Geophysical Journal International

SummaryNortheast China, with its complicated regional tectonic evolution, situated within the eastern Central Asian Orogenic Belt, is a key region for understanding lithospheric deformation and mantle dynamics. However, the ongoing debate surrounding its lithospheric structure and evolutionary processes remains, largely attributed to data limitations and methodological constraints. In this study, we integrate topography, geoid height, surface heat flow, and Rayleigh wave phase velocity dispersion curves to conduct a detailed imaging of the lithospheric thermal and compositional structure in Northeast China. We find a significant east-west gradient in lithospheric thickness, ranging from approximately 60 km in the east to 140 km in the west, and a compositional transition in the lithospheric mantle from fertile peridotite in the east to refractory peridotite in the west. By integrating analyses of upper mantle anisotropy and the spatiotemporal distribution of Mesozoic basalts, we argue that the lithospheric delamination and mantle upwelling may have combined to cause the lithospheric thinning in the region. This study highlights the significance of joint inversion of multiple datasets and integrated multidisciplinary analysis.

SummaryTraditional methods for measuring geopotential difference using optical fiber frequency transfer or satellite-based time-frequency transfer, based on general relativity, require the use of two clocks and the calibration of these clocks. Here we present a simplified clock transportation experiment using a single hydrogen clock to measure the geopotential difference between two time-frequency stations, separated by 129 km with a height difference of 1,245 m, by GNSS precise point positioning time-frequency transfer. Taking the reference clock of International GNSS Service (IGS) time as a ‘bridge’, we extract the gravity frequency shift between the two stations by comparing the fractional frequency differences between the hydrogen clock and the ‘bridge’ before and after clock transportation. The determined geopotential difference between the two stations is 12075.9$\pm $118.5 m²/s², which closely aligns with the value computed by the EIGEN-6C4 global gravity field model, with a difference of -78.7 m²/s². These results validate the feasibility of geopotential difference measurements with a single clock and highlight several advantages compared to the dual-clock method: elimination of inter-clock calibration, low operational complexity and equipment cost, high data utilization efficiency but similar precision of geopotential difference measurement. Furthermore, this method can be extended to other similar techniques to measure geopotential differences, provided that they enable users to connect to a stable time-frequency reference.

SummaryShale gas extraction could produce underground stress perturbation and local seismicity, which could put a threat human casualty. The Weiyuan area in the Sichuan province, China, underwent massive gas production and a significant increase of earthquake since 2015. In this study, we focus on human-induced subsurface hydrofracturing, calculate cumulative underground Coulomb-stress changes using a 3D numerical model, and probe the main cause of recent seismic activity in the Weiyuan area based on continuous regional stress/displacement loading. The simulation reveals the regional extent of positive Coulomb stress change with fractures matches the distribution of the moderate and micro seismicity in the past ten years. Background regional tectonic stress in the vicinity of the active fault likely resulted in earthquake preparation within and around the active faults; hydraulic fracturing changes mainly the displacement and stress pattern in the vicinity of the fracturing wells, and enhanced fracturing intensity (fracture volume-to-model volume ratio (θ), causes more obvious difference; faults may be locked prior to fracturing, and even small fracturing intensity may trigger the earthquakes near faults and fracturing wells; the seismic risk will be significantly increased near the two faults and fracturing wells in the next 50 years.

