site effects may arise in soils during strong ground motion. This translates into a decrease in propagation velocity, shift of resonance frequencies, increased material damping, and lessened ground motion amplification. In this study, we introduce a time-frequency resonance analysis (TFRA) technique to unveil nonlinear site response by computing broadband resonance frequencies derived from waveforms recorded across a wide range of ground motions at 567 stations of the Kiban Kyoshin network (KiK-net) in Japan. The found resonance frequencies follow closely those computed from surface to borehole spectral ratios. Furthermore, we quantify the coseismic frequency changes, which shift towards lower values as the earthquake ground shaking increases. At some stations, the extracted resonance frequencies attain up to a 60% decrease, representing a shear modulus reduction (μ/μmax) of 0.16 (84% decrease) assuming a homogeneous layer over a halfspace model. To ensure independence from specific seismic events, we establish peak ground acceleration (PGA) thresholds corresponding to 5% and 10% frequency shifts to identify regions where sites are prone to soil nonlinearity. We find that for a 5% frequency shift, some sites require a relatively small PGA (∼ 30 cm/s2) to trigger this effect, with most places needing a PGA ∼ 50 cm/s2. Furthermore, the computed μ/μmax values are categorized by the time-averaged shear-wave velocity to a depth of 30 and 5 meters (VS30 and VS05), which are proxies commonly used in earthquake engineering studies for characterizing site effects. We do not observe a clear correlation between the time-averaged shear-wave velocity to a certain depth and the computed shear modulus reduction (nonlinear site effect). Furthermore, no evident correlation was found between the nonlinear site effect and the earthquake magnitude, the distance from the earthquake to the site. This study suggests that nonlinear soil behavior is site-specific. This complicates the use of proxies or equations to take into account these effects, making it difficult to include soil nonlinearity in regional seismic hazard studies.

SummaryThe conventional Liouville equation (LE), commonly used to elucidate Earth's polar motion (PM), has subsequently developed into the observable LE (OLE); however, the OLE is not applicable for full-band PM investigations. By integrating the OLE with relationship between the theory and observations of PM, we obtained a comprehensive resolution to the LE, which is referred to as the complete LE (CLE) solution. In the CLE solution, the effects of the incalculable excitation derivative term ${\rm{d}}\psi /{\rm{d}}t$ have been incorporated and can now be easily estimated. Afterwards, we did an extensive review of the geophysical implications of the CLE solution in order to figure out how this excitation derivative term impacts the full-band PM. According to this CLE solution, we have established a solid relationship between the observed PM and the celestial intermediate pole (CIP). Furthermore, we recreated the PM by employing both the simulated and real geophysical fluid excitation series. The simulation example demonstrates the application of the CLE solution for full-band PM examinations with a numerical accuracy of less than 1${\rm{\mu as}}$. Real data reconstructions have shown that the CLE solution outperforms the OLE solution in both low- and high-frequency PM bands, with amplitude enhancements of around hundreds${\rm{\ \mu as}}$ and several${\rm{\ \mu as}}$, respectively. These findings enable us to precisely evaluate the comprehensive PM investigations.

SummaryIn recent years, Full-Waveform Inversion (FWI) has been extensively used to derive high-resolution subsurface velocity models from seismic data. However, due to the nonlinearity and ill-posed nature of the problem, FWI requires a good starting model to avoid producing non-physical solutions (i.e., being trapped in local minima). Moreover, traditional optimization methods often struggle to effectively quantify the uncertainty associated with the recovered solution, which is critical for decision-making processes. Bayesian inference offers an alternative approach as it directly or indirectly evaluates the posterior probability density function using Bayes’ theorem. For example, Markov Chain Monte Carlo (MCMC) methods generate multiple sample chains to characterize the solution’s uncertainty. Despite their ability to theoretically handle any form of distribution, MCMC methods require many sampling steps; this limits their usage in high-dimensional problems with computationally intensive forward modeling, as is the FWI case. Variational Inference (VI), on the other hand, approximates the posterior distribution in the form of a parametric or non-parametric proposal distribution. Among the various algorithms used in VI, Stein Variational Gradient Descent (SVGD) is characterized for its ability to iteratively refine a set of samples (commonly referred to as particles) to approximate the target distribution through an optimization process. However, mode and variance-collapse issues affect SVGD in high-dimensional inverse problems. In this study, we propose to improve the performance of SVGD within the context of FWI by combining an annealed variant of the SVGD algorithm with a multi-scale strategy, a common practice in deterministic FWI settings. Additionally, we demonstrate that Principal Component Analysis (PCA) can help to evaluate the performance of the optimization process and gain insights into the behavior of the output particles and their overall distribution. Clustering techniques are also employed to provide more rigorous and meaningful statistical analysis of the particles in the presence of multi-modal distributions (as is usually the case in FWI). Numerical tests, performed on a portion of the acoustic Marmousi model using both single and multiple frequency ranges, reveal the benefits of annealed SVGD compared to vanilla SVGD to enhance uncertainty estimation using a limited number of particles and thus address the challenges of dimensionality and computational constraints.

