Incorporating advanced DFT methods is essential for accurately describing compression-driven magnetic and spin transitions in materials relevant to planetary interiors. Here, we present a first-principles investigation of the pressure-induced spin crossover in face-centered cubic (fcc) FeH using density functional theory with Hubbard U corrections (DFT + U). By evaluating how the choice of U affects the relative stability, volume, and magnetic moment of high-spin (HS) and low-spin (LS) solutions, we find that FeH remains in an HS ferromagnetic state up to ∼43 GPa with U = 3.00 eV, followed by a gradual crossover to an LS nonmagnetic state described by U = 1.07 eV and completed near ∼60 GPa. The static HS–LS endmember contrast produces distinct changes in volume, elastic stiffness, compressibility, density, and acoustic velocity, with the largest response observed in C11. The calculated bulk moduli of 164.3 GPa for the HS state and 227.4 GPa for the LS state, together with an approximately 13% density increase across the crossover, highlight the geophysical significance of FeH at core pressures. Enthalpy and cohesive-energy trends further support stabilization of the compressed LS state, and the static HS–LS endmember branches show a change in the pressure dependence of compressional velocity across the inferred crossover interval. These findings underscore the critical role of electron-correlation effects in controlling the spin-state energetics and static endmember elastic response of FeH, and establish FeH as a useful analogue for assessing the behavior of hydrogen-bearing iron phases in terrestrial planetary cores.
In this study, a regional geomagnetic field model based on the Revised Spherical Cap Harmonic Analysis (R-SCHA) method was developed to investigate the detailed evolution of the magnetic field over the second minimum region of the South Atlantic Anomaly (SAA). The model was developed using satellite and ground-based measurements and evaluated against the most recent global CHAOS-8.1 model. Statistical results show that the R-SCHA model achieves better agreement with the input data than CHAOS-8.1, with root-mean-square error (RMSE) values of 2.1, 1.1, and 0.8 nT for the X, Y, and Z components, respectively, while CHAOS-8.1 yields corresponding RMSE values of 2.5, 1.8, and 1.3 nT. Gaussian fits to the residuals further confirm this improvement, with the R-SCHA model yielding mean values of −0.4, 0.3, and −0.1 nT and standard deviations of 2.0, 1.3, and 1.0 nT for X, Y, and Z, respectively. Comparatively, CHAOS-8.1 yields corresponding mean values of −0.5, 0.4, and −0.2 nT and standard deviations of 3.0, 1.9, and 2.0 nT. The R-SCHA model was then used to track the evolution of the second minimum in total magnetic field intensity from 2014.5 to 2024.5. The results reveal a complex migration pattern, involving both longitudinal and latitudinal changes and non-uniform motion after epoch 2021.0. An unexpected finding is that the fastest decrease in the total field intensity occurs at a position (4.55°E, 32.61°S) different from the current position of the second minimum (0.55°W, 40.60°S). Analysis of secular variation shows that this rapid decrease is mainly driven by the Z component, followed by the Y and X components with their contributions averaging at about 81 nT/yr, 29 nT/yr and 22 nT/yr, respectively. The extrapolation of the total field, based on estimated constant averaged secular acceleration (SA) values at both positions over the study period, indicates that the position with the fastest decrease of the field will record the lowest total field intensity in this region around 2048. The analysis of the temporal variation of the field at the first and second SAA minima using the CHAOS-8.1 model indicates positive SA and reduced rate of decrease of the field at the first minimum, while the second minimum shows a negative SA with an increasing rate of decrease of the field.
The composition and interior structure of giant planets are still a mystery. Data from the space missions Cassini and Juno suggest that the traditional three-layer model, comprising an inner ice and rock core, an intermediate metallic hydrogen dynamo region, and an outer molecular hydrogen envelope, is too simplistic. Instead, models with multiple layers and sublayers throughout the interior better match observations. These layers can be either stably stratified or chemically and/or thermally convecting. Understanding how these layers form is important for constraining and constructing accurate interior models. Here, I use direct numerical simulations (DNS) of rotating thermal convection in a cylindrical domain to mimic the outer molecular envelope in the polar region of a gas giant. Crucially, I employ a kinematic viscosity ν and thermal diffusivity κ that vary with either temperature or height, similar to their dependencies within gas giants. Thus, the Prandtl number Pr=ν/κ varies locally throughout the fluid layer, permitting oscillatory thermal-inertial convection if Pr<1. I show that distinct layers with different mixing properties appear naturally in this system. This finding challenges the common belief that pure thermal convection always produces homogeneously mixed single layers and that only double-diffusive convection is able to create sublayers by staircase formation. Instead, the results reveal that thermal convection with realistic non-constant material properties provides an alternative route to the generation of layers and sublayers inside the molecular envelope. This highlights the importance of considering different possible mechanisms to explain the distribution of heavy elements inside gas giants and to gain deeper insights into their composition.