SummaryIn this study, we compare the usability of a simplified microtremor-based empirical method and a conventional microtremor method based on an inversion analysis of a subsurface velocity structure model for constructing a map of average S-wave velocity (AVS) values. In the simplified (empirical) method, the phase velocities of Rayleigh waves, which can be obtained by processing a microtremor array, at wavelengths of 13, 25, and 40 m are regarded as AVS values from the ground surface to depths of 10, 20, and 30 m (${\overline {Vs} }_{10},{\overline {Vs} }_{20},\ {\rm{and}}\ {\overline {Vs} }_{30}$), respectively. Microtremor array surveys were conducted at 173 observation points within a 15 km × 17 km area east of Aso caldera, Kyushu, Japan (target area). AVS values are obtained by applying the empirical method to the phase velocities obtained at each observation point. The AVS values at an observation point (located near the centre of the target area) with velocity logging data are verified by a comparison with those based on the velocity logging data (i.e. overestimations by 6 per cent at maximum). It is found that for the entire target area, the spatial distribution of the obtained AVS values is consistent with the geological distribution. The AVS values within areas of the Aso-3 ignimbrite are 30–40 per cent larger than those within areas of thick soil and tephra on the strongly consolidated Aso-4 ignimbrite. In addition, the AVS values of the Aso-3 deposits are more than 10 per cent larger than those of the Aso-4 deposits and about 10 per cent smaller than those of geological units older than the Aso-3 deposits. We also apply a conventional (i.e. inversion) method to the phase velocity data at each observation point to obtain a one-dimensional S-wave velocity (Vs) structure model from which we deduce AVS values. The deduced AVS values at the velocity logging point are underestimated by -8 per cent, with differences from the AVS values obtained using the empirical method reaching 13 per cent. The average systematic difference between the two methods is 15 per cent, as determined from a statistical analysis. None the less, a strong correlation is found between the methods, with an average correlation coefficient of 0.94, with no evidence showing that either method is more accurate. The empirical method can be used to construct an AVS map if overestimation is carefully considered. This analysis also reveals that the average maximum survey depths of the one-dimensional Vs structures based on the inversion method are only 23±10 m, making them often insufficient to map ${\overline {Vs} }_{20}$ and ${\overline {Vs} }_{30}$ (the ratios of the available to total numbers of data points are only 60 and 21 per cent, respectively). In contrast, the empirical method can determine ${\overline {Vs} }_{10},{\overline {Vs} }_{20},\ \ {\rm{and}}\ {\overline {Vs} }_{30}$ at more than 80 per cent of all sites. The construction of AVS maps using the empirical method is effective in terms of the simplicity and reliability of planning, observational efficiency, and simplicity of data processing, which support a practical and objective approach to seismic assessments.

SummaryIn this study we obtain 35 903 high-quality P-wave receiver functions from 1737 teleseismic events recorded at 120 dense broadband TanluArray temporary stations deployed in and around the Tanlu fault zone (TLFZ). After station azimuth and sediment correction are made, a detailed Moho depth distribution is obtained by CCP stacking. Our results show a sharp change in the Moho depth across the TLFZ from the west to east, which well corresponds to the surface geological structure. The deepest Moho (38.0 ∼ 40.0 km) occurs beneath the Dabie orogenic belt and the Sulu orogenic belt. The Moho beneath the Luxi uplift, Jiangnan orogenic belt and Jiaodong uplift is deeper (36.0 ∼ 37.0 km), whereas the Subei basin and the southern basin of the South Yellow Sea have a shallow Moho (28.0 ∼ 30.0 km). There is an obvious Moho uplift near Weifang, which corresponds to the Changle ancient volcano on the surface and may be a channel for upwelling of hot mantle material. The Moho is unclear under the fault zone near Tancheng, which is speculated to be a channel for upwelling of hot mantle material. It may be related to upwelling of hot and wet flows in the big mantle wedge above the subducted Pacific slab that is stagnant in the mantle transition zone beneath East Asia, which is a possible cause of the 1668 M8.5 Tancheng earthquake.

SummaryThe Limpopo transform margin offshore southern Mozambique results from the separation of Gondwana along the East Africa continental margin. Over the last three decades, more than thirty different reconstruction models have been proposed, sometimes contradicting each other. Here, we present results from the travel-time tomography of wide-angle seismic data acquired during the second China-Mozambique Joint Cruise, allowing the interpretation of the crustal structure and magmatism in the Limpopo Corridor and the Mozambique Basin. Using these results, we determine the extent of the Continent Ocean Transition and the location of the Continent Ocean Boundary on the southern Mozambique margin. The seismic profile is 442-km long, extending from the eastern part of the North Natal Valley in the west and crossing the Limpopo Corridor and the Mozambique Basin to the east. Based on the tomographic velocity model, we delineated three distinct domains from west to east along the profile: (1) a western transitional domain with anomalous or mixed crust, bounded by the Mozambique Fracture Zone to the east, where the crust gradually thins eastward from ∼14 km at distance 45 km to ∼10.8 km at distance 140 km; (2) a domain of thickened oceanic crust resulting from enhanced magmatism, where the crust thins eastward, from ∼10.8 km to ∼8.5 km over ∼100 km distance; and (3) an eastern domain of normal oceanic crust, where the average crustal thickness is ∼8 km. We suggest that (1) the western transitional domain roughly corresponds to the Limpopo Corridor and is of continental crustal origin but was affected and modified by strike-slip motion and magmatic activity, resulting in anomalous or mixed crust. The eastern Continent Ocean Boundary of the Limpopo Margin is close to the Mozambique Fracture Zone; (2) The thickened oceanic domain thins eastward, and the crustal velocity and thickness change dramatically compared to the oceanic domain. This domain seems to have strongly interacted and contaminated by the Limpopo Corridor during the opening of Mozambique Basin and seafloor spreading; (3) The eastern oceanic domain shows a relatively uniform oceanic crust of ∼8 km and high velocity up to 7.4 km/s in the lower crust, suggestive of a hotter mantle that produces more MgO-rich melts probably due to the influence of a thermal mantle anomaly.