SummaryThe ability to detect low frequency sounds from distant energetic events depends on the temperature and wind structure of the atmosphere. Thus, from time to time surface-based acoustic detectors may not be able to capture sounds arriving from certain directions. However, the temperature minimum at the tropopause may create an acoustic duct called the “AtmoSOFAR” channel that could transmit acoustic waves laterally – but perhaps not to the ground. If true, then elevated sensors such as those borne aloft by balloons would record the signatures even in regions where ground based sensors cannot. This has been difficult to prove because high altitude acoustic sources are rare and balloon deployments are sporadic. This work describes the detection and characterization of powerful acoustic waves generated during the launch and terminal explosion of the SpaceX Starship rocket on 20 April 2023 using a pair of microbarometers on a stratospheric balloon. The signals traveled through the AtmoSOFAR channel, carrying information about the size and nature of their source. This channel also appears to leak some acoustic energy to the ground, in agreement with previous studies. The acoustic yield of the Starship terminal explosion was on the order of 103 tons TNT equivalent, which agrees with the estimated fuel load of the vehicle to about a factor of 2, but is two orders of magnitude larger than optical estimates. These results support an earlier study that claimed lateral transmission of sound from a smaller rocket through the AtmoSOFAR channel. The transmission of source information through the AtmoSOFAR channel motivates its use for monitoring other natural and anthropogenic events using balloon-borne sensors. This may become increasingly important as more and more private and government entities conduct spacecraft launches and reentries. It may also provide a means of monitoring clear air turbulence and other sound-generating atmospheric phenomena at a distance.

SummaryThe Sichuan-Yunnan region is a crucial area for studying the deformation and tectonic evolution of the lithosphere within the Tibetan Plateau. However, a significant controversy exists about the spatial distribution of the low-viscosity zones in its mid-lower crust. Herein, we utilized a combination of topography, geoid height, surface heat flow, and Rayleigh wave phase velocity dispersion curves to ascertain the lithospheric temperature, seismic wave velocity, and density structure in this region. By correlating the inverted velocity and density structures with laboratory measurements of rock velocity and density, we further deduced the lithospheric lithology structure and rheological property of the Sichuan-Yunnan region. Our findings suggest that the lower crust of this region is predominantly composed of felsic granulite. The lower crust of the Qiangtang Block, the Chuan-Xi Block, the Dian-Zhong Block west of the Lvzhijiang fault, and the IndoChina Block exhibit extensive areas with low-viscosity characteristics (<1021 Pa·s). In contrast, the Sichuan Basin, the Eastern Himalayan Syntaxis, and the central region of the Emeishan Igneous Province are characterized by high strength. We argue that the collision between the Indian and Eurasian plates led to the thickening of the Qiangtang Block's crust, producing a large low-viscosity area within the mid-lower crust. The delamination of the IndoChina lithosphere may cause the upwelling of mantle material, thereby weakening the lithosphere of the Dian-Zhong Block west of the Lvzhijiang fault and the IndoChina Block. This study delineates the spatial distribution of low-viscosity zones within the mid-lower crust of the Sichuan-Yunnan region, offering a foundational rheological model that can be instrumental for subsequent seismological and dynamic analyses.