We present an improved version of the method of Le Goff et al. (2002) for tracing directional variations of the geomagnetic field over millennia. This spherical approach calculates mean directions over time using sliding windows of variable durations, combined with a bivariate extension of Fisher statistics. Window duration is set to include a minimum weighted number (WT) of data points, and each mean direction is computed independently at a constant time step. We apply the method to the French archeomagnetic database derived from the Thellier protocol and assess the influence of key parameters, including WT, data age uncertainties, and relocation errors. Calculations can also be performed using fixed window durations while allowing the weighted number of data points within each window to vary, yielding similar results. The method is further applied to different data selections within circular regions of variable radii across Europe. We identify regions where directional secular variations are best documented, highlighting gaps in southern and eastern Europe. Comparison with the SCHA.DIF.4k regional geomagnetic field model reveals moderate differences in accuracy and variability, likely reflecting the inherently different methodological approaches. We also present three archeomagnetic dating options based on the spherical framework. In particular, we introduce a method for computing probability density functions separately for declinations and inclinations. These options yield comparable dating results and can be selected according to the application context. Combined with our method for constructing reference geomagnetic directional curves, this framework enables consistent archeomagnetic dating across Europe and allows a detailed assessment of the sensitivity of dating results to modeling choices.
In recent Martian core composition models, the sulfur content in the Martian core is estimated to 6.6-30 wt%. In the range of S-rich compositions from the eutectic Fe-S, FeS is a potential liquidus phase under the core conditions. Therefore, the melting temperature of FeS could play an important role in constraining the thermal profile and internal structure of the Martian core. In this study, the melting temperature of FeS (precisely Fe0.95-0.96S) was determined at 13.9-36.7 GPa based on in situ X-ray diffraction (XRD) measurements using a laser-heated diamond anvil cell. Melting detection was evaluated based on the sample XRD peaks with information of estimated temperature gradient in the sample and laser power-temperature relation. The melting curve of FeS was obtained to 2130-2160 K at 20 GPa (top of the core) and 2520-2600 K at 40 GPa (center of the core) using the Simon and the Kraut-Kennedy equations. Under Martian core conditions (20-40 GPa), the melting curve of FeS is almost consistent or lower than that of Fe at 20-40 GPa. In recent core thermal models, except for the highest-temperature model, the core temperature is lower than the melting curve of FeS and it is close at near the core-mantle boundary condition (similar to 20 GPa). The slope of the FeS melting curve is found to be steeper than that of the core temperature profiles. These results suggest that FeS would crystallize from the deeper part of the core during core secular cooling.
The southeastern Tibetan Plateau is a key region for understanding the eastward extrusion of Tibetan mantle material. However, the pattern and extent of asthenospheric flow remain debated. We present new shear-wave splitting (SWS) measurements of XKS phases recorded by a dense east-west broadband seismic array (similar to 15 km spacing) across the western Yangtze Block, from the Tibetan margin to the Yangtze interior. The results show predominantly east-west fast polarization directions with delay times of similar to 1.0 s, consistent with horizontal asthenospheric flow. Spatial coherence analysis places the anisotropy source at similar to 170 km depth, supporting an asthenospheric origin. Notably, fast directions rotate from NWW-SEE to NEE-SWW across the Xuefeng Mountain, indicating mantle flow deflection by lithospheric topography. Integrating our data with previous shear-wave splitting results from the southeastern Tibetan Plateau, we propose that the corner flow induced by the eastward advance of the Indian slab, together with mantle flow originating from the interior of the plateau, jointly perturbs the regional mantle flow field beneath the southeastern Tibetan Plateau. In contrast, within the western Yangtze Block, due to the lack of corner flow influence, a stable east-west asthenospheric flow prevails.
