Geophysical Journal International

SummaryThis study investigates the complex tectonic interactions and crustal deformation within the Weihe Basin and its surrounding regions, encompassing the northeastern Tibetan Plateau, Ordos Craton, and Qinling Orogenic Belt. By conducting a detailed analysis of GNSS data and employing a refined tectonic model, we explore relative motion patterns and fault activities in the area. Our findings highlight nuanced movement patterns, with a clockwise rotation observed in the western and central parts of basin, contrasting with an anticlockwise rotation in the eastern part. Secondary block motion decreases from west to east, with the western region showing southeastward motion and the eastern region exhibiting subtle eastward deflection. Fault activities within the Weihe Basin generally feature low slip rates, often below 1 mm/a. Intriguingly, faults in the northern basin predominantly exhibit dextral and extensional movement, while those in the southern region display sinistral and compressive movement. The Weihe fault is identified as a critical boundary between the Ordos block and the Qinling Orogenic Belt. This study offers valuable insights into the tectonic complexities of the Weihe Basin, enhancing our understanding of its kinematic behavior.

SummaryWe analyze data from 48 seismic stations located in the western part of the Makran Subduction Zone to gain a detailed knowledge of the crustal and uppermost mantle structure in that region. The Makran is a flat subduction zone with a very thick accretionary wedge. It is a major tsunami hazard of the Indian Ocean but remains one of the world's least studied subduction zones. Its structure and evolution is increasingly becoming a subject of research interest as it can help to better understand the dynamics of flat subduction zones. Our P- and S-wave receiver function analyses reveal that the Arabian oceanic plate is currently dipping north-ward beneath the onshore accretionary wedge at a very low angle of 3°. The depth of the oceanic Moho in the coastal region is ∼30 km due to the presence of ∼22-24 km of sedimentary cover. It increases to ∼60 km beneath the Jazmurian Depression and further deepens to ∼80 km beneath the Bazman and Taftan volcanoes. The change from a relatively flat to a steeper subduction occurs just south of the Qasr-e Qand thrust fault. From the combined results of the receiver function stacking and joint inversion of P-wave receiver functions and Rayleigh wave group dispersion data, we infer that the continental Moho varies within a depth range of 40 to 56 km, with the shallowest part beneath the Sistan Suture Zone and the deepest beneath the Taftan volcano. Based on shear-wave velocity models, the sedimentary cover thickness in the onshore accretionary wedge varies from Coastal Makran to 34 km in Inner Makran. The lower-than-normal mantle wedge shear-wave velocities suggest that the mantle wedge might have undergone at least 25 per cent serpentinization. From the velocity models we conclude that the crust of the Jazmurian Depression is more likely of continental origin.

AbstractA Slow-Slip Event (SSE) is a slow release of tectonic stress along a fault zone, over periods ranging from hours to months. SSEs have been recorded in most of the geodetically well-instrumented subduction zones. Although these transient events observed by geodesy are typically excluded from probabilistic seismic hazard analysis (PSHA), they might play a crucial role in the seismic cycle by reducing the seismic slip rate (slip rate discounting the aseismic process). This effective reduction implies that incorporating SSEs into PSHA may improve the reliability of hazard assessments. Costa Rica, located at the southern end of the Middle American Trench, hosts large earthquakes as well as SSEs. Shallow and deep SSEs have long been detected at the Nicoya peninsula, in northern Costa Rica, and recently, also in the southern part of the country at the Osa peninsula. In this study, we first collect geodetic and SSE observations in Costa Rica. Then, we propose a method to incorporate them into PSHA, based on identifying regions where SSEs occur, inferring slip deficits and estimating seismic slip rates in each subduction segment. Next, we analyze the implications for PSHA and its epistemic uncertainty, using these seismic slip rates, the resulting seismic moment rate budgets, and determining earthquake rates and maximum magnitudes with different approaches. Finally, we compute a countrywide PSHA following the 2022 Costa Rica Seismic Hazard Model (CRSHM 2022) but modifying the seismic source characterization using geodetic information for the regions where SSEs occur. Compared to the CRSHM 2022, this approach leads to reductions of the resulting peak ground acceleration at return period of 475 years (PGA-475) of up to ∼15 per cent in the Nicoya peninsula, but also to an increase up to ∼40 per cent in the Central Pacific region and ∼30 per cent in the Osa peninsula. Moreover, we find that, under a geodetic-based approach and disregarding SSEs, the PGA-475 would increase by up to ∼10 per cent. Our novel approach underscores the relevance of incorporating geodetic observations and particularly SSEs into PSHA, especially in subduction margins near the coast.

