Geophysical Journal International

SummaryContinental intraplate regions exhibit slow deformation yet host destructive earthquakes, posing fundamental questions about strain partitioning far from plate boundaries. Here we combine a refined GNSS velocity field and elastic block modeling to investigate the Datong basin–range system in North China, a key intraplate deformation zone at the intersection of major fault systems and Quaternary volcanism. We find that left-lateral shear (∼2 mm/yr) and NW–SE extension (∼1 mm/yr) are accommodated across multiple faults and coherent block rotations, indicating broadly distributed deformation rather than localization on a single structure. Integration with mantle tomography reveals a low-velocity anomaly beneath Datong, suggesting that upwelling-driven thermal weakening augments far-field stresses from India–Asia collision and Pacific plate rollback. This deep–shallow coupling implies non-negligible seismic hazard and highlights the role of mantle dynamics in intraplate deformation.

SummaryLaterally distributed conductive bodies and intervening resistive regions within the transient electromagnetic (TEM) sensitivity region can produce anomalously enhanced responses over resistive zones. When interpreted with standard one-dimensional (1D) inversion, this response pattern can lead to laterally misplaced conductive anomalies. We refer to this family of inversion artefacts as array effects, because they arise when a subsurface array of conductive and resistive features jointly contributes to the measured TEM response, causing 1D inversion to recover conductive anomalies at incorrect lateral positions. The underlying physical cause is the conductive–resistive alternation mechanism: spatially separated conductive regions, interleaved with resistive gaps, can jointly influence the measured response, causing 1D inversion to place conductive anomalies into, or towards, intervening resistive regions. This produces severe artefacts in inversion models and can mislead geological interpretation. Such conditions may occur in geological settings characterized by repeated lateral electrical heterogeneity, provided that the electrical contrast and spacing are appropriate. Using smoke-ring theory and 3D numerical modelling, we investigate how these effects arise and propose the Instantaneous Smoke Ring Footprint (InSR-Footprint) as a practical criterion for model design and effect identification. Finally, we present field evidence for array effects in SkyTEM data acquired over a coastal dune environment in the Netherlands.

SummarySeismic surface wave tomography uses surface wave information to obtain velocity structures in the subsurface. Due to data noise and nonlinearity of the problem, surface wave tomography often has non-unique solutions. It is therefore required to quantify uncertainty of the results in order to better interpret the resulting images. Bayesian inference is the most widely-used method for this purpose. However, the commonly-used Monte Carlo methods require huge computational cost and remains intractable in high-dimensional problems. Variational inference uses optimization to solve Bayesian inverse problems, and therefore can be more efficient in the case of large datasets and high-dimensional parameter spaces. Variational inference has been widely applied to 2-D phase velocity map inversion. In this study, we extend the method to 3-D surface wave tomography by directly inverting for 3-D spatial seismic velocity structures from frequency-dependent travel time measurements. Specifically, we apply three variational methods, mean-field automatic differential variational inference (mean-field ADVI), physically structured variational inference (PSVI) and stochastic Stein varational gradient descent (sSVGD) to surface wave tomographic problems using both synthetic data and real data in the Southwest China. The results show that all methods can provide accurate velocity mean estimates, while sSVGD produces more reasonable uncertainty estimates than mean-field ADVI and PSVI because of Gaussian assumption used in these methods. In the real data case, the variational methods provide more detailed velocity structures than those obtained using traditional linearized methods, along with reliable uncertainty estimates. We therefore conclude that variational surface wave tomography can be applied fruitfully to many realistic problems.

SummaryOn 21 May 2021, a moderate earthquake with a magnitude of M6.4 occurred in western Yunnan Province, China. Several felt earthquakes had occurred in the region in the three days preceding this event. We aimed to investigate whether there are indications of crustal fluid migration in this region and, if so, the extent to which regional seismicity might be influenced by crustal fluids. Using the epidemic-type aftershock sequence (ETAS) model, we analyzed the earthquake triggering process and the nonstationary background rate. In addition, seismicity rates were used to estimate Coulomb stress changes. Specifically, the results suggest that the background rate increased from 0.2 to 15 events per day during the Yangbi earthquake sequence, and the spatial evolution of the background rate was broadly consistent with a fluid diffusion pattern. The evaluated cumulative stress change indicates that the stress rate increased by approximately 50 per cent following the M6.4 earthquake. Spatial stress changes suggest migration and expansion of regions with increased stress. These results imply the presence of crustal fluid flow in the study area, with a tendency to diffuse southward.