The investigation of geomagnetic variations has revealed the presence in Earth’s core of a planetary-scale, axially columnar and eccentric gyre flow. Together with the magnetic anomaly of low intensity presently seen beneath the South Atlantic, these structures show that longitudinal hemisphericity is a common feature of the geodynamo. Here, we propose that these hemispherical features result from the onset properties of spherical shell rotating convection in presence of an imposed axial magnetic field, with spatially homogeneous fixed-flux thermal boundary conditions. For an Earth-like range of background magnetic field amplitudes, we find hemispherical critical convection modes that are largely supported by a magneto-Archimedes-Coriolis (MAC) balance and where viscosity plays a secondary role. The morphology of the critical modes is in agreement with the general circulation of the gyre. Pursuing this analysis with self-sustained dynamo simulations, we find that hemispherical modes inherited from convection onset can be maintained if the MAC balance is not perturbed by inertia, the force coming at the next order in the force balance. The presence of the eccentric gyre is therefore conditional to the magnetic energy matching or exceeding the kinetic energy in the system, the so-called strong-field dynamo regime. The simulations also feature low magnetic intensity anomalies that rotate westward together with the gyre flow. We highlight a strong correlation between the gyre longitudinal position, the low intensity focus of magnetic intensity, and the eccentricity of the dynamo-generated dipole, showing that these hemispherical structures are indeed linked by the properties of magnetic induction.
We perform waveform inversion for the three-dimensional radially anisotropic S-wave velocity (VS) structure of the lowermost 400 km of the mantle (the D '' region) beneath Central America. We find high-velocity anomalies beneath Central America, interpreted as the Farallon paleo-slab, and low-velocity anomalies extending from Colombia to the Caribbean Sea, interpreted as a upwelling plume. We also infer a 3-D model of the anisotropy parameter (xi). In the regions corresponding to the Farallon paleo-slab, delta xi is positive 400-300 km above the coremantle boundary (CMB), becomes negative to the southeast 300-100 km above the CMB, and returns to positive 100-0 km above the CMB. In the regions corresponding to a passive plume, we find generally positive delta xi anomalies. These results suggest that 1) the subducted paleo-slab exhibits complex flow patterns due to interaction with the CMB and dense material just above the CMB, and 2) the root of the upwelling plume is pushed eastward by the subducted paleo-slab. The ease of moving the root of the plume might promote the Earth's thermochemical evolution.
We apply high-resolution local earthquake tomography, fault dynamics, and magnetotelluric constraints to examine the role of crustal fluids, crack density, and saturation in intraplate seismicity of the Kachchh Rift Zone, western India. Using 28,928 P-and 28,691 S-wave arrivals from 4643 aftershocks of the 2001 Bhuj earthquake sequence (2001-2016), we resolve three-dimensional Vp, Vs, and Vp/Vs structures and estimate crack density (epsilon) and saturation rate (xi) from O'Connell-Budiansky theory. Results show that Mw > 5 earthquakes, including the Bhuj mainshock, cluster where epsilon >= 0.013, xi >= 0.4, and Vp/Vs > 1.72, conditions consistent with fluid-saturated, critically stressed rock volumes. Depth zonation of fluids indicates meteoric recharge at 0-6 km, and metamorphic/aqueous fluids with mantle-derived CO2 at 6-25 km, providing a mechanism for pore pressure maintenance and fault reactivation. Interpretations for depths >25 km are presented with appropriate caution given resolution limitations. High-velocity mafic plutons (Vp 6.0-7.2 km/s, Vs 3.0-4.0 km/s) at 5-35 km act as stress amplifiers. Independent magnetotelluric imaging reveals deep conductive, fluid-rich zones coincident with the steeply dipping North Wagad Fault, supporting episodic fault-valve behavior during compressional inversion. Temporal analysis reveals fluctuations in epsilon and xi between 2001 and 2013, consistent with cyclic fluid redistribution. We interpret Mw > 5 recurrence as driven by episodic fluid overpressuring, reducing effective stress and facilitating rupture on otherwise unfavorably oriented faults. Comparison with the New Madrid, Charlevoix, and other global intraplate zones demonstrates universal fluid-fault coupling mechanisms. This integrated framework demonstrates the fundamental role of fluids in sustaining intraplate earthquake sequences in ancient rift zones, while acknowledging important methodological limitations. The approach provides a physically consistent basis for assessing seismic hazard in other intraplate regions worldwide where similar fluid-crustal interactions prevail.