SummaryInfrasonic signals of interest can occur during periods with persistent, coherent, background noise, which may be natural or anthropogenic. For high signal-to-noise (SNR) ratio transient signals, an “overprinting” of the coherent background may occur, and the signal may still be detected. However, this approach fails for low SNR signals of interest, which may be obscured by coherent noise. An infrasound beamforming method based on generalized least squares (GLS) is investigated for detecting transient signals of interest in the presence of coherent and incoherent background noise. This approach relies on an estimate of the noise covariance, captured in a covariance matrix, to effectively null contributions to the array response from noisy directions of arrival. Synthetic array data is used to investigate the performance of the GLS beamformer compared to the Bartlett beamformer when coherent and incoherent backgrounds are present. Additionally, the effects of array element number and relative strength of the interfering signal on the GLS estimates is investigated. GLS empirical area under the curve estimates suggest that the beamformer can recover coherent power for a signal of interest lower in amplitude than the coherent background, but this effectiveness degrades more quickly with SNR for a four element array compared to a six or eight element infrasound array. Finally, infrasound from the Forensic Surface Experiment, a bolide signal observed at IMS array I37NO, and a volcanic signal recorded at the Alaska Volcano Observatory array ADKI are used to evaluate GLS performance on recorded data. A ten minute window was used to capture the background noise, and the coherent background signal was nulled in all three examples.

SummarySeismic data acquisition can innovatively be implemented on the surface and within underground infrastructure to illuminate subsurface targets. In the seismic data processing and imaging phases, prior subsurface information, such as approximate interface dipping angles, can enhance reflection imaging in a target-oriented manner. We leverage a unique field dataset from an unconventional seismic acquisition setup to image a volcanogenic massive sulphide (VMS) deposit at the Neves-Corvo mining site in southern Portugal. The setup involved seismic sources positioned in a tunnel at a depth of approximately 650 m, from which the wavefields were recorded by surface receivers deployed along a 2D line directly above the tunnel. The data were marred by strong noise and limited acquisition aperture due to the tunnel length, resulting in significant smearing artifacts in images generated from conventional migration techniques, which impeded a detailed delineation of the deposit. By utilizing directional information from illumination vectors, derived from the gradients of source-side and receiver-side traveltime fields, we implemented a controlled-illumination strategy within the Kirchhoff prestack depth migration workflow. This approach resulted in enhanced imaging of the targeted Lombador VMS deposit. The improved image revealed a subtle discontinuity in the Lombador reflector, indicating a possible fault, which is also present in the area. The reflection imaging results highlight the advantages of employing underground infrastructure, such as tunnels, for seismic applications in supporting detailed in-mine exploration and drilling programs for resource estimations.

SummaryInterpolating scatter data obtained from discrete observations is essential for the continuous representation of subsurface media. Traditional interpolation algorithms typically rely on weighting the relationship between interpolation points and nearby known points, which makes it more difficult to incorporate multi-source data and prior constraints as the amount of information increases. This study explores the use of deep neural networks to replace traditional interpolants, constructing a deep learning-based scatter interpolation workflow that integrates prior information through isotropic or anisotropic smoothness loss functions, addressing traditional methods’ limitations. To enhance the capability of the deep neural network for sparse scatter interpolation, we synthesized a large number of scatter-velocity model pairs to pre-train it using supervised learning. The pre-trained network is further adapted to specific interpolation tasks by physics-guided unsupervised fine-tuning to achieve stable interpolation results. Due to the flexibility of incorporating multi-source information through input or supervised loss and imposing constraints of geophysical laws through unsupervised loss, our DL-based interpolation can be easily extended to solve geophysical inversion problems that jointly fits both data and geophysical laws. Our experiments validate the effectiveness of this workflow and demonstrate its potential in multi-information-constrained geophysical scatter interpolation, which forms the basis for multi-information inversion. This work not only advances machine learning algorithms for geophysical scatter interpolation but also provides valuable insights for deep learning geophysical inversion involving multiple data sources, and physical laws.

SummaryWe revisit the budget of 20th century true polar wander (∼1°/Myr in the direction of 70°W) using a state-of-the-art adjoint-based reconstruction of mantle convective flow and predictions of ongoing glacial isostatic adjustment that adopt two independent models of Pleistocene ice history. Both calculations are based on a mantle viscosity profile that simultaneously fits a suite of data sets related to glacial isostatic adjustment (Fennoscandian Relaxation Spectrum, post-glacial decay times) and a set of present-day observations associated with mantle convection (long-wavelength gravity-anomalies, plate motions, excess ellipticity of the core-mantle boundary). Our predictions reconcile both the magnitude and direction of the observed true polar wander rate, with convection and glacial isostatic adjustment contributing signals that are 25-30% and ∼75% of the observed rate, respectively. The former assumes that large-scale seismic velocity heterogeneities are purely thermal in origin, and we argue that our estimate of the convection signal likely represents an upper bound due to the neglect of hypothesized compositional variations within the large low shear velocity provinces in the deep mantle.

