Geophysical Journal International

SummaryOn 16 April 2013, an MW = 7.7 earthquake struck the border of Iran and Pakistan in the central part of the Makran subduction zone with a reported depth of 80 km by USGS. This rare event in this poorly instrumented region helps to shed light on the kinematics of the subducting slab. We investigate source parameters of the Saravan intra-slab normal earthquake using RADARSAT-2 SAR images in three ascending tracks, nine permanent GNSS sites, and teleseismic data. The maximum coseismic displacement occurred at the SRVN GNSS station with 54.1 mm southeast horizontal and 42.7 mm upward vertical displacements. The coseismic ascending InSAR displacement maps illustrate a continuous and smooth NE-trending elliptical shape deformation pattern with a maximum of ∼29 cm of displacement away from the satellite. We use 25 broadband teleseismic P-waveforms to estimate the focal mechanism of the mainshock. A joint uniform inversion of InSAR, GNSS, and teleseismic data reveals a NW-dipping SW-striking fault and a primarily normal-faulting earthquake with a minor right-lateral strike-slip component. The static slip distribution of the InSAR coseismic maps localizes variable slip at depths between 50-81 km with a maximum amplitude >3 m at 60–75.5 km depth, rupturing the oceanic crust of the subducted slab. The kinematic slip distribution exhibits a well-constrained slip pattern with a nucleation depth of 65 km. The source time function indicates that the earthquake reaches its maximum moment tensor release at ∼8 s and ∼16 s. The NE-trend of the Saravan earthquake slip pattern, the orientation of the volcanic arc, and the distribution of the intra-slab intermediate-depth normal earthquakes provide new insights into slab geometry in the central Makran subduction zone. We suggest that the slab bending at the hinge of subducting Arabian plate is oblique along a NE-SW direction parallel to the volcanic arc rather than the shoreline or deformation front, and it is likely to be the reason for an oblique volcanic arc in the Makran subduction zone. These new constraints on the Makran slab geometry will help further studies in establishing realistic coupling maps for seismic hazard assessment.

SummaryTaiwan, one of the most active orogenic belts in the world, undergoes orogenic processes that can be elucidated by the doubly-vergent wedge model, explaining the extensive island-wide geological deformation. To provide a clearer depiction of its cross-island orogenic architecture, we apply ambient noise tomography across an east-west linear seismic array in central Taiwan, constructing the first high-resolution 2-D shear velocity model of the upper crust in the region. We observe robust fundamental- and higher-mode Rayleigh waves, with the latter being mainly present in the western Coastal Plain. We develop a multi-mode double beamforming method to determine local phase velocities across the array between 2 and 5-sec periods. For each location, we jointly invert all available fundamental- and higher-mode phase velocities using a Bayesian-based inversion method to obtain a 1-D model. All 1-D models are then combined to form a final 2-D model from the surface to ∼10 km depth. Our newly developed 2-D model clearly delineates major structural boundaries and fault geometries across central Taiwan, thereby corroborating the previously proposed pro-wedge and retro-wedge models while offering insight into regional seismic hazards.

SummaryAccurate absolute palaeointensity is essential for understanding dynamo processes on the Earth and other planetary bodies. Although great efforts have been made to propose techniques to obtain magnetic field strength from rock samples, such as Thellier-series methods, the amount of high-fidelity palaeointensities remains limited. One primary reason for this is the thermal alteration of samples that pervasively occurred during palaeointensity experiments. In this study, we developed a comprehensive rock magnetic experiment, termed thermal rock magnetic cycling (TRMC), that can utilize measurements of critical rock magnetic properties at elevated temperatures during multiple heating-cooling cycles to track thermal changes in bulk samples and individual magnetic components with different Curie temperatures in samples for palaeointensity interpretations. We demonstrate this method on a Galapagos lava sample, GA 84.6. The results for this specimen revealed that GA 84.6v underwent thermophysical alteration throughout the TRMC experiment, resulting in changes in its remanence carrying capacity. These findings were then used to interpret the palaeointensity results of specimen GA 84.6c, which revealed that the two-slope Arai plot yielded two linear segments with distinct palaeointensity values that were both biased by thermophysical alteration. To further test the TRMC method, we selected another historical lava sample (HS 2) from Mt. Lassen, detecting slight thermal-physical changes after heating the specimen HS 2–8C to a target temperature of 400° C. We also isolated a stable magnetic component with a Curie temperature below 400° C using the TRMC method, which may provide a more reliable palaeointensity estimate of 51 μT. By providing a method for tracking thermal alteration independent of palaeointensity experiments, the TRMC method can explore subtle, unrecognizable thermal alteration processes in less detailed palaeointensity measurements, which can help to assess the thermal stability of the measured samples and interpret the changes in the TRM unblocking spectrum and palaeointensity estimates, facilitating the acquisition of more reliable records for constrain the formation of the inner core and the evolution of Earth's magnetic field.