The Gal & aacute;pagos archipelago is one of the regions with the highest rates of eruption and surface magma emission in the world. Knowledge about the subsurface structure is fundamental for a better understanding of the dynamics of the magmatic systems in the area. However, the details of the internal architecture beneath the archipelago remain poorly constrained at both shallow and deep levels. To shed light on these, we apply seismic interferometry by autocorrelation to records of earthquakes with epicentral distances greater than 30 degrees, obtained from 18 broadband stations deployed around two volcanoes (Cerro Azul and Sierra Negra) located in Western Gal & aacute;pagos, which is characterized by the highest volcanic activity of the archipelago. The methodology we apply results in the identification of the main mantle discontinuities at a wide range of depths with high resolution. We combine the individual results from all stations to construct a representative depth model for the entire region down to a depth of 800 km. Our results reveal coherent reflectivity patterns consistent with previously identified mantle structures and define the extent and location of the Gal & aacute;pagos mantle plume. These findings provide new insights into the mantle structure beneath the archipelago and can be used to improve our understanding of plume processes driving volcanism and its surface expression in the region.
Based on the local seismic events recorded by 29 stations of the Yunnan Earthquake Agency, this study uses the S-wave splitting analysis method to estimate the spatial distribution and temporal variation of seismic anisotropy in the upper-crust, in the region where the Yangbi MS6.4 earthquake occurred on May 21, 2021. Both deep-learning-generated catalogs and additional uncataloged events were incorporated to improve event completeness and reliability. The results reveal partitioned fast polarization directions and systematic variations in delay times. Spatially, stations near the mainshock region epicenter predominantly exhibit NS fast polarization directions, suggesting that the upper-crustal anisotropy is controlled by the regional stress field. Most stations located outside the aftershock zone show fault parallel orientations, indicating that the observed anisotropy is closely related to the fault structures. Data from stations located on the eastern side of the seismogenic fault display two fast orientations, which may reflect the different characteristics of S-wave splitting in different ray paths. The average normalized delay times of the stations in the source region are higher than those outside, suggesting substantial stress accumulation in the source region during the seismogenic process. Temporally, stations close to the source region show a marked increase in normalized delay time shortly before the mainshock, followed by a period of sustained high values and then eventually stabilising by late September 2021. This temporal pattern suggests that upper-crustal anisotropy is significantly affected by stress release and adjustments caused by mainshock and aftershock activities.
Origins of small-scale scattering heterogeneities in the lowermost mantle remain elusive. We analyze PKP precursors from earthquakes in the north of South America recorded by a linear seismic array in Northeast China to explore the detailed scattering heterogeneities in the lowermost mantle beneath northeast Asia. We find that PKP precursors present different characteristics, which implies small-scale lateral variations in scattering strengths in the lowermost mantle. Waveform modeling results further indicate that localized strong seismic scatterers mixed with weaker seismic heterogeneities can explain the observations. The complex distribution of these scatterers suggests that small-scale heterogeneities are compositionally distinct anomalies and likely linked with the subducted oceanic crust in the lower mantle. Moreover, the strong seismic scatterers with reduced P-wave velocity perturbations of similar to 6% could be ultra-low velocity zones (ULVZs) at the core-mantle boundary caused by partial melting of the piled-up ancient oceanic crust and/or core-mantle interactions.