SummaryDistributed Acoustic Sensing (DAS) is an emerging technology that turns existing optical-fiber cables into high-density seismic arrays, generating vast amounts of observational data in various contexts. Consequently, large-scale storage, transmission and processing of DAS data present both challenges and opportunities in seismology. In this study, we propose a data compression and seismic detection workflow based on compressive sensing (CS) and apply it to DAS data from the Taiwan Milun Fault Drilling and All-inclusive Sensing (MiDAS) project. Our algorithm achieves a compression ratio of at least 15, with reconstructed data from specific earthquakes compatible with standard seismological algorithms. Additionally, we migrate the detection algorithm to the compressed domain, enabling quasi-real-time, high-accuracy seismic detection. This approach demonstrates the feasibility of directly processing compressed data, reducing computational burdens in DAS processing. Furthermore, we discuss an empirical criterion for determining the maximum compression ratio of a given signal based on CS. Our workflow is compared with other mature compression algorithms across eight different datasets to demonstrate its advantages, limitations, and applicability. Generally, the CS algorithm is more effective for high-SNR event-oriented DAS applications rather than continuous noise-dominated monitoring. Collectively, these results highlight the potential of the CS algorithm in advancing the development of efficient, user-friendly DAS data products.

SummaryThe pore-crack network, characterized by the distribution of pore aspect ratios and porosity, controls the seismic properties of a rock. The increase in laboratory ultrasonic velocity with effective pressure reflects the closure of microcracks, thereby allowing the pore-aspect-ratio spectrum to be inverted through rock-physics modelling. Previous studies on sandstone samples have suggested that the pore-aspect-ratio spectrum (porosity distribution vs. pore-aspect-ratio distribution) follows a power-law distribution; however, its robustness across different lithologies, model dependence, and origin remain poorly understood. In this study, these questions are further investigated. We compile laboratory ultrasonic velocity-pressure data from 50 rock samples spanning a wide range of lithologies and porosities. Then we invert the power-law exponent (the slope of the power-law distribution) from these data by integrating the pore-aspect-ratio spectrum into two independent rock-physics models: the existing Kuster-Toksöz (KT) model and the multi-crack wave theory (MCWT) that incorporates the squirt-flow mechanism. The inversion results show that both models obtain power-law pore-aspect-ratio spectra across lithologies, with the MCWT showing improved modelling of the squirt-flow effect in saturated P-wave velocities, leading to smaller fitting errors. Theoretical modelling explains the differences between the two models in their fitting behavior and inverted power-law exponent values. Furthermore, we demonstrate that the power-law pore-aspect-ratio spectrum can be derived from widely observed power-law fracture length and aperture distributions of the crack system. The result also establishes a relationship between the power-law exponent and porosity that well explains the global data trend from inverting the velocity-pressure data of rock samples. The significance of the power-law exponent and the applicability of the E-porosity relationship are also discussed. Our findings support a robust power-law pore-aspect-ratio spectrum across different lithologies and highlight its simplicity and practical applicability for analyzing velocity-pressure data in cracked-porous rocks.

SummaryAccurate numerical computation of traveltimes is essential for seismic applications, including traveltime tomography and seismic imaging. Most existing numerical methods have been developed on rectangular grids, whose regular structure limits their ability to capture rugged topography and irregular subsurface interfaces. To address this limitation, we develop a fast sweeping method (FSM) combined with the discontinuous Galerkin (DG) method to efficiently calculate traveltimes on triangular meshes. In the proposed method, the FSM offers an efficient Gauss–Seidel iterative framework, whereas the DG method solves the eikonal equation with second-order accuracy. The combination of these two techniques enables efficient computation of transmitted-wave traveltimes in models with complex geometries. After computing the transmitted-wave traveltimes, those at the reflection interfaces are employed as the initial conditions to further compute the traveltimes of reflected PP and PS waves in the elastic medium. Numerical simulations confirm the validity and accuracy of the proposed method in handling models with complex geometry.