AbstractThe earthquake size distribution is well described by the Gutenberg Richter Law, controlled by the b-value parameter. In recent decades, a great variety of methods for estimating the b-value have been proposed by the scientific community, despite the simplicity of this relationship. All these methods underlie the different views of individual modelers and, therefore, often generate inconsistent results. In this study, we perform a seismological experiment in which we compare different, commonly adopted, methodologies, to estimate the completeness magnitude and the b-value, for seismicity in Central Italy. The inter-method differences are on average equal to 0.4 and 0.3, for Mc and b, respectively, but reach much larger values, especially during more intense seismic activity. This shows that epistemic uncertainty in the b-value plays a more crucial role than intra-method uncertainties, opening new perspectives in the interpretation of discrepant, single studies.

SummaryGeological Carbon Storage (GCS) is one of the most viable climate-change mitigating net-negative CO2-emission technologies for large-scale CO2 sequestration. However, subsurface complexities and reservoir heterogeneity demand a systematic approach to uncertainty quantification to ensure both containment and conformance, as well as to optimize operations. As a step toward a Digital Twin for monitoring and control of underground storage, we introduce a new machine-learning-based data-assimilation framework validated on realistic numerical simulations. The proposed Digital Shadow combines Simulation-Based Inference (SBI) with a novel neural adaptation of a recently developed nonlinear ensemble filtering technique. To characterize the posterior distribution of CO2 plume states (saturation and pressure) conditioned on multimodal time-lapse data, consisting of imaged surface seismic and well-log data, a generic recursive scheme is employed, where neural networks are trained on simulated ensembles for the time-advanced state and observations. Once trained, the Digital Shadow infers the state as time-lapse field data become available. Unlike ensemble Kalman filtering, corrections to predicted states are computed via a learned nonlinear prior-to-posterior mapping that supports non-Gaussian statistics and nonlinear models for the dynamics and observations. Training and inference are facilitated by the combined use of conditional invertible neural networks and bespoke physics-based summary statistics. Starting with a probabilistic permeability model derived from a baseline seismic survey, the Digital Shadow is validated against unseen simulated ground-truth time-lapse data. Results show that injection-site-specific uncertainty in permeability can be incorporated into state uncertainty, and the highest reconstruction quality is achieved when conditioning on both seismic and wellbore data. Despite incomplete permeability knowledge, the Digital Shadow accurately tracks the subsurface state throughout a realistic CO2 injection project. This work establishes the first proof-of-concept for an uncertainty-aware, scalable Digital Shadow, laying the foundation for a Digital Twin to optimize underground storage operations.

SummaryThe Ecuadorian forearc, formed by the accretion of oceanic plateaus, island arcs, subduction of an aseismic ridge, records a history of long-lived subduction. The modern system includes subduction of the Carnegie Ridge and seamounts, young forearc coastal ranges, and translation of a forearc sliver from oblique subduction of the Nazca Plate beneath South America. The margin has experienced large megathrust earthquakes and exhibits slow-slip events and earthquake swarms. We present results from joint tomographic inversion of local earthquakes for 3D velocity structure and earthquake location. Our joint inversion uses seismic arrival-time data from local earthquakes recorded by permanent stations and dense seismic temporary networks deployed near the coast after the 2016 Mw 7.8 Pedernales megathrust rupture and across the entire northern forearc into the foothills of the Andes in 2021-2022. Our results show that seismicity distribution and megathrust rupture are controlled by inherited and modern structures in the upper plate forearc and subducting Nazca Plate. Forearc sedimentary basins observed as low-velocities (Vp < 5.8 km/s, Vs < 3.2 km/s) are dissected by forearc basement highs observed as fast velocities (Vp 6.6-7.2 km/s, Vs. 3.6-4.0 km/s). Localized deep depocenters adjacent to basement highs preserve older sedimentary sections beneath younger forearc deposits. Differences in velocity allow discrimination between oceanic plateau basement associated with the Piñón terrane beneath the forearc and accreted island arc terranes along the eastern forearc boundary with the Andes. Along the coast, basement velocities are consistent with a hydrated upper plate. We observe an apparent transient in Vp/Vs (higher to lower) in the upper plate after the 2016 megathrust rupture, representing a transient flux of fluids from the subducting slab into the upper plate triggered by the earthquake. We observe variable thickness of the subducting Nazca plate from ∼10 km north of the Carnegie Ridge reaching 20-25 km where the Carnegie Ridge subducts beneath the forearc. Lateral velocity variations in the subducting plate indicate heterogeneity along strike and dip associated with magmatic evolution of the ridge. High-velocity domains at depth correlate with seamounts and subducted relief along the Carnegie Ridge. A low-velocity zone marks the boundary between the subducting and overriding plates. The downdip termination of the Pedernales megathrust rupture coincides with structure of the Carnegie Ridge and along strike changes in the plate interface. The downdip edge of the rupture occurs where the low-velocity zone is absent, and the subducting Carnegie Ridge intersects the overlying mantle wedge. Earthquakes located with the joint inversion focus into tight clusters controlled by relief at the top of the subducting slab and basement structure in the overriding plate. Along the coast, seismicity shallows from south to north across the east-west striking Canandé Fault. South of the fault, seismicity locates predominantly within the subducting plate and plate interface. To the north, seismicity concentrates within the plate interface and upper plate. The northward shift in hypocenter depths and an offset in the eastern limit of thick subducting Nazca plate across the Canandé fault marks a significant transition in the forearc across the fault.