SummaryDynamic material constants obtained by wave-based methods are different from their static counterparts. Constraining rock's elastic constants’ dynamic-to-static ratios (Rij) are important for understanding the geomechanical properties of earth's materials, particularly in the context of hydraulic fracturing that requires the knowledge of shale's static elastic constants. Conducting experiments with dynamic and elastic constants’ anisotropy, on top of their pressure dependency, properly accounted for is challenging. Here, we measure suites of dynamic and static elastic constants, with anisotropy fully accounted for, on the shale samples extracted from the Duvernay unconventional reservoir; a comprehensive set of geochemical/petrophysical measurements are obtained too. We observe that the dynamic-to-static ratios are generally not sensitive to the increasing pressures at σ > 50 MPa; we do not find a correlation with the samples’ mineral contents either. However, we find that Rij strongly correlates to the dynamic elastic constants except for the R11. The correlation between Rij, particularly Ri3, and the dynamic elastic constants can be explained by the sedimentary rocks’ compactness and the horizontal void spaces parallel to the rock's laminated bedding planes.

SummaryWe present an approach, based on cross-coupling of quadrupole and monopole borehole acoustic modes caused by anisotropy, to investigate the in-situ stress state, a critical parameter for effective CO2 sequestration and for determining subsurface injection bounds in general. We focus on in-situ stress states where the vertical direction is a principal stress direction, and we aim at determining the minimum and maximum horizontal stresses. Because of non-linear elastic effects, three unequal principal stresses in an otherwise isotropic rock may create three orthogonal planes of symmetry, which characterize an orthorhombic elastic medium. Near a wellbore, where the stress field is perturbed, a stress sensitive material causes material axes and moduli to form a spatial distribution to which sonic logging is sensitive. We present a method for differentiating between stress induced and intrinsic anisotropy. Using finite element modelling, we demonstrate that in either case the quadrupole fundamental mode includes an axis-symmetric (monopole) component. We demonstrate that the acoustic amplitude at the borehole centre divided by the maximum acoustic amplitude at the wellbore periphery (dominated by the acoustic profile cos(2θ)) is an indicator of elastic anisotropy. We denote this ratio IA and argue that IA > 0 when the elastic anisotropy is of entirely intrinsic origin (meaning the elastic moduli are in-sensitive to stress), and further that IA increases for decreasing frequency for such cases. We demonstrate that IA attains negative values for increasing frequency in stress-sensitive formations where a cross-over (from negative to positive values) is attributed to the perturbed velocity/moduli/stress fields near the wellbore. In synthetic data, we show that the ratio IA, in combination with the phase velocity dispersion, uniquely determines the state of stress in stress-sensitive formations. In stress in-sensitive formations, we argue that IA at lower frequencies, i.e., at frequencies slightly above the cut-off frequency, is very sensitive to elastic anisotropy. We argue that in quadrupole eigenmodes, evidence of intrinsic anisotropy is present at low frequencies whereas stress induced anisotropy is better gauged at moderate to high frequencies. Finally, we discuss the practical implications of these findings.

SummaryThe analysis of seismic events recorded by NASA’s InSight seismometer remains challenging, given their commonly low magnitudes and large epicentral distances, and concurrently, strongly varying background noise. These factors collectively result in low signal-to-noise ratios (SNR) across most event recordings. We use a deep learning denoising approach to mitigate the noise contamination, aiming to enhance the data analysis and the seismic event catalogue. Our systematic tests demonstrate that denoising performs comparable to fine-tuned bandpass filtering at high SNRs, but clearly outperforms it at low SNRs with respect to accurate waveform and amplitude retrieval, as well as onset picking. We review the denoised waveform data of all 98 low frequency events in the Marsquake Service catalogue version 14, and improve their location when possible through the identification of phase picks and back azimuths, while ensuring consistency with the raw data. We demonstrate that several event waveforms can be explained by marsquake doublets - two similarly strong quakes in spatio-temporal proximity that result in overlapping waveforms at InSight - and we locate them in Cerberus Fossae. Additionally, we identify and investigate aftershocks and an event sequence consisting of numerous relatively high magnitude marsquakes occurring within hours at epicentral distances beyond Cerberus Fossae. As a result of this review and interpretation, we extend the catalogue in event numbers (+8%), in events with epicentral distances and magnitudes (+50%), and events with back azimuths and a resulting full locations (+46%), leading to a more comprehensive description of Martian seismicity.