SummaryFull-waveform inversion (FWI) is a key tool for velocity model building. Multiparameter elastic FWI suffers from parameter coupling and unbalanced radiation sensitivities, which generate crosstalk and hinder convergence. The Gauss–Newton (GN) method alleviates these limitations by incorporating second-order curvature information, but its computational cost remains a limiting factor. We present an adaptive local inverse-Hessian preconditioning approach for elastic FWI that approximates the GN update without solving the global linear system. The method constructs a local inverse mapping between curvature responses and model perturbations using reference perturbations and associated curvature, which is defined as the Hessian-vector product in sense of GN. This formulation captures both diagonal and off-diagonal contributions of the inverse Hessian, providing an explicit treatment of parameter coupling. The local systems are solved by singular value decomposition with Tikhonov regularization to improve stability. Numerical experiments on synthetic models, including a crosstalk-sensitive anomaly test and the Marmousi II model, indicate that the proposed method yields update directions consistent with those of a fully converged GN solution at substantially lower cost. Compared with block-diagonal pseudo-Hessian and truncated GN implementations, the method shows reduced crosstalk, faster misfit reduction, and improved reconstruction under comparable computational constraints.

SummaryIn near-surface porous media containing aligned fractures and fluids, the propagation of Rayleigh waves is influenced by both fracture geometry and wave-induced fluid flow. These processes introduce frequency-dependent dispersion and azimuthal anisotropy in both phase velocity and attenuation. To investigate these effects, this study applies Chapman-type fracture–pore effective medium theory to describe fluid-saturated fractured media as horizontally transversely isotropic (HTI) media with frequency-dependent complex stiffness. The complex dispersion relation of Rayleigh waves is obtained by combining this model with the Stroh formalism for HTI half-spaces. Numerical results show that Rayleigh-wave phase velocity exhibits clear frequency-dependent dispersion together with azimuthal anisotropy. The fast and slow propagation directions remain consistent with the orientation of the fracture strike. Rayleigh-wave attenuation exhibits a clear relaxation peak within the frequency band associated with wave-induced fluid flow and shows strong azimuthal variation. Parameter analysis further indicates that fracture density primarily controls the strength of azimuthal anisotropy, whereas porosity mainly affects the overall level of dispersion and attenuation. The relaxation time governs the frequency range over which dispersion and attenuation become significant. The combined frequency–azimuth variations of phase velocity and attenuation therefore provide potential constraints for estimating fracture–pore structures in fractured reservoirs using multi-frequency and multi-azimuth Rayleigh-wave observations.

SummaryAutonomous underwater vehicles (AUVs) provide high-resolution magnetic measurements essential for investigating crustal accretion, tectonic segmentation, and hydrothermal processes at mid-ocean ridges. However, raw magnetometer measurements are strongly influenced by platform-induced magnetic fields, attitude-dependent biases, and navigation uncertainties that must be corrected to recover geophysically meaningful anomaly fields. We apply a procedure to measurements from AUVs which includes platform independent scalar calibration, transformation into geographic reference frames, spin-based removal of permanent and induced vehicle magnetization, and computation of magnetic anomalies relative to the International Geomagnetic Reference Field. The framework also incorporates a two-dimensional spectral upward continuation to reconcile altitude variations, thereby recovering anomalies at a uniform reference level. Application of this system to AUV datasets from the East Pacific Rise and Mid-Atlantic Ridge demonstrates effective suppression of platform biases, improved crossover consistency, and the generation of internally coherent geomagnetic anomaly fields suitable for subsequent analysis and mapping.