SummaryCharacterizing the hydraulic and geomechanical behavior of crystalline rocks is of importance for a wide range of geological and engineering applications. Geophysical methods in general and seismic techniques in particular are extensively used for these purposes due to their cost-effective and non-invasive nature. In this study, we combine legacy seismic observations to analyze the seismic attenuation and velocity characteristics in macroscopically intact regions of the granodiorite hosting the underground Grimsel Test Site in the central Swiss Alps across a wide frequency range. By focusing on data from the intact rock volumes we aim to assess the importance of viscoelastic effects in the crystalline host rock. Our results show consistent frequency-dependent characteristics of the seismic velocity and attenuation. We illustrate that it is possible to fit a microcrack-related wave-induced fluid flow (WIFF) model to the data over the entire frequency spectrum under examination extending from the Hertz to the Megahertz range. Utilizing complementary pressure-dependent ultrasonic measurements, we infer microcrack properties that validate the key parameters of the proposed WIFF model. These findings deepen our understanding of dispersion and attenuation mechanisms at the microscopic scale in crystalline environments, which is critical for a coherent analysis and integration of data from different seismic techniques as well as for the identification of dispersion and attenuation effects related to macroscale heterogeneities, such as fractures and faults.

SummaryThe South China block (SCB) formed after the collision between the Yangtze craton and the Cathaysia block along the Jiangnan orogenic belt at 980–820 Ma. Afterward, intense tectonic activity occurred in the SCB in the early Paleozoic and early Mesozoic. In the Mid–Late Jurassic and Early Cretaceous, the SCB experienced vigorous magmatic activity, which resulted in assemblage of mineralogenetic materials in the Youjiang basin (YB), Southeast coastal metallogenic belt (SCMB) and Wuyishan metallogenic belt (WMB). The mechanisms involved in the formation of various types of metallic ore deposits have attracted considerable attention. However, the crustal and uppermost mantle dynamics of the metallogenic mechanisms are still controversial. To address this issue, we conducted seismic tomography to image the velocity and azimuthal anisotropy of the crust and uppermost mantle beneath the SCB. In this study, an eikonal equation-based traveltime tomography method was used to invert a total of 143,473 high-quality P-wave first arrivals, which were obtained by manually picking the seismic waveforms of 3615 regional earthquakes recorded by 892 broadband seismic stations. After the inversion, we derived high-resolution images, in which we identified a strong low-velocity anomaly and weak azimuthal anisotropy in the uppermost mantle of the northern YB. Below the SCMB, a low-velocity body extends from the uppermost mantle to the bottom of the crust; the azimuthal anisotropy of the uppermost mantle is weak and does not exhibit a consistent fast-velocity direction (FVD). These characteristics can be attributed to the upwelling of hot materials and crustal partial melting. For both the northern YB and SCMB, the low-velocity anomaly is probably related to hot property of the uppermost mantle and weak azimuthal anisotropy may be due to the nearly vertical α-axis of olivine. These features indicate the upwelling of hot materials beneath the YB. The upwelling of hot materials carried metal elements from deep mantle to shallow crust, resulting in metal deposits. Beneath the WMB, the lower crust and uppermost mantle show high-velocity anomalies and moderate strong azimuthal anisotropy with a consistent NE–SW-oriented FVD. The high-velocity anomalies reflect cold and rigid properties of the lower crust and the uppermost mantle beneath the WMB; consistent FVD of azimuthal anisotropy may indicate ancient fossil anisotropy. These features suggest ancient continental relicts of the Cathaysia block under the WMB.

