Geophysical Journal International

SummaryLower crustal high-velocity bodies (LCHBs) are key indicators of deep magmatic addition and lithospheric modification at rifted continental margins. Integrating 3D gravity modeling with regional geophysical and geological constraints, we identify a prominent LCHB beneath the Xihu Sag of the East China Sea (ECS) shelf basin. This body is NNE–SSW elongated, ~5–7 km thick, and spatially coincides with major depocenters and fault systems. We propose a two-stage mafic emplacement model linking its formation to the tectonic transition from fore-arc compression to back-arc extension. During the early–mid Cretaceous, compressional subduction of the Paleo-Pacific Plate facilitated arc-related underplating and accumulation of mafic material in the lower crust. In the early Cenozoic, slab rollback and asthenospheric upwelling during back-arc extension renewed melt supply, further thickening the lower crust. The absence of surface volcanism indicates that magmas were largely trapped and crystallized at depth, forming dense mafic cumulates. Present-day low shallow-mantle temperatures and high densities beneath the Xihu Sag suggest that preservation of these cumulates was sustained not solely by mantle thermal conditions, but also by prolonged subsidence, sedimentary insulation, and inherited compressional structures. These results underscore the need to integrate tectonic, thermal, and structural factors to fully understand deep magmatic processes in marginal basins.

SummaryWe develop a novel comprehensive theoretical framework, the Biot-patchy-spherical-squirt (BIPSSQ) model, for wave propagation in partially saturated dual-porosity media. This model simultaneously incorporates three key fluid flow mechanisms: macroscopic flow (Biot flow or global flow), mesoscopic flow, and microscopic flow (three-dimensional spherical squirt flow). The constitutive relations and fluid pressure expressions for the BIPSSQ model are first derived and then the governing wave equations are established using a Lagrangian approach based on the system’s kinetic energy, potential energy, and dissipation functions. Through plane wave analysis, we obtain the phase velocity and attenuation of the fast P-wave. Numerical examples demonstrate that the BIPSSQ model predicts multiple dispersion transition bands and corresponding attenuation peaks, attributed to two squirt flows and two Biot global flows from two immiscible fluids. Furthermore, the presence of squirt flow significantly suppresses the mesoscopic patchy saturation effect, leading to the disappear of mesoscopic dispersion transition band and attenuation peak. The influences of permeability, saturation, porosity, squirt-flow length and inclusion radius on velocity dispersion and attenuation are also analyzed. Finally, excellent agreement between theoretical predictions and experimental measurements from an Aksu outcrop rock sample (800 kHz), a gas-water-saturated Estaillades limestone (1 kHz), and an oil-brine-saturated Vosgian sandstone (350 kHz), validates the applicability and effectiveness of the BIPSSQ model. Moreover, the BIPSSQ model can degenerate to other theories (i.e. Biot, BISSQ, BR) under certain conditions. Our proposed model provides a unified and robust tool for interpreting wave propagation phenomena in complex, partially saturated reservoir rocks.

SummaryThe maximum possible earthquake magnitude (${{M}_{MAX}}$) is a consequential parameter that is difficult to quantify. In this paper, order statistics concepts are adapted to infer ${{M}_{MAX}}$ from an earthquake catalogue. Examining jumps in the ordered sequence of largest events significantly improves inferences of ${{M}_{MAX}}$ truncation; I continue this improvement by considering deeper metrics (i.e., jumps in the second, third, fourth, … largest events). I begin by providing a theoretical foundation for these deeper metrics, while highlighting special cases. Synthetic tests are performed to quantify the improvements gained. While the largest information gains arise from the largest event sequence, appreciable gains are found to depths of ten. This approach is also validated on real-data cases, such as Groningen and FORGE, demonstrating their utility. Overall, this approach will contribute to better understanding earthquake hazards and discerning the physical processes that allow earthquakes to grow large.