SummaryThe downward continuation of gravity field can provide valuable information for 3-D gravity-field modeling, shallow-layer geological interpretation, source depth estimation, and so on. However, downward continuation is ill-posed, and traditional approaches often suffer from computational instability, poor noise resistance, and limited continuation depth, making it a longstanding challenge in gravity data processing. We present a new approach for fast, stable and large-depth downward continuation of gravity anomalies by using frequency-domain 3-D imaging. First, we utilize the frequency-domain 3-D imaging approach to invert the gravity anomalies at the original observational plane to quickly obtain the equivalent density model in the subsurface. Then, we apply the optimized strategy of frequency-domain 3-D forward calculation on the equivalent density model to rapidly obtain high-precision gravity anomalies at the downward-continuation plane. The synthetic data tests prove the effectiveness of our approach, and demonstrate that our approach enables fast, stable, robust noise resistance and large-depth downward continuation of large-scale gravity anomalies data, and has superior performance compared to the traditional regularized filtering approach and spatial-domain equivalent-source approach. The real data test of the free-air gravity anomalies data in the central South China Sea also verifies the fast, stable and reliable downward continuations of large depths by our approach. The 3-D gravity-field model built by our approach will provide significant support for the tectonic studies and resource exploration in this area.

SummaryModelling and inversion of controlled-source electromagnetic data requires elaborate numerical tools. The major challenge is the high computational cost of computing solutions to numerous forward problems (for the forward responses as well as the sensitivity matrix). Forward modelling is accomplished using either a direct or an iterative solver. Current modelling suites predominantly employ direct solution methods in the forward operator since multiple solutions are easily accessible using inexpensive and quick forward-backward substitution after an initial resource-demanding matrix factorisation step. Iterative techniques, on the other hand, require little resources for single forward solutions, and are yet very time consuming if solutions for many right-hand sides are to be computed. Evaluations of different solution techniques for modelling and inverse problems are only sparsely investigated. In light of this, we integrated an iterative solver as alternative in the forward and and inversion operators of the open-source software custEM and pyGIMLi. In particular, we implemented a two-level iterative scheme where the outer solver employs a generalised conjugate residual algorithm preconditioned with a highly efficient block-based preconditioner for square blocks. The inner-level solver is either of the same type as the outer solver, but preconditioned with the auxiliary-space Maxwell preconditioner, or may alternatively be a direct solver. In this paper, we evaluate the described iterative forward operator for forward modelling tasks for the Marlim R3D model for a single as well as numerous right-hand side vectors and compare the performance to the direct solver MUMPS. We further investigate the solver’s applicability on small and medium-sized computing platforms. We then examine the iterative solver for inversions of synthetic land-based and semi-airborne data in terms of computational requirements. Our results demonstrate that forward modelling tasks are best performed using an iterative approach for single source problems. Moreover, simulations of large and complex problems are accessible on even on small computing platforms such as laptops in very reasonable time. For inversions, the iterative forward operator, in particular the mixed iterative-direct-based one, performs equally well in terms of time as the direct one while reducing the memory demands for the computations of the forward responses and the data sensitivities.