SummaryApplications of spectral induced polarization (SIP) require electrodes that maintain hydrologic contact with surrounding soils to capture small electrical responses, often observed as phase shifts in milliradians. For unsaturated soils, electrodes must overcome the increased electrical contact impedance due to reduced pore fluid. Traditional designs use a ceramic membrane electrode (CME) with a water reservoir and metal conductor, requiring periodic maintenance to retain electrolytic solution. For field applications where maintenance is impractical, alternative designs are needed. This study evaluated a new electrode design (silica flour electrode, SFE) alongside a CME design. SFEs use packed silica flour to store water via capillary forces against a metal conductor. The study examined both designs in three variably saturated soils at soil suctions up to 700 mbar and soil water contents below 1 per cent, with SIP measurements across 0.01 to 10,000 Hz frequencies. SFEs match CMEs at high frequencies and perform better at lower frequencies, without requiring ongoing maintenance, making them ideal for field use. In water-only experiments, CMEs produced errors and high noise below 1.5 Hz, whereas SFEs were more accurate. However, CMEs performed better above 300 Hz. In fine sand, SFEs performed better due to the relatively lower contact impedance as compared to CMEs. Both electrode types performed comparably in silty sand and silt loam soils, although CMEs required ongoing maintenance, suggesting potential for long-term reliability issues.

SummaryThe finite-difference method (FDM), limited by uniform grids, often encounters severe oversampling in high-velocity regions when applied to multi-scale subsurface structures, leading to reduced computational efficiency. A feasible solution to this issue is the use of non-uniform grids. However, previous discontinuous grid approaches required careful consideration of interpolation operations in transition regions, while single-block continuous grids lacked flexibility. This paper proposes a novel approach using multi-block stretched grids with positive and negative singularities to achieve non-uniform grids, the numerical simulation of seismic waves is realized by combining it with the curvilinear grid finite-difference method (CGFDM). Our method facilitates seamless information exchange between coarse and fine grids without additional interpolation or data processing and allows for flexible grid configurations by adjusting singularity pairs.The effectiveness of our approach is verified through comparisons with the generalized reflection/transmission method (GRTM) and the finite-element method (FEM). Numerical experiments demonstrate the method's reliable accuracy and significant reduction in grid points compared to uniform grids. Although the stability of our method has not been rigorously mathematically proven, we demonstrate that the algorithm remains applicable for sufficiently long simulations to address realistic scenarios.

SummaryFor a weakly anisotropic medium, Rayleigh and Love wave phase speeds at angular frequency ω and propagation azimuth ψ are given approximately by V(ω, ψ) = A0 + A2ccos 2ψ + A2ssin 2ψ + A4ccos 4ψ + A4ssin 4ψ. Earlier theories of the propagation of surface waves in anisotropic media based on non-degenerate perturbation theory predict that the dominant components are expected to be 2ψ for Rayleigh waves and 4ψ for Love waves. This paper is motivated by recent observations of the the 2ψ component for Love waves and 4ψ for Rayleigh waves, referred to here as “unexpected anisotropy”. To explain these observations, we present a quasi-degenerate theory of Rayleigh-Love coupling in a weakly anisotropic medium based on Hamilton’s Principle in Cartesian coordinates, benchmarking this theory with numerical results based on SPECFEM3D. We show that unexpected anisotropy is expected to be present when Rayleigh-Love coupling is strong and recent observations of Rayleigh and Love wave 2ψ and 4ψ anisotropy can be fit successfully with physically plausible models of a depth-dependent tilted transversely isotropic (TTI) medium. In addition, when observations of the 2ψ and 4ψ components of Rayleigh and Love anisotropy are used in the inversion, the ellipticity parameter ηX, introduced here, is better constrained, we can constrain the absolute dip direction based on polarization measurements, and we provide evidence that the mantle should be modeled as a tilted orthorhombic medium rather than a TTI medium. Ignoring observations of unexpected anisotropy may bias the estimated seismic model significantly. We also provide information about the polarization of the quasi-Love waves and coupling between fundamental mode Love and overtone Rayleigh waves in both continental and oceanic settings. The theory of SV-SH coupling for horizontally propagating body waves is presented for comparison with the surface wave theory, with emphasis on results for a TTI medium.