SummarySatellite altimetry data, with its increasing density and quality, has become the primary source for marine deflection of the vertical (DOV) and gravity anomaly modeling. Limited by orbital inclinations, the precision of the meridian component of the gridded deflection of the vertical (GDOV) calculated by traditional altimetry satellites is significantly better than that of the prime vertical component, and the excessive precision difference between these two components restricts the inversion precision of marine gravity anomaly model. The study of cross-track deflection of the vertical (CTDOV) is enabled by the multi-beam synchronous observation mode of the new laser altimetry satellite, Ice, Cloud, and Land Elevation Satellite-2 (ICESat-2). Based on the remove-restore method, residual geoid gradients are first calculated in this paper using three approaches: along-track (A-T), cross-track (C-T), and an integration of along-track and cross-track. Vertical deflections are then computed on a 1'×1' grid using the least squares collocation (LSC) method, and the precision is verified against the SIO V32.1_DOV model. An optimized combination is proposed to address the issue of precision differences between the meridian and prime vertical components, and to enhance the precision of DOV inversion. A new DOV combination is formed by combining the meridian component from along-track deflection of the vertical (ATDOV) with the prime vertical component from cross-track deflection of the vertical (CTDOV) based on the remove-restore method. The Philippine Sea (0°-35°N, 120°-150°E) is selected as the test area to verify the feasibility of the optimized combination. The results indicate that the optimized combination of the meridian and prime vertical components achieved test precision of 2.63 urad and 3.33 urad, respectively, when compared against the SIO V32.1_DOV model. The precision gap between the components is effectively narrowed by this approach, which maintains the precision of the meridian component and enhances that of the prime vertical component, thereby achieving optimal inversion precision for gravity anomalies.

SummaryFundamental mode surface wave data have often been used to construct global shear velocity models of the upper mantle under the so-called “path average approximation”, an efficient approach from the computational point of view. With the advent of full-waveform inversion and numerical wavefield computations, such as afforded by the spectral element method, accounting for the effects of the crust accurately becomes challenging. Here, we assess the merits of accounting for crustal and uppermost mantle effects on surface and body waveforms using fundamental mode dispersion data and a smooth representation of the shallow structure. For this we take as reference a model obtained by full waveform inversion and wavefield computations using the spectral element method, model SEMUCB-WM1 (French and Romanowicz, 2014) and compare the waveform fits of synthetics to different parts of three component observed teleseismic records, in the period band 32-300 s for body waves and 40-300 s for surface waves and their overtones for three different models. The latter are: a dispersion-only based model (model Disp_20s_iter5), and two models modified from SEMUCB-WM1 by successively replacing the top 200 km (model Merged _200km) and top 80 km (model Merged _80km), respectively, by a model constrained solely by fundamental mode surface wave dispersion data between periods of 20 and 150 s. The crustal part of these 3 models (resp. SEMUCB-WM1) is constrained from dispersion data in the period range 20-60 s (resp. 25-60 s), using the concept of homogenization (e.g., Backus 1962, Capdeville & Marigo 2007) which is tailored to simplify complex geological features, enhancing the computational efficiency of our seismic modeling. We evaluate the fits to observed waveforms provided by these 3 models compared to those of SEMUCB-WM1 by computing three component synthetics using the spectral element method for 5 globally distributed events recorded at 200+ stations, using several measures of misfit. While fits to waveforms for model 3 are similar to those for SEMUCB-WM1, the other two models provide increasingly poorer fits as the distance travelled by the corresponding seismic wave increases and/or as it samples deeper in the mantle. In particular, models 1 and 2 are biased towards fast shear velocities, on average. Our results suggest that, given a comparable frequency band, models constructed using fundamental mode surface wave data alone and the path average approximation, fail to provide acceptable fits to the corresponding waveforms. However, the shallow part of such a 3D radially anisotropic model can be a good starting model for further full waveform inversion using numerical wavefield computations. Moreover, the shallow part of such a model, including its smooth crustal model, and down to a maximum depth that depends on the frequency band considered, can be fixed in FWI iterations for deeper structure. This can save significant computational time when higher resolution is sought in the deeper mantle. In the future, additional constraints for the construction of the homogenized model of the crust can be implemented from independent short period studies, either globally or regionally.