SummaryThe Ojos del Salado Volcano, the highest active volcano in the world, is located at the southern end of the Puna plateau in central Chile. Here, the subduction angle of the down-going Nazca plate shallows, causing volcanism to move inland marking the southern end of the Central Andean Volcanic Zone (CAVZ). Little is known about the current volcanic activity at this southern edge or the dominant crustal stresses at these volcanic centres. In this study, we use a temporary network of 29 geophones to record local seismicity at the Ojos del Salado volcano. The type of seismic event, number of events per day, location, and magnitudes of events all provide insight into the structure, material properties, and activity level of the volcano. Between February 6th and 28th 2024, this network recorded 93 events with local magnitudes larger than 0, the largest having local magnitude 2.8. The events formed two main clusters, one on the western flank of Ojos del Salado itself near the summit, and a smaller cluster to the north. Most events in the northern cluster occurred within a 35 minute seismic swarm on February 8th. Twenty-one fault plane solutions were determined for events within the network. Six of these occurred during the northern swarm and showed steep oblique faulting and fifteen in the summit cluster, which mainly show normal faulting with strikes comparable to E-W oriented mapped faults in the area. Fault plane solutions at both clusters indicate a north-south extensional stress state. This agrees with the regional stress axes of the southern Puna plateau found in other studies, suggesting that the local crustal stresses at the Ojos del Salado volcano mainly follow the regional stresses with some variation in fault planes near the summit and in the northern swarm that could be due to locally high magmatic or geothermal fluid stresses. Heavy rain in the days preceding the northern swarm may have increased the amount of fluid available, potentially inducing the swarm on February 8th. No seismicity was observed near the Laguna Verde, or the two smaller volcanoes within the network: the Barrancas Blancas and Mulas Muertas. Ojos del Salado is therefore the main source of seismic activity and likely heat source within the study area. The level of seismicity and the occurrence of a seismic swarm to the north and five small seismic swarms near the summit suggest that there is still volcanic activity at Ojos del Salado and it could benefit from monitoring.

SummaryThe eastwards extrusion of Tibetan Plateau (TP) materials has led to complex tectonic deformation and frequent seismicity in the western Sichuan Basin (SCB). To elucidate crustal deformation mechanisms and seismogenic structures, we inverted broadband (3–60 s) Rayleigh wave dispersion curves using ambient noise tomography from 448 stations and constructed a 3-D S-wave velocity (Vs) structure for the upper to middle crust beneath the SCB and adjacent regions. Our model revealed a thick, low-velocity sedimentary layer within the SCB that extends to 15 km depth along its northwestern margin, likely resulting from the accumulation of eroded materials from surrounding orogenic belts. The three-dimensional velocity model resolved sedimentary cover thicknesses ranging from 6 to 13 km within the basin and yielded average Vs values of 3.08, 3.17, and 3.25 km/s for Mesozoic, Palaeozoic, and Proterozoic strata, respectively, thereby calibrating the basement burial depths of major geological units in the sedimentary layers. Notably, this study identified deep low-velocity anomalies beneath the Dayi seismic gap (DSG) and segmented velocity structures along the Kangding–Shimian section, providing crucial deep structural constraints for evaluating the seismogenic environment and future earthquake hazards of major seismic gaps in the Sichuan–Yunnan region. The velocity structure clearly delineates the formation and evolutionary characteristics of multiple foreland basin development episodes since the Late Triassic, offering important constraints for understanding the deep structure of the SCB, assessing seismic hazard risks, and guiding petroleum resource exploration.

SummaryHigh-resolution P-wave velocity tomography of the Japan subduction zone down to 700 km depth is determined by conducting a joint inversion of arrival-time data of local earthquakes and teleseismic events, which were recorded at land-based Hi-net seismic stations and seafloor S-net stations. Our inversion results show the high‐velocity subducting Pacific slab and low‐velocity zones in the mantle wedge beneath active arc volcanoes. Subslab low-velocity anomalies (SLVAs) are revealed in the mantle below the Pacific slab, which may reflect hot and wet mantle upwelling derived from return flow associated with the slab deep subduction. The SLVAs at depths of ~150-260 km exhibit a bimodal distribution, where interplate slow earthquakes occur. There is a SLVA gap below the mainshock hypocenter and rupture zone of the great 2011 Tohoku-oki earthquake (Mw 9.0). The SLVAs may influence the megathrust segmentation by their buoyancy, heat, and melt, and so affect the generation of megathrust and intraslab earthquakes. These results shed new light on the structural heterogeneity and mantle dynamics of the Japan subduction zone.