SummaryThe Southwest Subbasin (SWSB) of the South China Sea represents a young oceanic basin formed by continental rifting and subsequent seafloor spreading between ∼23.6 and 16 Ma. Yet the deep crustal structures of its continental margins and the oceanic crustal accretion processes remain poorly understood. This study presents integrated two-dimensional seismic and density models along the OBS2020-3 profile, which traverses the continental margins and oceanic basin, using seismic refraction, multichannel seismic reflection, and free-air gravity data. Our results reveal prominent intra-crustal reflectors at ∼11.5 km depth beneath the northwestern continental margin, which we interpret as the brittle-ductile transition marking depth-dependent extension during rifting, without any evidence of high-velocity lower crust layers. Within the oceanic basin, the thin magmatic crust overlying serpentinized mantle suggests reduced magmatic supply during its opening. Combined with published seismic profiles crossing the ridge axis to the northeast and southwest, our results reveal a systematic southwestward decrease in crustal thickness in the ridge-parallel direction, suggesting a progressive reduction in melt supply toward the southwestern termination of the SWSB. The ridge axis (Longmen Seamount) is characterized by pronounced crustal low-velocity anomalies accompanied by apparent crustal thickening. We interpret that these features resulted from enhanced porosity and mantle serpentinization, both facilitated by extensive faulting during the final, tectonically dominated spreading stage. Together, these findings provide new constraints on continental extension along the northwestern margin of the SWSB and oceanic crustal accretion within the basin, and highlight comparable fault-controlled processes in other marginal seas.

SummaryThis study presents the moment tensor catalog of 40 small-to-moderate earthquakes (MW ≥3.5) in the Garhwal-Kumaon region of the northwestern Himalayas. We use the cut-and-paste approach to estimate the moment tensor and to quantify uncertainty in solutions. Observed data from local and regional broadband networks are used to see the impact of station distribution on moment tensor solutions. A 1-dimensional local velocity model is used to compute Green’s functions for generating the synthetic waveforms. The analysis reveals that combining local and regional networks significantly reduces the moment tensor uncertainties, with the local data set generally yielding a more stable solution than the regional one. Additionally, incorporating the first-motion polarity information along with waveform data provides robust solutions. Our moment tensor catalog has 21 earthquakes with thrust mechanisms, 4 with normal mechanisms, and 15 with oblique mechanisms. The systematically observed negative time shifts across most stations suggest that the actual velocity model is faster than the one assumed. This is most likely due to the unaccounted heterogeneous structure and inaccuracy in the 1D velocity model. The findings of this study will be beneficial in the advancement of 3D seismic imaging of the Himalayas and offer significant insights into the seismotectonics of the Garhwal-Kumaon Himalayas.

SummaryAnomalous medium-scale gravity gradient changes reported prior to the 2011 $\rm M_w$ 9.0 Tohoku and the 2010 $\rm M_w$ 8.8 Maule earthquakes have been attributed to transient extensional deformations of the subducting slabs at depths of ~150–300 km. With a regional-scale extent and weak associated surface displacements, these signals and their deformation sources are not well understood. Our aim here is to improve their modelling by taking into account the 3D elastic structure of the subduction zone, and by assessing how the surface observables respond to variations in the spatial distribution and the depth of the deformation source. Taking the case of the pre-Tohoku signals, and representing slab extension with ensembles of dislocations, we investigate the corresponding gravitational and surface displacement signals using fully three-dimensional spectral-infinite-element simulations implemented in the numerical code SPECFEM-X, incorporating a realistic subduction-zone geometry. For the considered deep deformations, our results show a limited impact of the lateral elastic structure on the surface displacement and geoid signals, which differ by less than ~2% and ~3% respectively from the purely radially layered case. The degree of the spatial distribution of the deformation and the depth more strongly impact the relative amplitudes and the smoothness of the gravity and surface displacement signals. Broadly distributed slab deformation leads to a reduction in the extremum amplitude of the vertical surface displacements by up to ~40% compared to the results obtained for highly localized slab deformation of similar magnitude, whereas the amplitude of the medium-scale gravity gradient signals decreases by ≤20% only. These surface displacement and gravity gradient signals are attenuated by ~90% and ~60%, respectively, when moving the source depth from 100 to 500 km. In all cases, broader-scale gravity gradient signals are obtained. Thus, while distributed deformations contribute to smoothing the gravity signals and reducing the amplitude of the corresponding surface displacements, they still do not account for the required sub-centimetric level of ground motions. Our results finally provide a quantitative framework for interpreting intermediate-scale GRACE pre-seismic anomalies at subduction zones.