India is the only country that has data on the past geomagnetic field (GMF) strength in the entire South Asia, therefore construction of new regional intensity secular variation curve is essential to describe the past Earth magnetic field behavior in this region. Two palaeo secular variation (PSV) curves for the last 3000 years are proposed; one for the whole Indian Subcontinent and the other for the Southern India using data respectively within 1600 km and 1000 km radius circles. The Subcontinent and the Southern India data are relocated to their geographically centered reference points (Karaundi village and Bangalore city respectively). All data were weighted based on the thermal intensity protocol, the number of samples per site and the type of studied materials. Selected data with weight (>= 6 and >= 8) were used to calculate the intensity PSV curves using the sliding window technique. The converted Virtual Axial Dipole Moment (VADM) PSV curves of both data sets are similar with respect to age, but values of the Southern India are little higher than those for the Indian Subcontinent with differences being somewhat conspicuous similar to 400 BCE and between 1100 and 1600 CE. Such difference can be due to (i) input data difference for the two curves and/or (ii) varying geomagnetic effects between the Indian Subcontinent and South India. The intensity PSV curve also indicates that the maximum PSV rate of change per century is observed between 900 CE and 1000 CE time window (similar to 8 mu T/century for 1600 km; 10 mu T/century for 1000 km radius circles). Intensity PSV predictions by global geomagnetic models (ARCH10k.1, CALS10k.2, SHAWQ2k and SHAWQ-Iron Age and ArchKalmag14k.r) for India do not reflect the regional intensity PSV pattern obtained in the current study and thus suggest that regional GMF curves are more meaningful rather than global models to describe regional/local variations. The converted VADM PSV curve trend of the Indian Subcontinent, considering the error in VADM and age, closely agree in short time spans with Central Asia (Uzbekistan), Western and Eastern Europe, and Mexico VADM PSV curve trends but shows no match at all with the VADM patterns of Eastern Asia (China, Japan and Korea), Central-Western Europe. The differences in regional VADMs behavior could be mainly due to variable non-dipolar contributions. The present results provide a good opportunity to describe some of the main features of the upper Holocene (0-3000 Yrs) GMF for the Indian Subcontinent and it needs further refinement as the present analysis is based on limited data.
Reliable reconstruction of missing or contaminated geomagnetic records is essential for maintaining continuous observatory time series. Here we benchmark three methods for reconstructing target-station variations from neighboring observatories: a genetic-algorithm-optimized back-propagation neural network (GA-BP), standard back propagation (BP), and the spherical elementary current system (SECS) method. Using multi-station data from 2013, we reconstruct the LYH record from TAN, YUL, MCH, and QIX stations under four representative scenarios (quiet days, disturbed days, magnetic storms, and geomagnetic pulsations). Performance is first assessed in the time domain using mean absolute residuals and the agreement score (AS), together with repeatedrun runtime statistics. Based on these time-domain results, GA-BP is identified as the best-performing method overall; we then conduct time-frequency spectrogram validation on the GA-BP reconstructions to examine whether the selected reconstruction preserves dominant spectral signatures. GA-BP delivers the most accurate reconstructions during disturbed conditions, with its clearest advantage during storms, while its performance is comparable to SECS for quiet days and low-amplitude pulsations. In an activity-dependent assessment, SECS shows an approximately linear increase in residuals with the Ap index, whereas GA-BP and BP exhibit a much weaker dependence. Time-frequency validation confirms that the GA-BP reconstruction retains the dominant spectral characteristics of both full-day variability and event-scale disturbances, with remaining mismatches primarily expressed as differences in background noise level and localized spectral deviations. Overall, this benchmark clarifies the activity-dependent strengths and limitations of data-driven and physics-based reconstruction strategies for regional geomagnetic records; GA-BP improves reconstruction accuracy at the cost of longer runtime due to GA-based optimization.
We construct a new global lithospheric magnetic field model using the latest datasets from the Macau Science Satellite-1 (MSS-1), complemented by measurements from the Swarm and CHAMP missions. The model is developed through a sequential inversion approach that combine satellite magnetic field observations and gradient data. After removing contributions from the core field and large-scale magnetospheric sources, the satellite data still contain residual noise arising from unmodeled external fields (e.g., Polar Electrojet Current, PEJ), orbital errors, oceanic magnetic signals, and track misfits. To isolate the lithospheric magnetic signals as accurately as possible, we first correct the satellite data for real-time tidal and ocean circulation effects during geomagnetically quiet periods. An along-track spherical harmonic high-pass filtering scheme, combined with Singular Spectrum Analysis (SSA) in polar regions, is applied to suppress high-frequency anomalies. To further enhance internal consistency, line leveling is performed between adjacent orbital tracks to minimize cross-track discrepancies. The final model parameters are estimated using Iteratively Reweighted Least Squares (IRLS) with L2 regularization. The resulting MSS-1, Swarm, and CHAMP Lithospheric (MSCL) model shows excellent agreement with existing global lithospheric field models, with correlation coefficients exceeding 0.75 for spherical harmonic degrees N <= 100. MSS-1 data effectively capture small-scale magnetic field variations at midto low-latitudes, providing a valuable complement to the current constellation of geomagnetic satellite datasets.