SummaryExtensional faults in Southern Calabria (Italy) have been widely studied for their capability of generating high magnitude earthquakes (Mw 7-7.2). An example is the historical seismic sequence occurred in 1783, which caused numerous fatalities near the villages located along the longest faults of this region: the Cittanova and the Serre faults. In this work, we estimated the seismic potential of these two faults by a kinematic block modeling approach using GNSS data of both campaign points and permanent stations. Our results indicate that both faults are accommodating the recognized extensional velocity gradient (∼ 1 mm/yr) by long-term slip rates (∼ 2 mm/yr). To estimate the back slip distribution and the interseismic coupling degree of the Cittanova and Serre faults, we discretised these by a triangular dislocation elements (TDEs) mesh. This approach has allowed us to distinguish the fault areas where elastic seismic rupture is more likely to happen from those affected by aseismic creeping behaviour. The obtained results show that the highest values of coupling are located near the shallow portion of the fault planes and near the southern tip of the Cittanova fault. We therefore estimated a set of possible rupture scenarios finding that the Southern Calabria domain is accumulating an interseimic moment rate at most equal to 2.16 ×1016 Nm/yr, the equivalent of an earthquake of Mw 4.86 for each year.

SummaryChemical potentials are defined as the partial derivatives of the Helmholtz energy with respect to moles of chemical components under conditions of zero domain strain and fixed temperature. Under hydrostatic conditions, chemical potentials are dependent only on state properties. Under nonhydrostatic conditions, they also depend on a “chemical expansivity tensor” - a second-order tensor with unit trace that characterises how the elastic network is compressed to accommodate new material within the local domain element. The five degrees of freedom of this tensor generate a class of chemical potentials. An important group within this class are the “uniaxial chemical potentials”, which quantify the Helmholtz energy change when new material is incorporated via compression along a single axis. Chemical and mechanical equilibrium is achieved when all uniaxial chemical potentials remain constant along their respective axes. The derived expressions apply to both crystalline and amorphous materials. Their utility is demonstrated through solutions to classic phase-equilibrium problems.

SummaryWith advancements in deep learning (DL) technology, many scholars have applied it to bathymetry inversion, gradually revealing its potential. However, most current studies focus primarily on data-driven approaches, using various gravity data combinations for bathymetry inversion, without fully exploring the models′ capabilities or understanding the relationship between gravity and bathymetry. This study proposes a novel Attention Residual Physical Enhanced Neural Network (ARPENN), an architecture integrating attention mechanisms, residual modules, and physical constraints to help the model better understand the physical context, which enhances the utilization of shipborne data and effectively addresses the divergence issues faced by traditional algorithms in areas without shipborne measurements. The experimental results demonstrate that ARPENN achieves a root mean square (RMS) of 77.37 meters based on single-beam testing, outperforming the CNN method by 17.21 per cent and the classical Smith and Sandwell (SAS) method by 40.11 per cent. In complex regions, multi-beam evaluation shows ARPENN improves over SAS by 14.4 per cent. Further analysis reveals that the residual modules and physical constraints are identified as critical for improving accuracy, while attention mechanisms enhance robustness. ARPENN effectively reduces depth anomalies compared to GGM and SASA, achieving a reduction in anomaly rates by approximately 8.00 per cent and bringing them closer to zero. In evaluations using SIO_V25.1 as a reference, ARPENN demonstrates better stability and consistency. The ARPENN model offers promising potential for advancing global bathymetry prediction, particularly in improving depth estimation in areas surrounding continental margins.

SummaryDistributed Acoustic Sensing (DAS) systems are increasingly used for earthquake monitoring due to their cost-effectiveness and high spatial resolution. However, signals exceeding the dynamic range in DAS systems lead to signal clipping and data loss during strong ground motion and near-fault observations. In this study, we investigated the saturation effects of DAS signal clipping using two collocated DAS arrays with a looped setup in Hualien City, drawing on seismic data from the 2022 MW 7.06 Taitung earthquake sequence. The two DAS arrays, connected to different interrogators, simultaneously recorded the earthquake signals and exhibited different dynamic ranges, allowing for direct comparisons of clipped and unclipped signals. Our results indicate that the primary factors contributing to signal clipping in DAS can be categorized as (1) strong ground motion induced by earthquake magnitude and cable installations and (2) the limited dynamic range of the interrogator. Furthermore, our analysis reveals that signal clipping leads to an amplitude increase across all frequencies in the spectra, resembling the addition of a white-noise-like signal that contaminates the waveform spectra. To address this issue, we develop a frequency-based detection approach using spectral coherence estimation on collocated channels to identify clipped signals. Our findings demonstrate that coherencegrams can be employed to detect clipped signals to ensure the reliability of DAS data during strong ground motion and enhance applications that rely on near-real-time high-quality data, such as earthquake early warning systems.