SummaryThe cycle-skipping problem that plagues full waveform inversion (FWI) can be at least partially mitigated if low frequencies (which encode the kinematics of wave propagation in seismic data) are recorded. However, seismic sources and receivers are band-limited, so seismic data doesn’t generally include signals down to 0 Hz. To improve our ability to solve the seismic inverse problem, one can synthesize this missing low-frequency (LF) content from the recorded high-frequency (HF) data using machine learning (ML) models. Deep learning models such as convolutional neural networks (CNNs) demonstrate impressive ability to perform low frequency extrapolation. However, such models require powerful hardware (GPU machines) and careful training. We assess the extrapolation capabilities of three different ML models that do not require GPU machines, namely, random forest, Gaussian process regression, and gradient boosting, on both synthetic and real data. Experimental results on two synthetic datasets (generated from a low velocity lens embedded in a homogeneous medium, and the Marmousi model) demonstrate that FWI applied to the extrapolated data consistently improves inversion accuracy relative to FWI applied to the original datasets that do not contain low frequencies. Application of low-frequency extrapolation to real data from the Northwest Shelf of Australia demonstrates that tree-based ML models such as gradient boosting can outperform CNNs in terms of both accuracy and computational cost on non-GPU architectures.

SummaryThis study quantifies the spatial heterogeneity of nonlinear signals, background noise, and vertical velocities in GNSS vertical time series across the Tibetan Plateau (TP), using multi-source loading corrections to isolate tectonic deformation. We analyzed 20 years of GNSS data (2002–2021) from CMONOC and NGL networks, processed via GipsyX and referenced to ITRF2014. Non-tidal atmospheric (NTAL), oceanic (NTOL), and hydrological (HYDL) loading effects were applied utilizing operational models from GFZ and GRACE mascon data (CSR/JPL/GSFC), followed by common mode error (CME) filtering. The findings highlight significant spatial heterogeneity: Monsoon-dominated southern TP exhibits 10–20 per cent RMS reduction after non-tidal atmospheric-oceanic (AO) loading corrections, while northern TP shows minimal improvement (<10 per cent), highlighting non-atmospheric noise dominance. Integration of AO and GRACE-modeled hydrological (AOG) loading corrections outperform soil moisture-based models (AOH), achieving 25–35 per cent RMS reduction in glacier-covered Himalayas by resolving cryospheric mass loss. Spectral and principal component analysis (PCA) analyses confirm AOG’s superiority in suppressing interannual signals (PC1 variance: 62.7 per cent vs. AOH’s 60.3 per cent), particularly in monsoon-ENSO-affected regions. Noise modeling demonstrates high spatiotemporal correlations (63.1 per cent WN + FN in raw data), with flicker noise (FN > 5.2 mm) linked to seismic activity in southeastern TP and power-law noise (PL > 3.5 mm) to permafrost dynamics in the north. Post-AOG_CME processing simplifies noise structures (WN + GGM dominance: 32.9 per cent), reducing velocity uncertainties by 26.9 per cent and revealing a residual + 1.2 mm/yr uplift in the southern inner TP, indicative of mid-crustal flow. Persistent uncertainties (>0.55 mm/yr) along the Himalayan thrust front correlate with deep lithospheric boundaries. Our findings demonstrate the necessity of integrating GRACE-derived corrections with CME filtering to accurately delineate tectonic signals within the intricate suture zones of the TP, offering crucial insights into plateau-wide geodynamic processes.