SummaryThe Korean infrasound catalog (KIC) covers 1999 to 2022 and characterizes a rich variety of source types as well as document the effects of the time-varying atmosphere on event detection and location across the Korean Peninsula. The KIC is produced using data from six South Korean infrasound arrays that are cooperatively operated by Southern Methodist University and Korea Institute of Geoscience and Mineral Resources. Signal detection relies on an Adaptive F-Detector (Arrowsmith et al., 2009) that estimates arrival time and backazimuth, which draws a distinction between detection and parameter estimation. Detections and associated parameters are input into a Bayesian Infrasonic Source Location procedure (Modrak et al., 2010). The resulting KIC contains 38,455 infrasound events and documents repeated events from several locations. The catalog includes many anthropogenic sources such as an industrial chemical explosion, explosions at limestone open-pit mines and quarries, North Korean underground nuclear explosions, and other atmospheric or underwater events of unknown origin. Most events in the KIC occur during working hours and days, suggesting a dominance of human-related signals. The expansion of infrasound arrays over the years in South Korea and the inclusion of data from the International Monitoring System infrasound stations in Russia and Japan increase the number of infrasound events and improve location accuracy because of the increase in azimuthal station coverage. A review of selected events and associated signals at multiple arrays provides a location quality assessment. We quantify infrasound events that have accompanying seismic arrivals (seismoacoustic events) to support the source type assessment. Ray tracing using the Ground-to-Space (G2S) atmospheric model generally predicts observed arrivals when strong stratospheric winds exist, although the predicted arrival times have significant discrepancies. In some cases, local atmospheric data better captures small-scale variations in the wind velocity of the shallow atmosphere and can improve arrival time predictions that are not well matched by the G2S model. The analysis of selected events also illustrates the importance of topographic effects on tropospheric infrasound propagation at local distances. The KIC is the first infrasound catalog compiled in this region, and it can serve as a valuable dataset in developing more robust infrasound source localization and characterization methods.

SummaryGlobally, there is now a growing evidence for a low velocity layer in the deeper parts of the upper mantle, above the 410 km discontinuity (hereafter called LVL-410). The origin of this layer is primarily attributed to interaction of slabs or plumes with a hydrous mantle transition zone (MTZ) that results in dehydration melting induced by water transport upward out of the MTZ. However, the ubiquitous nature of this layer and its causative remain contentious. In this study, we use high quality receiver functions (RFs) sampling diverse tectonic units of the Indian sub-continent to identify Ps conversions from the LVL-410. Bootstrap and differential slowness stacking of RFs migrated to depth using a 3D velocity model reveal unequivocal presence of a deep low velocity layer at depths varying from 290 to 400 km. This layer appears more pervasive and deeper beneath the Himalaya, where detached subducted slabs in the MTZ have been previously reported. Interestingly, the layer is shallower in plume affected regions like the Deccan Volcanic Province and Southern Granulite Terrane. Even though a common explanation does not appear currently feasible, our observations reaffirm deep low velocity layers in the bottom part of the upper mantle and add to the list of regions that show strong presence of such layers above the 410 km discontinuity.

SummaryWe present our third and final generation joint P and S global adjoint tomography (GLAD) model, GLAD-M35, and quantify its uncertainty based on a low-rank approximation of the inverse Hessian. Starting from our second-generation model, GLAD-M25, we added 680 new earthquakes to the database for a total of 2,160 events. New P-wave categories are included to compensate for the imbalance between P- and S-wave measurements, and we enhanced the window selection algorithm to include more major-arc phases, providing better constraints on the structure of the deep mantle and more than doubling the number of measurement windows to 40 million. Two stages of a Broyden-Fletcher-Goldfarb-Shanno (BFGS) quasi-Newton inversion were performed, each comprising five iterations. With this BFGS update history, we determine the model’s standard deviation and resolution length through randomized singular value decomposition.