SummaryNumerical simulations of earthquakes and seismic wave propagation require accurate material models of the solid Earth. In contrast to purely elastic rheology, poroelasticity accounts for pore fluid pressure and fluid flow in porous media. Poroelastic effects can alter both the seismic wave field and the dynamic rupture characteristics of earthquakes. For example, the presence of fluids may affect cascading multi-fault ruptures, potentially leading to larger-than-expected earthquakes. However, incorporating poroelastic coupling into the elastodynamic wave equations increases the computational complexity of numerical simulations compared to elastic or viscoelastic material models, as the underlying partial differential equations become stiff. In this study, we use a Discontinuous Galerkin solver with Arbitrary High-Order DERivative time stepping (ADER-DG) of the poroelastic wave equations implemented in the open-source software SeisSol to simulate 3D complex seismic wave propagation and 3D dynamic rupture in poroelastic media. We verify our approach for double-couple point sources using independent methods including a semi-analytical solution and a finite-difference scheme and a homogeneous full-space and a poroelastic layer-over-half-space model, respectively. In a realistic carbon capture and storage (CCS) reservoir scenario at the Sleipner site in the Utsira Formation, Norway, we model 3D wave propagation through poroelastic sandstone layers separated by impermeable shale. Our results show a sudden change in the pressure field across material interfaces, which manifests as a discontinuity when viewed at the length scale of the dominant wavelengths of S- or fast P-waves. Accurately resolving the resulting steep pressure gradient dramatically increases the computational demands, requiring high-resolution modeling. We show that the Gassmann elastic equivalent model yields almost identical results to the fully poroelastic model when focusing solely on solid particle velocities. We extend this approach using suitable numerical fluxes to 3D dynamic rupture simulations in complex fault systems, presenting the first 3D scenarios that combine poroelastic media with geometrically complex, multi-fault rupture dynamics and tetrahedral meshes. Our findings reveal that, in contrast to modeling wave propagation only, poroelastic materials significantly alter rupture characteristics compared to using elastic equivalent media since the elastic equivalent fails to capture the evolution of pore pressure. Particularly in fault branching scenarios, the Biot coefficient plays a key role in either promoting or inhibiting fault activation. In some cases, ruptures are diverted to secondary faults, while in others, poroelastic effects induce rupture arrest. In a fault zone dynamic rupture model, we find poroelasticity aiding pulse-like rupture. A healing front is induced by the reduced pore pressure due to reflected waves from the boundaries of the poroelastic damage zone. Our results highlight that poroelastic effects are important for realistic simulations of seismic waves and earthquake rupture dynamics. In particular, our poroelastic simulations may offer new insights on the complexity of multi-fault rupture dynamics, fault-to-fault interaction and seismic wave propagation in realistic models of the Earth’s subsurface.

SummaryIn this work we apply a dedicated 4D full waveform inversion workflow to short offset streamer data from the Sleipner CO2 storage field in the North Sea. We consider a baseline dataset acquired in 1994 and a monitor dataset acquired in 2008. Accessing to only short offset data raises significant difficulties for full waveform inversion. In this case the penetration of diving waves, which controls the depth where quantitative updates of the velocity can be expected, do not reach the zone of interest where the CO2 is injected. For this reason, we propose to combine an efficient time-lapse full waveform inversion strategy, which we call simultaneous, with a reflection oriented full waveform inversion workflow. The latter has been introduced in the literature as a way to circumvent short-offset limitation and increase the ability of full waveform inversion to update the macro-velocity model at depth by exploiting the reflection paths, using a prior step of impedance reconstruction. We first illustrate the interest of this combined strategy on a 2D synthetic model inspired from the Sleipner area. Then we apply it to the Sleipner field data, using as baseline model the one we present in a companion paper, where our reflection oriented workflow is presented. Our combined approach yields reliable estimates of the changes due to the CO2 injection, characterized by velocity reductions of up to 400 m.s−1 and strong impedance contrasts at depths of 800-1000 m, which consistent with previous FWI studies. Furthermore, the spatial distribution of CO2 changes aligns with conventional seismic time-migration results from earlier studies, following a north-south migration trend.