SummaryThe Mediterranean and its surrounding regions are characterized by strong interactions of the Eurasian, African and Arabian Plates, as well as several microplates, resulting in significant seismic and volcanic activities. In addition, this region has a complex history of plate movements, leading to the formation of distinct orogenies such as the Alps, Apennines, and Carpathians, as well as a complex distribution of subducted slabs in the mantle. Intermediate-depth earthquakes actively occur in the subducting Vrancea slab beneath Romania. Furthermore, volcanic activity in the Caucasus and Arabian Peninsula may be stimulated by mantle plumes. To better understand the complex tectonics and seismic and volcanic activities, we need to study the detailed 3-D structure of the crust and mantle beneath this region. However, previous studies did not thoroughly investigate the 3-D structure of the whole mantle, particularly the lower mantle. Here we apply the updated global tomography method to reveal the 3-D P-wave velocity (${V}_P$) structure of the whole mantle beneath this region. We use ∼7 million P, pP, PP, PcP, and Pdiff wave arrival times of 21,629 earthquakes recorded at 14,283 seismograph stations worldwide. The resulting ${V}_P$ tomography clearly shows stagnation of the subducted African and Tethys slabs above the 660-km discontinuity, although a portion of the slab penetrates the discontinuity and sinks into the lower mantle. A big mantle wedge (BMW) has formed above the stagnant slab, which may affect the surface topography and seismic activity such as the Vrancea intermediate-depth earthquakes. A window appears between the subducting Hellenic and Cyprus slabs. Given the development of subslab hot mantle upwelling (SHMU) beneath this region, an extensive volcanic eruption in the Aegean Sea area might be powered by a mixture of island arc magma and SHMU through the slab window. Intraplate volcanoes in the Arabian Peninsula and Caucasus may be fed by hot mantle plumes rising from the core-mantle boundary.

SummaryThe Central African Plateau records multiple stages of continental extension and assembly between the Congo and Kalahari cratons in south-central Africa. Of significant interest is the formation of the Neoproterozoic Katangan Basin which was subsequently closed during the Pan-African assembly of Gondwana — a region that contains some of the world’s largest sediment-hosted copper and cobalt deposits. Whether Katangan Basin development only involved continental extension or progressed to incipient sea-floor spreading is uncertain; so too the extent to which mafic magmatism has modified bulk-crustal structure. Also debated is whether crustal re-working during overprinting by the Pan-African Orogeny to form the Lufilian Arc, was localised or broadly distributed across the entire Katangan Basin. To address these questions, we calculate crustal thickness (H) and bulk-crustal VP/VS ratio (κ) using H-κ stacking of teleseismic receiver functions recorded by seismograph networks situated across the Central African Plateau, including the new Copper Basin Exploration Science (CuBES) network. Crustal thickness is 45–48 km below the Congo Craton margin, Mesoproterozoic Irumide belt, and Domes region of the Lufilian Arc, 38–42 km below the Bangweulu Craton and 35–40 km below the Pan-African Zambezi Belt in southeastern Zambia. Bulk-crustal VP/VS is generally low (<1.76) across the majority of the Plateau, indicating a dominantly felsic bulk-crustal composition. The formation of the Katangan Basin in the Neoproterozoic is thus unlikely to have been accompanied by voluminous mafic magmatism, significant lower crustal intrusions and/or the formation of oceanic crust. The early-Paleozoic overprinting of the basin by the Pan-African Orogeny, forming the Lufilian Arc, appears to have been most intense in the Domes region, where a deep and highly variable (38–48 km) Moho topography at short length-scales (<100 km), is evident in our H-κ stacking results. In contrast, shallow and flat Moho architecture with consistently low bulk crustal VP/VS ratios, are observed further south. This flat region includes the Mwembeshi Shear Zone, which is also not associated with a VP/VS ratio contrast, suggesting the fault likely separates two very similar crustal domains.