Internally-heated convection for a material whose viscosity depends on temperature is studied, using a large set of 2-D Cartesian numerical simulations. A Newtonian temperature-dependent viscosity is used, obeying either an Arrhenius law, with realistic parameters for an Earth-like mantle, or the so-called Frank-Kamenetskii (FK) approximation. When the viscosity depends very strongly on temperature, a rigid stagnant lid develops the surface of the model. In the absence of plastic yielding, some surface deformation is still possible when the dependence of the viscosity on the temperature is weak. The transition between these two behaviors has been extensively studied with the FK approximation, but not with the Arrhenius law. In the present study, when the latter is used, the stagnant-lid regime is very stable. Non-negligible surface motion is limited to very low values of the activation energy E alpha, well below estimates for terrestrial mantles, or to unrealistically high values of the surface temperature. A new scaling approach is proposed to predict the internal temperature below the stagnant lid, as well as the thicknesses of the rigid lid and of the active thermal boundary layer (TBL) below the lid. The predicted values, with the two viscosity formulations used here, fit the observed results of numerical simulations. It is shown that surface deformation is non-negligible when the active TBL thicker than the stagnant lid. While this criterion can be met when the FK approximation is used, even when considering otherwise realistic parameters for a terrestrial mantle, it is not observed in numerical simulations, nor predicted, when an Arrhenius viscosity with an activation energy larger than 40 kJ/mol is used.
Investigations of paleosecular variation (PSV) have utilized paleomagnetic records from geological materials. However, the detailed time-averaged field (TAF) structure is limited by the uneven spatio-temporal distribution of paleomagnetic data. These limitations hinder our ability to understand geomagnetic field evolution, especially in assessing the persistence of anomalous field features over the past few million years. We report new paleo directional records from Quaternary lava flows collected from the Tromen Volcanic System and the Zapala Volcanic Field, both located in Neuquen Province, Argentina. Rock magnetic analysis revealed that titano magnetite with variable titanium concentrations constitutes the main magnetic carrier, predominantly charac terized by vortex domain structures. From the 20 sampled sites corresponding to 19 lava flows, N =17 paleodirectional sites met the selection criteria with n >= 5 and k >= 50. After the exclusion of transitional sites using the Vandamme cutoff (53.4(degrees)), we obtain a mean direction for N =16 (D =346.9(degrees), I = 54.1 degrees and alpha(95 )= 10.1(degrees)), which is statistically indistinguishable from the expected inclination for a geocentric axial dipole (GAD) field (IGAD= 56.8(degrees))within the 95% confidence limits. The paleopole (Plat =80.9(degrees), Plon =196.5 degrees and A95 = 12.9(degrees)) coincides at the 95% confidence level with the geographic north pole. The estimated inclination anomaly (Delta I=2.7(7.4)(12.8 degrees )) is agreement with zonal TAF models with quadrupole and octupole contributions that are <5.5% of the GAD term for the 0-10 Ma interval. The VGP dispersion obtained (S-B=26.9(31.4 degrees) (22.3) ) is not statistically consistent with South American datasets at similar latitudes, nor with predictions from PSV models. These results expand the records of anomalous paleofield behavior as observed in other locations in the South America and the South Atlantic regions for the last 10 Myr.