SummaryThe origins of the diverse Mesozoic magmatism and present-day intraplate seismicity in central Brazil, part of the South American plate currently undergoing compression/shortening, have been the subjects of long-standing debate. Here, we investigate the structure of the lithosphere using a combined three-dimensional (3D) electrical resistivity and seismic P-wave velocity (Vp) tomography to understand the deformation patterns and the relationship to the distribution of magmatism and seismicity in the region. Magnetotelluric and geomagnetic depth sounding data and P-wave travel times from seismological arrays were inverted for the 3D resistivity and Vp variations in the ∼1500 × 1000 km2 area of study. The results reveal a basic structure consisting of an electrically resistive and seismically fast upper crust, a low-resistivity and low-wavespeed lower crust, a resistive and fast uppermost mantle and a basal low-resistivity and low-wavespeed asthenosphere. This layering is thinned in places and dissected by steep cross-cutting NE-SW and NW-SE shear-zones of low resistivity and slow wavespeeds yielding a fragmented lithosphere. Thick blocks of high resistivity and fast wavespeed bounded by the steep shear-zones correlate spatially with zones of gravity lows across the Neoproterozoic fold belts. The steep shear-zones correlate with the Middle Proterozoic and Mesozoic mafic dyke swarms (likely emplaced during the opening of Neoproterozoic and younger proto-Atlantic oceans) and Cretaceous-Eocene alkaline and alkaline-carbonatite magmatism, indicating that inherited structures controlled the lithospheric deformation and magmatism. They also correlate spatially with zones of high seismicity, suggesting that fluid migration plays a role in generating the ongoing seismicity.

SummaryThe buried Nawa Domain of the Gawler Craton, Australia, remains poorly understood, with its deep structure and connections to adjacent terranes largely unknown. We apply a trans-dimensional Bayesian joint inversion of receiver functions and surface wave dispersion along the Marla line, a dense passive seismic array that crosses the craton margin, to image a two-dimensional shear wave velocity profile across over 200 km distance and down to 60 km depth. Our model reveals two low-velocity anomalies: within the craton, where Vs is reduced by about 6 per cent, possibly from fault-related fracturing and elevated heat flow; and at the craton margin, where a 7 per cent low-velocity anomaly spans the crust and extends into the upper mantle, likely linked to the Norwest Fault and thick sediments. Moreover, an increased Vp/Vs ratio near the Norwest Fault, generally above the profile median and locally reaching high values of about 1.80–1.85, provides evidence for mantle-derived fluid migration. The Vs model also reveals Moho shallowing by 10-15 km toward the craton margin, from about 50 km beneath the craton interior to 35-40 km beneath the basin. Edge-driven convection caused by lithospheric steps may drive mantle migration along faults and Moho uplift.