SummaryEarthquake ground motion is strongly influenced by near-surface geology, which governs its amplification, duration, and spatial variability. Under intense shaking and depending on the material strength, sediments often exhibit nonlinear behaviour, producing large deformations that reduce shear-wave velocity, shift resonance frequencies, and increase damping. We analyse over two decades of borehole-surface recordings from 28 stations in Iwate Prefecture, Japan, collected by the Kiban Kyoshin network (KiK-net), to quantify these effects. Frequency-domain analysis (stacked Stockwell power spectral density) and time-domain interferometric methods (multitaper deconvolution and phase cross-correlation) provide consistent results, revealing systematic decreases in both resonance frequency and seismic velocity with increasing peak ground acceleration (PGA). Frequency shifts inferred from the surface data mainly reflect the shallowest layers, whereas velocity changes estimated with borehole-referenced methods capture a path-averaged perturbation between the surface and borehole sensors and therefore depend on borehole depth. The data set is divided into seven PGA bins based on surface recordings, with the 1-5 cm s−2 bin serving as a baseline for comparison, representing linear site conditions. Across all stations, relative velocity reductions average ∼ 12 per cent in the 200-400 cm s−2 PGA range, corresponding to a shear modulus reduction (μ/μ0) of about 23 per cent. Nonlinear effects are most pronounced at sites with thicker sedimentary deposits, which are mainly found in the central valley and northern foothills of Iwate Prefecture. In contrast, VS30 shows no clear correlation with the observed nonlinearity, as its averaging effect masks thin low-velocity layers near the surface that are prone to nonlinear response during strong shaking. These results underline that nonlinear site response is highly site-specific, and that large observational data sets are crucial for robust characterisation across a seismic network.

SummaryThe Xianshuihe-Xiaojiang Fault System (XXFS), with slip rates of centimeters per year, is a major tectonic boundary accommodating southeastward extrusion of the Tibetan Plateau. Stretching over ~1,000 km through the densely populated Sichuan and Yunnan provinces in western China, it is particularly important to evaluate its potential for generating destructive earthquakes. This study systematically evaluates the XXFS within a physically grounded probabilistic framework by integrating geodetically modeled interseismic coupling, seismicity, empirical magnitude-area scaling laws, and barrier effects of creeping zones during dynamic rupture. We assess a range of rupture scenarios and obtain most probable maximum magnitudes of Mw 7.4 for the Xianshuihe fault, Mw 7.3 for the Anninghe-Zemuhe Fault, and Mw 7.2 for the Xiaojiang Fault, with corresponding fault-level recurrence of ~300, ~1 500 and ~170 years. The probabilities of occurrences of Mw 7.0 earthquake are higher along the northern and southern Xianshuihe, southern Anninghe, northern Zemuhe, and southern Xiaojiang segments. By assimilating geodetic and seismic data into a probabilistic framework that incorporates moment balance and rupture dynamics, our study provides a physics-based foundation for assessing regional seismic hazard in this tectonically active area. The approach is generalizable and can be applied to other fault systems where seismicity, basic geometry and geodetic coupling are constrained.

SummaryFrom late-December 2024 to mid-March 2025, a 50-km-long dyke intrusion triggered over 300 earthquakes (magnitude 4 to 5.9) between Fentale and Dofen volcanoes along the Northern Main Ethiopian Rift. Dyke intrusions periodically occur along the Fentale-Dofen magmatic segment and are an expression of ongoing rift extension. Preliminary analyses using interferometric synthetic aperture radar revealed extensive ground deformation (up to 60 cm), which closely matched the temporal and spatial evolution of surface manifestations and earthquake locations from global catalogs. While global catalogs are critical for real-time monitoring, the precision of locations in remote and or sparsely instrumented regions can be low. In this investigation, we present surface-wave relocation results of the dyking episode that began near Fentale volcano in December 2024. We estimate relative locations using differential travel times measured from regional-to-teleseismic distance surface-wave observations of earthquakes reported by the U.S. Geological Survey. Relative relocations reduce the initial region of diffuse seismicity to a 50-km-long narrow band bounding the strike of surface manifestations and the zone of maximum surface deformation. We demonstrate the precision of surface-wave relocations over incremental time periods, capturing the progression of dyking from seismic onset through seismic migration and caldera subsidence. Results showcase the utility of surface-wave relocations in the characterization of dyking episodes and provide complementary insights into the current understanding of the Fentale-Dofen volcanic plumbing system.