SummarySeismic velocity models provide important constraints on Greenland’s deep structure, which, in turn, has profound implications for our understanding of the tectonic history of this region. However, the resolution of seismic models has been limited by a sparse network, particularly in northern and central Greenland. We address these limitations by generating new high-resolution Rayleigh-wave phase velocity maps encompassing Greenland and northeastern Canada by processing over three decades of teleseismic earthquake records and incorporating recently added stations in Greenland and Arctic Canada. These phase velocity maps are sensitive to structure from the lower crust down to the sub-lithospheric mantle (25-185 s period). We find significant heterogeneity and a strong correlation between isotropic and anisotropic seismic velocities with inferred geological structure.High seismic velocities associated with cratonic lithosphere are broadly divided into two regions, with a belt of reduced velocity spanning central Greenland, which we interpret as lithospheric erosion resulting from interaction between the Greenland continental keel and the Iceland plume. Within each region, we identify tectonic subdivisions that suggest fundamental differences between the blocks that make up Precambrian Greenland. In the south, the North Atlantic craton (NAC) has a high-velocity keel exhibiting anisotropic stratification. Between the NAC and the cratonic lithosphere further north, the Proterozoic Nagssugtoqidian orogenic belt shows a distinct signature of reduced seismic velocity to ∼75 s period, but then appears to pinch out at depth. The northern Greenland lithosphere exhibits significant isotropic heterogeneity, with a distinct core of high velocities in the northwest (∼55-75 s period) giving way to a set of distinct east-west trending high-velocity belts at longer periods. At all periods sensitive to the lithospheric mantle in this region, anisotropic fast orientations are E-W, consistent with a north-south Precambrian assembly of the Greenland shield. In contrast to the NAC, there is no evidence of anisotropic stratification in the northern part of the cratonic keel.Based on both isotropic and anisotropic phase-velocity anomalies, we suggest that the Phanerozoic Caledonian and Ellesmerian-Franklinian fold belts are relatively thin-skinned features onshore Greenland, though the Caledonian belt may have a stronger signature off the east coast. At the longest periods, a prominent low-velocity anomaly initially centred on Iceland migrates northwards and spreads beneath central-eastern Greenland. Coupled with NW-SE trending anisotropy, this feature is interpreted as the effect of mantle flow radiating outward from the Iceland plume and interacting with the eroded Greenland lithosphere.

SummaryFault rupture dynamics is expected to be significantly affected by the geometry of fault system, especially for orthogonal faults. However, the rupture behaviors of orthogonal faults especially the coseismic interactions are far from fully understood. Here, we present experimental results from a series of laboratory earthquakes to elucidate the effect of the stress state and initiation location on the rupture behaviors of orthogonal faults. Our results reveal a phase diagram of rupture behaviors of orthogonal faults, which is collectively controlled by stress state and rupture initiation location. For events initiating from the main fault, the rupture cannot jump to the branch, which may be due to the clamping effect or the inhibited shear stress accumulation on the branch. On the contrary, events initiating from the branch can persistently trigger ruptures of the main fault. This difference highlights the directional effect associated with the rupture of orthogonal faults. Further, the rupture length of triggered ruptures on the main fault is controlled by the stress state of the fault system. With the increase of the ratio between the shear stress and normal stress, the rupture length of the main fault increases. Our results reproduce the rupture behaviors of orthogonal faults, which may provide insights into the rupture characteristics of natural earthquakes.

SummaryBedrock geology from Antarctica remains largely unknown since it is hidden beneath thick ice sheets. Geophysical methods such as gravity and magnetic inverse modelling provide a framework to infer crustal rock properties indirectly in Antarctica. However, due to limited availability of rock samples, validation against direct geological information is challenging. We present a new rock property catalogue containing density and susceptibility measurements on 320 rock samples from northern Victoria Land. This catalogue is used to assess the reliability of local and regional scale inverse results, including a new local high resolution magnetic inversion in the Mesa Range region and a previously published regional scale joint inversion of gravity and magnetic data in northern Victoria Land and the Wilkes Subglacial Basin. We compare our density and susceptibility measurements to global and local measurements from the literature to access the correlation to rock types and geological units. Furthermore, the measured values are compared against inverted values. The close correspondence between inverted and measured rock properties allows us to predict locations of rock types where currently such information is missing. The utility of measured susceptibility and density relationships for interpreting inversion output provides a strong incentive to incorporate local rock samples into geophysical studies of subglacial geology across Antarctica.