SummaryThe Turkey earthquake on February 6, 2023, Mw7.8 and Mw7.6, generated substantial concern about its effect on regional seismicity, especially in neighbouring Iran. Iran is located at the intersection of many major tectonic plates, including the Arabian, Eurasian, and Indian plates. This complex tectonic environment produces a high frequency of seismic activity, making it critical to understand how major earthquakes affect the behaviour of faults in the region. The study examines the dynamic triggering of seismicity in Iran following the Turkish earthquakes using seismic waveform and catalogue data. The bandpass filter, STA/LTA detection, β-statistics, and matched filter technique were used to analyse continuous waveform data of 99 seismic stations (IIEES, IRSC and AZAR networks) to detect small triggered seismic events. This study examines these earthquakes' effects on the Iranian plateau’s seismic patterns, including the potential for dynamic stress transfer and future seismic activity. We observed that tremors and earthquakes occurred at 14 stations. Six stations in the Persian Block, five stations in the Main Recent Fault zone, and one station each in eastern, southeast Iran, western Alborz and central Iran recorded dynamically triggered seismic activity. The triggered earthquakes were located at hypocentral distances of approximately 20 to 200 km relative to the recording stations. The triggered stations are largely concentrated around regions with high concentration of hot springs which implies that crustal fluids can raise pore pressure and contribute to the occurrence of delayed seismic events. Following the Mw7.8 mainshock, there was a rapid increase in the seismic activity and it continued for ∼5 hours before returning back to the background level. The second large event Mw7.6 did not cause any triggering. These observations imply that transitory seismic activation in Iran occurs through dynamic stress transfer. In addition, the findings suggest that subsurface fluids would have been crucial to delayed triggering events, especially in fluid-rich fault zones. This work shows the importance of remote dynamic triggering in Iran’s tectonically complicated and seismically active areas.

SummarySeismic ambient-noise horizontal-to-vertical spectral ratios (H/V, HVSR) are widely used to characterise near-surface structure and to identify site resonant frequencies. In this article, we propose a nonparametric statistical descriptor of ambient-noise HVSRs. In single-station practice, HVSR curves are typically represented by the geometric mean of window-wise ratios at each frequency, i.e., by the median of a lognormal model. However, distributions of window ensembles are frequently skewed or multimodal. The lognormal median and symmetric uncertainty bands can then depart from the most probable amplitudes and distort the spread. The lognormal mode can mitigate this bias in near-lognormal cases, but it remains tied to a unimodal parametric form. As a consequence, many workflows rely on strict window selection or rejection to justify lognormal assumption. To address this problem, we present a data-driven, nonparametric alternative that provides a marked improvement when ensembles deviate from lognormality. At each frequency, we estimate the probability density of the log-amplitude distribution using kernel density estimation (KDE), transform it to linear space, and take the mode of the linear-domain density as the representative curve. Uncertainty is quantified by highest-density intervals (HDIs), which naturally accommodate asymmetry and multi-branch distributions. Using an example of a near-lognormal microtremor, we demonstrate that the KDE mode closely tracks the lognormal mode while revealing a systematic upward shift of the commonly used lognormal median. Using a microtremor at a structurally complex site, we observe strongly multimodal amplitude and directional statistics. Both the lognormal-median and lognormal-mode curves fall between competing modes. However, the KDE-based modes and intervals follow the dominant branches and capture multimodal spread. Because the density is inferred directly from window-wise data, the method reduces the need for strong window rejection. We also compare the KDE-mode descriptor with the energy-ratio estimator which averages horizontal and vertical component energies before taking their ratio. Agreement between the energy-ratio estimator and the KDE mode is consistent with a stable single HVSR population, whereas discrepancies help identify frequency bands affected by multimodality, non-stationarity or directional effects. Finally, we extend the same framework to directional HVSR using the horizontal spectral matrix and quantify directional variability through a $\pi $-periodic circular KDE of the axial principal horizontal directions. We show that KDE-based modes, highest-density intervals and directional spread provide robust distribution-aware observational diagnostics that can guide data selection, uncertainty assignment and frequency weighting in site-characterisation and inversion workflows.