SummaryTraditional bathymetry inversion methods often fail to capture the complex nonlinear relationship between gravity data and bathymetry and lack the capability to quantify prediction uncertainty. To address these limitations, we investigated deep learning and Bayesian methods that enhance prediction accuracy and provide estimates of prediction uncertainty. Three methods—the Fully Connected Neural Network (FCNN), normalizing Flow model (Flow) and FCNN-Markov Chain Monte Carlo (FCNN-MCMC)—were developed to construct high-resolution (1′×1′) bathymetry models (FCNN, Flow, and FCNN-MCMC models) of the South China Sea (113°E-119°E, 12°N-19°N). The input data included positional, topographic, and gravity information from 4′×4′ grid points surrounding each training, validation and prediction point, while the output data corresponded to the measured bathymetry at training and validation points. At the check points, the standard deviation (STD) of the FCNN, Flow, and FCNN-MCMC models decreased by 7.13 m, 14.24 m, and 15.51 m, respectively, compared with topo_27.1, and by 18.19 m, 25.30 m, and 26.57 m, respectively, compared with ETOPO2022. The distribution of prediction uncertainties (STD) showed that over 90 per cent of the area exhibited an STD below 130 m. The prediction uncertainties exhibited a spatial distribution similar to the predicted results, with higher uncertainties mainly concentrated in shallow waters and steep areas.

SummaryHydraulic shear stimulation is a method to enhance permeability and heat extraction efficiency of geothermal systems. However, such reservoir treatments can have the risk of injection-induced seismicity. To address this issue, a novel Traffic Light System (TLS) is proposed based on the change of the seismic injection efficiency rate which is the ratio between seismic energy and hydraulic energy. In a first step, we numerically investigate the behavior of a naturally rough, slowly slipping, velocity-strengthening fracture in a granite at laboratory scale. The model is formulated on an evolution law for fractures within a rate-and-state friction framework, with fracture aperture varying as a function of both slip displacement and slip velocity. We compare the effects of the proposed Energy-based Traffic Light System (ETLS) injection protocol against those of modeled monotonic and cyclic injection, focusing particularly on aperture evolution and slip velocity. We show how implementing the ETLS criteria can reduce slip velocity by 30% compared to monotonic injection, while it was increased by 51% for cyclic injection. Even with lower slip velocity, the ETLS injection scheme sustains a similar aperture per injected volume as the monotonic scheme once larger volumes are reached. Overall, our simulations suggest that an ETLS approach could provide a safer hydraulic shear stimulation strategy for enhanced geothermal systems by minimizing slip velocity while maximizing permeability, compared to monotonic or cyclic injection.

SummaryEarth’s magnetic field has exhibited erratic polarity reversals over much of its history; however, the processes that cause polarity transitions are still poorly understood. Dipole reversals have been found in many numerical dynamo simulations and often occur close to the transition between dipole-dominated and multipolar dynamo regimes. However, the physical conditions used in reversing simulations are necessarily far from those in Earth’s liquid iron core because of the long runtimes needed to capture polarity transitions and because a systematic exploration of parameter space is needed to find the dipole-multipole transition. Here, we use the theory of distinguished limits in an attempt to simplify the search for the dipole-multipole transition at increasingly realistic physical conditions. We consider three limits that are all built from the requirements of a constant magnetic Reynolds number $\mathit {Rm}$; one limit further attempts to impose balance between Magnetic, Coriolis, and Archimedean forces (a QG-MAC balance) while the other two seek to constrain solutions to an inertia-MAC, or QG-IMAC, balance. The presence of inertia, although not geophysically realistic, allows us to build limits that more closely follow the conditions where simulated reversals have been found to date. Numerical simulations along paths in parameter space defined by these limits show some consistencies with the assumed dynamical balances within the accessible parameter space, but also important discrepancies from predicted behaviour for certain diagnostic quantities, particularly the magnetic field strength and the magnetic/kinetic energy ratio. Furthermore, the paths do not follow the dipole-multipole transition; starting from reversing conditions, simulations move into the dipolar non-reversing regime as they are advanced along the path. By increasing the Rayleigh number, a measure of the buoyancy driving convection, above the values predicted by the distinguished limit, we are able to bound the dipole-multipole transition down to an Ekman number $\mathit {E}\sim 10^{-6}$, comparable to the most extreme conditions reported to date. Our results, therefore, demonstrate that using distinguished limits is an efficient method for seeking the dipole-multipole transition in rapidly rotating dynamos. However, the conditions under which we bound the dipole-multipole transition become increasingly hard to access numerically and also increasingly unrealistic because $\mathit {Rm}$ rises beyond plausible bounds inferred from geophysical observations. Future work combining the theory of distinguished limits with variations in the core buoyancy distribution, as suggested by recent studies, appears a promising approach to accessing the dipole-multiple transition at extreme physical conditions.