SummaryEarthquake cycle modelling is critical to help us understand the underlying physical mechanisms of earthquake processes. However, it is a very challenging scientific problem because of the variety of spatial and temporal scales involved in fault friction behaviour. Scholars have researched this problem based on different numerical methods, but there is still an urgent need to develop more rigorous and robust numerical methods. We construct a new finite-difference operator to approximate the variable-coefficient second derivatives by combining the central-difference method with the equivalent medium parametrisation method. Using the method of manufactured solutions, we perform rigorous convergence tests, and the results show that the new finite-difference operator achieves second-order convergence. We use this new method in 2D earthquake cycle simulation and the geometric multi-grid method as an iterative solver to accelerate the computation while optimising the code on a GPU platform to improve computational efficiency further. We simulate the earthquake sequences on a vertical fault in homogeneous and heterogeneous basin models using our method and SCycle, respectively. The comparison of results shows good agreement. Our method can be utilised to study the long-term slip histories of large-scale faults in complicated mediums, as demonstrated by these results.

SummaryAs seismic imaging moves towards the imaging of more complex media, properly modelling elastic effects in the subsurface is becoming of increasing interest. In this context, elastic wave conversion, where acoustic, pressure (P-) waves are converted into elastic, shear (S-) waves, is of great importance. Accounting for these wave conversions, in the framework of forward and inverse modelling of elastic waves, is crucial to creating accurate images of the subsurface in complex media. The underlying mechanism of wave conversion is well understood and described by the Zoeppritz equations. However, as these equations are highly non-linear, approximations are commonly used. The most well-known of these approximations is Shuey’s approximation. However, this approximation only holds for small angles and small contrasts, making it insufficient for realistic forward and inverse modelling scenarios, where angles and contrasts may be large. In this paper we present a novel set of approximations, based on Taylor expansions of the Zoeppritz equations, which we name the extended Shuey approximations. We examine the quality of these approximations to the Zoeppritz equations and compare them to existing approximations described in literature. We then apply these extended Shuey approximations to the elastic Full-Wavefield Modelling algorithm for a simple, synthetic, 1.5D example, where we show that we can accurately model the P- and S-wavefields in a forward modelling case. Finally, we apply our approximations to the elastic Full-Wavefield Migration algorithm for a simple, synthetic, 1.5D example, where we show that we can recover an accurate image in an inverse modelling case.

SummaryOn 22 January 2024, a MW 7.0 earthquake struck the southern sector of the Tian Shan Mountains in Wushi County, northwestern China, causing damage and casualties. In this study, using Interferometric Synthetic Aperture Radar (InSAR) measurements (Sentinel-1 satellites), we constrained the geometry of the fault segment responsible for the seismic event, the coseismic slip distribution, and the source of the subsequent MW 5.7 aftershock deformation. Finally, we evaluated the potential state of stress of the unruptured portions of the causative fault as well as of adjacent fault segments, using the Coulomb stress failure function variations. Our findings indicate rupture along a transpressive left-lateral NNW dipping high-angle fault, associated with the Southern Tian Shan Fault (STF) alignment, likely the Maidan fault, with slip up to 3.5 m only occurring between 10 and 20 km depth. The position of the hypocenter with respect to our estimated slip distribution supports the evidence of a marked bilateral ENE-WSW rupture directivity during the mainshock. The modeling of the postseismic deformation that includes the MW 5.7 aftershock occurred on 29 January 2024, and that is located about 15 km to the south of the mainshock, indicates a main patch with up to 90 cm of slip that may have occurred on a shallow back-thrust segment, in agreement with the observed surface breaks. We propose a potential structural and/or lithological influence on the coseismic rupture extent, consistent with observations from other intracontinental earthquakes. Finally, based on the Coulomb stress distribution computation, we find that the MW 5.7 aftershock was likely triggered by the preceding mainshock and that the Wushi earthquake also increased the stress level at both terminations of the modeled fault plane, particularly along the southwestward continuation of the Maidan fault. In addition, we also find that a wide up-dip fault patch remained unruptured, and considering that these areas have been dynamically loaded it could represent potential further aseismic deformation and/or future significant ruptures, posing a continuing seismic hazard to Wushi County and surroundings areas.