SummaryIn a growing number of studies, researchers have presented induced polarization as a dielectric-related phenomenon arising from the complex nature of the permittivity in the constitutive law associated with the displacement current. Using Ampères’s law, they then write the conductivity as a complex number with an imaginary part related to the real part of the permittivity mixing Maxwell-Wagner-Sillars polarization and induced polarization phenomena. This presentation may be misleading to students and new researchers in the field. In terms of underlying physics, induced polarization phenomena have nothing to do with the displacement current density and Maxwell-Wagner-Sillars polarization should not be considered as an induced polarization mechanism. At the microscopic level, low-frequency polarization arises because the ionic fluxes responsible for the local electrical current are coupled not only through the long-range Coulombic effect but also through their physical interactions associated with diffusion phenomena driven by the random thermal motion of charge carriers. The thermodynamic force associated with ionic migration is not the electrical field alone but electrochemical potential gradients (induction effect can be accounted for, if needed, in the electrical field component of the Nernst-Planck equation governing the fate of ions in the pore space and along the surface of the mineral grains in their so-called electrical double layer). This has been known since the seminal papers of D.J. Marshall & T.R Madden in the 50s and Vinegar & Waxman in the 80s. This point seems, however, forgotten in recent studies leading to some misconceptions preventing a mechanistic understanding of the induced polarization processes and broad-band spectroscopic data.

SummarySufficiently accurate 3-D seismic wave modelling in the presence of topography remains computationally demanding, particularly when curvilinear discretizations are applied throughout the full domain. We develop an overset Virieux-Lebedev finite-difference framework that confines the curvilinear Lebedev-grid (LG) discretization to a near-surface patch and couples it to a standard staggered-grid (SSG) scheme in the deeper region through wavefield exchange across an overlap zone. The approach aims to reproduce traction-free free-surface effects and topographic scattering while reducing the cost associated with domain-wide curvilinear LG finite-difference operators. We first verify the underlying mimetic finite-difference implementation using homogeneous half-space benchmarks with both flat and Gaussian topographic free surfaces by comparison with reference solutions. We then assess the overset coupling using a series of verification tests, including a homogeneous Cartesian benchmark,a layered model with topography and impedance-contrast interfaces, and a modified GO_3D_OBS crustal model. Across these experiments, the receiver seismograms and wavefield snapshots exhibit no discernible artefacts attributable to the overset coupling interface, and the time-frequency envelope and phase misfit measures indicate close waveform agreement. Runtime profiling further indicates substantial efficiency gains when the LG overset thickness is held fixed, and a representative GO_3D_OBS experiment achieves a marked reduction in wall-clock time relative to a full-domain LG simulation. These results demonstrate that the proposed overset strategy enables efficient and sufficiently accurate 3-D wave propagation modelling for complex topographic settings relevant to waveform-based imaging and inversion.

SummaryKnowing and monitoring the spatial distribution of ambient seismic noise sources is essential for correlation-based investigations in seismology. The omnipresent primary and secondary microseismic noise characterizes the ocean-generated noise of the seismic spectrum and has been studied through observations and modeling to better constrain the source mechanisms, the source regions, and their temporal variability. In this study, we demonstrate the potential of a single six-degree-of-freedom (6-DoF) station, observing co-located translational and rotational ground motions, to determine and continuously monitor the source directions of the secondary microseismic wavefield. We harness direct rotational ground motion data obtained with the ROMY ring laser array. The effective array-like capability of a single 6-DoF station to separate Love and Rayleigh waves, both constituents of the secondary microseismic wavefield, by their polarization enables the independent estimation of the dominant source directions and the monitoring of their seasonal evolution. An anticipated seasonality is evident, with a dominant source direction for Love (254○) and Rayleigh (277○) waves in winter, whereas in summer no clear dominant direction emerges due to an absence of strong nearby sources. We compare 6-DoF-based backazimuth estimates with those of seismic array beamforming and find close agreement concerning the dominant source directions, particularly in the case of migrating pelagic storms in the North Atlantic associated with strong ocean wave activity over several days. In winter, a systematic bias of about 25○ to 35○ for Rayleigh waves and 50○ for Love waves is identified between the 6-DoF-based backazimuth estimates and those inferred for significant wave heights obtained from satellite altimetry and model-based seismic noise source maps. Our results provide a proof-of-concept for the direction monitoring of the secondary microseismic noise for both Love and Rayleigh waves using direct rotational observations, thereby reducing reliance on array-derived estimates. The advent of portable high-sensitive rotation sensors will facilitate constraining spatial seismic noise source distributions and temporal variations therein with future 6-DoF station networks potentially replacing or complementing traditional seismic array deployments.