SummaryThis study focuses on improving the seafloor compliance noise removal method, which relies on estimates of the compliance transfer function frequency response (the deformation of the seafloor under long-period pressure waves). We first propose a new multi-scale deviation analysis of broadband ocean bottom seismometer data to evaluate stationarity properties that are key to the subsequent analysis. We then propose a new approach to removing the compliance noise from the vertical channel data, by stacking daily estimated transfer function frequency responses over a period of time. We also propose an automated transient event detection and data selection method based on a cross-correlation criterion. As an example, we apply the method to data from the Cascadia Initiative (network 7D2011). We find that the spectral extent of long-period forcing waves varies significantly over time so that standard daily transfer function calculation techniques poorly estimate the transfer function frequency response at the lowest frequencies, resulting in poor denoising performance. The proposed method more accurately removes noise at these lower frequencies, especially where coherence is low, reducing the mean deviation of the signal in our test case by 27 % or more. We also show that our calculated transfer functions can be transfered across time periods. The method should allow better estimates of seafloor compliance and help to remove compliance noise at stations with low pressure-acceleration coherence. Our results can be reproduced using the BRUIT-FM Python toolbox, available at https://gitlab.ifremer.fr/anr-bruitfm/bruit-fm-toolbox.

SummarySeismic and tsunami hazard modelling and preparedness are challenged by uncertainties in the earthquake source process. Important parameters such as the recurrence interval of earthquakes of a given magnitude at a particular location, the probability of multi-fault rupture, earthquake clustering, rupture directivity and slip distribution are often poorly constrained. Physics-based earthquake simulators, such as RSQSim, offer a means of probing uncertainties in these parameters by generating long-term catalogues of earthquake ruptures on a system of known faults. The fault initial stress state in these simulations is typically prescribed as a single uniform value, which can promote characteristic earthquake behaviours and reduce variability in modelled events. Here, we test the role of spatial heterogeneity in the distribution of the initial stresses and frictional properties on earthquake cycle simulations. We focus on the Hikurangi-Kermadec subduction zone, which may produce Mw > 9.0 earthquakes and likely poses a major hazard and risk to Aotearoa New Zealand. We explore RSQSim simulations of Hikurangi-Kermadec subduction earthquake cycles in which we vary the rate and state coefficients (a and b). The results are compared with the magnitude-frequency distribution (MFD) of the instrumental earthquake catalogue and with empirical slip scaling laws from global earthquakes. Our results suggest stress heterogeneity produces more realistic and less characteristic synthetic catalogues, making them particularly well suited for hazard and risk assessment. We further find that the initial stress effects are dominated by the initial effective normal stresses, since the normal stresses evolve more slowly than the shear stresses. A heterogeneous stress model with a constant pore-fluid pressure ratio and a constant state coefficient (b) of 0.003 produces the best fit to MFDs and empirical scaling laws, while the model with variable frictional properties produces the best fit to earthquake depth distribution and empirical scaling laws. This model is our preferred initial stress state and frictional property settings for earthquake modelling of the Hikurangi-Kermadec subduction interface. Introducing heterogeneity of other parameters within RSQSim (e.g. friction coefficient, reference slip rate, characteristic distance, initial state variable, etc) could further improve the applicability of the synthetic earthquake catalogues to seismic hazard problems and form the focus of future research.

SummaryThe amalgamation and breakup of the West Gondwana shaped the South American platform. The dynamics during the processes can be reflected by crust anisotropy of the platform, but there are no specialized crustal anisotropic measurements yet. Splitting analysis of Moho-converted shear waves in P-wave receiver functions (Pms) can reveal crustal-scale anisotropy, which is important for understanding the dynamic evolution of the crust and for the interpretation of mantle anisotropy from splitting analysis of core–mantle refracted shear waves (XKS phases). This study measured crustal anisotropy for the old and stable South American platform by Pms splitting analysis. The splitting times vary mainly in the range of 0–0.5 s, with a regional mean of 0.2 s, slightly lower than that observed in tectonically active regions. The detected crustal anisotropy shows distinct characteristics and spatial zoning, providing insights into tectonic processes. (1) Fast polarization directions at stations close to the Transbrasiliano Lineament (TBL) are oriented NNE–SSW, generally consistent with the strike of the TBL but inconsistent with the maximum horizontal compressive stress, implying that they might be formed by dynamic metamorphism during the formation of the TBL. (2) Crustal anisotropy along the passive continental margin in the east and northeast is weak. Still, the fast polarization directions tend to be oriented along the margin, implying the existence of fossil extensional crustal fabrics formed during the continental rifting of West Gondwana. (3) The Paraná Basin, one of the world's largest Large Igneous Provinces (LIP) covered by continental flood basalts, shows distinctively strong anisotropy, with fast polarization directions highly aligned with mantle anisotropy, implying that synchronous crust–mantle deformation occurred in these regions as a result of magmatism during the breakup of West Gondwana.