SummaryLouis Néel’s theory of magnetism, which describes a rock’s magnetization as being carried by “ideal” uniformly magnetized or single domain (SD) particles, has been a cornerstone of paleomagnetic studies for over seven decades and has enabled paleomagnetists to make plate tectonic reconstructions, date geological and archaeological materials, and recover the history of Earth’s magnetic field. Unfortunately, many geological samples produce experimental results that disagree with the predictions made by Néel theory. This “non-ideal” behavior is often associated with larger particles that are non-uniformly magnetized. In this paper, we use simulations based on Néel theory to demonstrate that non-ideal behavior is also expected in assemblages of SD particles with a range of sizes and shapes previously assumed to be ideal. This effect occurs over a relatively small range of shapes and sizes for magnetite, but a much wider range for titanomagnetites. Our results call into question the typical interpretation of SD particles as ideal magnetic carriers. Instead, we suggest that the geological stability of a rock’s magnetization is influenced not only by the domain state of the magnetic carriers, but also by their shape and composition. This has important implications for paleointensities (and cooling rate corrections thereof), paleodirections, and the dating of viscous magnetizations.

SummaryThis paper presents a novel thermodynamically consistent constitutive model for partially saturated porous rocks across a wide range of conditions. The material states generated behind the shock wave from an explosive source can vary significantly, ranging from crushed and melted rock near the source to a poroelastic medium in the far field. In the model, rock strength is determined by the effective pressure, which is calculated using two independent equations of state: one for the solid rock and another for the pore fluid. The model accounts for shock-induced liquefaction resulting from fluid pressure buildup in the pore spaces near the explosive source. Simultaneously, it describes the increase in wave propagation speed due to elastic pore contraction in both dry and partially saturated rocks. This model is applied to investigate how fluid saturation affects the amplitude and shape of the generated waves, as well as the residual stress surrounding the cavity formed by spherical explosions.

SummaryThe Mendocino Triple Junction (MTJ), where the Gorda, North American, and Pacific plates meet, is one of the most seismically active regions in California. The tectonic movements along the Mendocino transform fault zone (MTFZ), Gorda slab (GS), and northern San Andreas Fault systems (NSAF) lead to high background seismicity rates but relatively low aftershock productivity. To improve the understanding of earthquake processes in the area, we analyze relations between background seismicity, aftershock productivity, and stress parameters. We apply the nearest-neighbor approach to investigate the spatial distributions and properties of background and clustered seismicity, and invert focal mechanisms of events in Voronoi cells for features of the deviatoric stress field. The results indicate that the intensity of background seismicity and aftershock productivity decrease with distance from the MTJ, defined here for simplicity as the hypocenter of the 1992 Mw7.2 mainshock. We also find that the stress regime is the most compressive in the area directly surrounding the MTJ. In the MTFZ and GS, the compressive stress decreases with increasing distance from the MTJ, correlating with the reduced aftershock productivity and background seismicity. In the NSAF, the observed relations between the stress, aftershock productivity, and background seismicity are not clear, possibly due to crustal extension related to the slab window and elevated heat flow. Compared to the MTFZ and GS, the NSAF has a higher foreshock proportion, lower aftershock proportion, and small-to-medium mainshock magnitudes, indicating more swarm-like clusters in this region. The inverted stress regimes in the MTFZ and NSAF are dominated by strike-slip faulting. The GS exhibits mostly strike-slip and normal mechanisms despite the subduction environment, which may reflect slab bending and reactivation of preexisting normal faults.