Seismic hazard analyses in India have traditionally relied on empirical methods that overlook the complexities of the earthquake source, medium, and path of wave propagation. While source-mechanism-based models are gaining popularity in the seismic hazard modeling community, a comprehensive, well-established material model or an in-depth analysis of the Indian plate's crustal structure for accurate ground motion simulations has been lacking. Hence, ground motion prediction is always reliant on simple site proxies or one-dimensional velocity models. Thus, the present study focuses on the development of a 3D computational model of India using a spectral element framework for ground motion simulation. In the absence of 3D tomographic inversion results, we develop a geology-based 3D crustal velocity model for India, spanning from longitude 680-980 and latitude 7.50-380. The developed model accounts for the complexities of the rupture process, material property variations, basins, ridges, bathymetry, and topography. The computational model contains 4.404 million spectral elements and approximately 34.4 million nodes, with an average surface nodal spacing of 1.5 km. The model is validated both qualitatively and quantitatively (in the frequency range 0.02-0.5Hz) using recorded data from the 2001 Mw 7.6 Bhuj event, the 2014 Mw 6.1 Bay of Bengal event, and 2015 Mw 7.9 Nepal event. The calibrated model is then used to simulate ground motions for three hypothetical seismic events located around Indo-Gangetic basin. The results highlight significant spatial variability in peak ground motions and amplification patterns within the Indo-Gangetic basin, influenced by the location of the seismic event and local geological features along the wave path. These findings underscore the importance of considering detailed geological features in regional simulations, as they have a substantial impact on ground motion characteristics. Consequently, the outcomes of this study offer more reliable insights and have practical implications for seismic hazard assessment in the region.
Based on ambient noise records from two short-period dense seismic arrays deployed in the Sanshui Basin, South China, we obtained the high-resolution S-wave velocity images with lateral resolution of 0.04 degrees & times; 0.04 degrees within a 5.5 km depth range of the Sanshui Basin. Combined with spatial distributions of the Gekeng salt mine and mantle-derived hydrogen H2, the deep structure control mechanism and fundamental controls on subsurface resource formation are discussed. Our results show that the Gekeng salt mine buried at a depth of 1.2-1.5 km corresponds to a distinct low-velocity anomaly in the shallow layer (0.5-2.5 km), and that its low-density and high-porosity characteristics are consistent with the loose water-bearing property of the salt rock reservoir. Shallow earthquakes of ML4.4 and Ms3.4 that occurred in the salt mining area in 1997 and 2023, respectively, have sources located near the transition zone of high-low velocity anomalies. Based on these results, it is speculated that salt mining activities such as high-pressure water injection and mined-out area collapses may lead to the stress imbalance of secondary faults in the mining area, superimposing the main tectonic stress direction in the area, and potentially inducing medium to small magnitude earthquakes. By comparing our work with previous results about the transmission channel of the natural hydrogen H2, it is inferred that geothermal activity promotes hydrothermal fluid to rise along the fault, causing recrystallization of surrounding rock and hydrothermal diagenesis, and resulting in high-velocity anomalies of shear wave velocity in geothermal fields. This study reveals the accumulation mechanism of salt mine and natural hydrogen: salt mines are controlled by sedimentary environment and tectonic activities of fault-depression basins, while hydrogen enrichment depends on the rift-fault-hydrothermal coupling system, which provides helpful information for safe development of salt mines and carbon-free energy exploration.
To celebrate over 55 years of Physics of the Earth and Planetary Interiors providing a venue for communicating advancements in deep Earth seismology, we examine the 1D Preliminary Reference Earth Model (PREM) by Dziewonski and Anderson (1981) and the 3D P-wave tomography model from Zhao (2004) in light of the expansion of seismic data and proliferation of computational resources since their publication. PREM has been a resource for a wide range of applications, including the geodynamical and thermochemical interpretation of Earth’s mantle and core, as well as comparisons to other planetary bodies. Moreover, radially symmetric models are still commonly used as reference models in wave simulations and seismic inversions, as well as starting models in tomography. However, differences in gradients, absolute velocities, and densities in radially symmetric models have consequences when investigating mantle composition and dynamics. The Zhao 2004 P-wave tomography model demonstrated the capacity of P waves to reveal features of plumes across the mantle. While S-wave tomography models have generally reached consensus on the long-wavelength low- and high-velocity anomalies in the mantle, we observe that P-wave models exhibit a greater degree of variability, which influences interpretations based on their structure since both P- and S-wave models are necessary to constrain material properties such as temperature, composition, and mineral phase. In this review, we discuss the current status of tomography with a focus on P-wave models and potential approaches to improve future tomographic images of the mantle.