SummaryGeomagnetic field data from six magnetic observatories in and adjacent to the South Atlantic Anomaly were individually analysed to detect abrupt secular variation changes occurring on time scales of less than a year and to explore any correlation with the evolution of the South Atlantic Anomaly. After applying external field corrections by means of the CHAOS-7 model, twelve-month differences of the respective observatory monthly mean of the eastward component Y revealed evidence of several geomagnetic jerks with varying amplitudes during the period 2000-2020. These observations were compared to the CHAOS-7 spherical harmonic model and previous studies of the South Atlantic Anomaly’s evolution. It emerged from this study that global field models like CHAOS are not very effective in identifying rapid localised changes in the geomagnetic field, highlighting the importance of using observatory data in conjunction with satellite data when studying geomagnetic jerks.

SummaryUtilizing over 31,000 Lg waveforms from 136 crustal earthquakes recorded at 346 regional stations, we conduct detailed tomographic mappings of the Lg Q structure across Eastern Asia in a frequency range from 0.5 to 4.0Hz. By improving the standard two-station (TS) method, we effectively correct non-unity site response ratios using site responses estimated at individual stations. This innovative approach combines the flexible recording geometry of the TS method with the precision of reversed two-station (RTS) and reversed two-event (RTE) methods, producing a comprehensive dataset devoid of source and site effects for Q tomography. To address unsolvable 3D structural effects in the Lg spectral amplitude modeling, we justify these as modeling errors with a Gaussian distribution. This approach supports our SVD-based tomographic method, allowing for effective inversion of attenuation parameters and quantitative assessment of model resolution and errors. Our results reveal a complex relationship between Lg Q and the tectonic characteristics of Eastern Asia. In well-resolved regions, low Qo (1-Hz Q) values correspond to areas with high heat flow, partial melt, thick sediment, and recent tectonic-thermal activities, in contrast to high Qo values in stable, ancient crusts lacking recent tectonic activity. Rift basins are characterized by low Lg Qo, whereas flexural basins generally have high Qo basements. We also note that post-formation factors, such as sedimentation and crustal flow intrusion, significantly impact Qo values. Furthermore, Lg Q shows a complex frequency relationship, though the power-law approximation with positive power η remains useful. The frequency dependence power η is inversely related to Qo: the regions with low Qo typically have high η and vice versa. This study provides reliable attenuation tomographic and relative site response models for Lg waves in Eastern Asia, pertinent for relative geophysical studies.

SummaryThe Epidemic Type Aftershock Sequence (ETAS) model describes how an earthquake generates its own aftershocks. The regular ETAS model assumes that distribution F of the number of direct aftershocks is Poissonian, however there is evidence suggesting that a geometric distribution might be more adequate. Let ${V}_M({m}_ \bullet )$ be the number of $m > M$ aftershocks generated by ${m}_ \bullet $event. In this study we consider the ${V}_M({m}_ \bullet )$ distribution within Epidemic-type Seismicity models, ETAS(F). These models include the Gutenberg-Richter law for magnitude and Utsu law for average ${m}_ \bullet $- productivity, but differ in the type of F distribution for the number $v({m}_ \bullet )$ of direct aftershocks. The class of F is quite broad and includes both the Poisson distribution, which is the basis for the regular ETAS model, and its possible alternative, the Geometric distribution. We replace the traditional $M = {m}_ \bullet - \Delta $ threshold in $\Delta $-analysis with $M = {m}_a - \Delta $ where ${m}_a$ is the distribution mode of the strongest aftershocks. Under these conditions we find the limit ${V}_M({m}_ \bullet )$ distribution at ${m}_ \bullet > > 1$. In the subcritical case, the limit distribution is extremely simple and identical to the $v({m}_\Delta )$ distribution with a suitable magnitude ${m}_\Delta $. This result allows us to validate both the priority of the geometric distribution of F for direct aftershocks and the very concept of epidemic-type clustering on global seismicity data.