SummaryUnderstanding when and where strong earthquakes occur is crucial for assessing seismic hazard. Changes in the b-value, which describes how frequently earthquakes of different sizes happen, have been investigated as possible indicators in the space-time vicinity of upcoming large earthquakes. In this study, we investigate short-term b-value variations in the central-northern Apennines (Italy) by comparing an observed earthquake catalogue spanning the period 1987–2025 with a 10 000-year synthetic catalogue generated by a physics-based earthquake simulator. The synthetic seismicity is produced using a three-dimensional seismotectonic model derived from the DISS database and an elastic-rebound framework for earthquake nucleation. We apply a stacking procedure to compute average b-values within symmetrical time windows of ± 15 days and 30 km distance from selected pivot events of moderate to large magnitude. The same methodology is consistently applied to both observed and simulated datasets, enabling a direct comparison of their temporal behaviour. The observed catalogue shows a statistically significant decrease in the b-value in the days preceding earthquakes with Mw ≥ 4.0, followed by a post-event recovery. A comparable pattern is reproduced by the synthetic catalogue, where a pronounced b-value drop precedes pivot events of Mw ≥ 4.5 and is systematically followed by an increase after rupture. The persistence of this behaviour across different magnitude thresholds in the simulated data supports its robustness. These results indicate that physics-based simulations can reproduce short-term b-value variations associated with earthquake nucleation, supporting the relevance of this parameter for investigating the physical processes governing seismicity.

SummaryBasaltic flows and sills of the Central Atlantic Magmatic Province (CAMP) along the eastern North American seaboard have been proposed to be present in buried Mesozoic basins. Their offshore distribution is poorly constrained, yet the strong magnetic and gravity signature produced by basaltic rocks means it should be possible to map them using magnetic and gravity surveys. We conducted forward modeling using existing magnetic and gravity data to identify Mesozoic basins and basaltic units offshore. Onshore and offshore basins containing CAMP basalts in forward models generally predict the best fit with observed magnetic and gravity data. A positive magnetic anomaly over the New York Bight Basin suggests it may contain multiple basalt flows at depths > 2500 m, and scenario testing indicates the Long Island Basin possibly hosts at least one flow. The newly identified Central Bight Basin is unlikely to contain basaltic units, although the adjacent East Coast Magnetic Anomaly may be overwhelming potential basalt signatures within the basin. Deeper basement structures and/or possible interbasinal basalt likely influence existing data, therefore higher-resolution aeromagnetic and marine gravity surveys are needed to constrain CAMP basalt presence in offshore basins.

SummaryGeocenter motion, defined as the displacement of Earth’s center of mass relative to its center of figure, is crucial for maintaining the International Terrestrial Reference Frame origin and quantifying large-scale mass redistribution. However, whether observing geocenter motion by tracking satellite orbits or inferring it using geophysical models, accurately acquiring such subtle motions imposes stringent requirements on the consistency and precision of both tracking data and geophysical models. This study improves geocenter motion estimates derived from the combination of GRACE/GRACE-FO time-variable gravity (TVG) and Ocean Bottom Pressure (OBP) models (the GRACE-OBP method) in two ways. First, we apply a forward modelling technique to mitigate land–ocean leakage in GRACE/GRACE-FO TVG fields, which demonstrably outperforms empirical coastline buffer-zone corrections in controlled simulation experiments. Second, we introduce the Bayesian Three-Cornered Hat (BTCH) method to optimally combine geocenter series derived from multiple GRACE solutions and two independent OBP models (ECCO2 and MPIOM), producing an improved geocenter product without requiring a ground-truth reference. Uncertainty analysis shows that the noise level is governed primarily by the GRACE solution, and that BTCH provides a clearer advantage over equal-weighted averaging when the number of input series is limited, reducing the noise level by about 30 per cent. After restoring atmospheric and oceanic contributions, our improved geocenter series shows good agreement with the CSR SLR-derived geocenter product. Although uncertainty levels vary among individual solutions, the estimated annual and secular trend signals are broadly consistent and show limited sensitivity to the choice of GRACE TVG solution and OBP model. Using the improved geocenter series, we revisit the annual geocenter oscillation and its drivers; the results indicate that cryospheric mass variability and land-ocean mass exchange (i.e. sea-level fingerprints) provide non-negligible contributions to the annual geocenter cycle and improve consistency with observations. Finally, the improved geocenter series yields the lowest uncertainty in degree-1 mass variations, with a global RMS of 0.55 mm. Incorporating these degree-1 terms into mass budget assessments yields secular trends of 38.8 Gt/yr for the Antarctic Ice Sheet and 0.57 mm/yr for global mean ocean mass, highlighting the need for accurate geocenter corrections to support reliable long-term climate monitoring.