SummaryIn a region of complex geology, we examine the influence of spatial resolution of conductivity models on Geomagnetically Induced Currents (GICs) estimations. We focus on the southern region of Portugal mainland, for which magnetotelluric (MT) sounding measurements have been obtained with lower noise from human activity. Using two conductivity models inverted from sets of MT soundings with different sampling distance, we look for an interpretation of the differences in GIC estimations at substation grounding resistances. We make use of two different proxies, the Local Effective Field (LEF) and the Regional Electromotive Source (RES), built from the electric induced field at each substation site and the sum of electromotive forces along all transmission lines connected to that substation, respectively. We compare different time signals associated to GICs using a parameter that combines Pearson correlation and linear regression slope, the Correlation Regression Coefficient (CRC). Our main conclusion is that spatially detailed information on lateral heterogeneities of the conductivity associated to complex geology is crucial for a rigorous assessment of GIC hazard, leading to relative differences in GIC standard deviation and in GIC peak values that can amount to more than 100% in certain cases. Additionally, using LEF and RES, we emphasise the non-locality of GIC drivers and bring new input concerning the choice of proxies used to monitor and forecast this kind of hazard.

SummaryThe Scandinavian Peninsula and its vicinity comprise highly tectonically diverse blocks, including the Baltic Shield, the continental margin, and the North Sea Basin. The crustal rheology is a critical constraint to understanding the tectonic evolution in this region. Based on 19 416 Lg waveforms from 233 earthquakes and 560 broadband digital stations, using an inversion method combining both single- and two-station ray paths, we constructed a broadband (0.05 and 10.0 Hz) Lg wave attenuation model in the study region, with the resolution approaches to 110 km (∼1°) or higher in areas with dense ray path coverages. The QLg distributions correlate well with regional geological features. The Baltic Shield exhibits the highest QLg, consistent with its thick Precambrian crust and high rheological rigidity developed through Archean Svecofennian orogeny. In contrast, passive margins with crustal thinning, magmatic modification, and thick sedimentary sequences exhibit strong attenuation, reflecting a reduction in rheological strength resulting from interactions with mantle plumes and extensional tectonics. The North Sea Basin exhibits the lowest QLg values and the presence of hydrocarbon-bearing sediments. The extremely high QLg distribution reveals the ancient cratonic core of the Baltic Shield, particularly in areas where the surface rock dating sample cannot be collected due to seawater coverage.

SummaryWe analysed infrasound waves associated with the Gyeongju earthquake (ML 5.8) that occurred on September 12, 2016, in the southeastern Korean Peninsula. For infrasound wave detection, the Progressive Multi-channel Correlation method was applied to the infrasound dataset recorded at 7 arrays operating in South Korea at epicentral distances ranging from 178 to 472 km. Based on the back-projection method constrained by array-dependent celerity and azimuth deviation models, the source regions were identified in both the epicentral and nonepicentral regions. Remarkably, the nonepicentral secondary sources of this earthquake were located in regions with shallow water depths: i) the western coastal area in the Yellow Sea and ii) the shallow ocean basin and bank in the East Sea. The location results obtained from the earthquake could be corroborated through its foreshock (ML 5.1), yielding location results consistent with those of the mainshock. The generation of infrasound waves over shallow water depths was fortuitously validated by direct recordings of dominant single-frequency (∼0.3 Hz) infrasound waves at close range via temporary sensors near the ocean basin and bank. We interpreted that low-frequency infrasound signals could be generated from interactions among the ocean floor, shallow seawater, and atmosphere. We performed numerical simulations of seismoacoustic fields to predict ground motions on the seafloor and acoustic transmission efficiency between the water and air interface. The simulations quantified the energy transfer through different media and clarified our observational results. We found that because this solid Earth‒water‒atmosphere coupled air wave has a relatively low frequency (∼0.3 Hz), it can survive propagation over long distances compared with high-frequency infrasound waves generated in inland and mountain regions. In this study, we extend our understanding of water‒atmosphere coupling and the monitoring framework for earthquake-associated nonepicentral infrasound waves, encompassing not only inland ground shaking but also shallow sea regions located far from the epicentre.

SUMMARYIn recent years, machine learning (ML) techniques have emerged as a powerful tool in seismology, enabling the detection of small-magnitude seismic events that typically go unnoticed by traditional methods. Here, we apply ML-based methods to improve the characterization of normal fault systems and aftershock activity in the Central Apennines, using data from the 2009 L'Aquila seismic sequence. By processing data from both permanent and temporary seismic stations, we identified approximately 191 000 events—with a local magnitude range of -1.83 and 5.96, recorded during January-December 2009—nearly ten times more than the standard catalog maintained by INGV. These events were relocated using a combination of absolute and relative location techniques, resulting in a high-resolution catalog of 148 000 earthquakes. This catalog is distinguished by an increased number of S-wave pickings, which significantly reduces localization errors and enhances the accuracy of fault geometry reconstruction. Compared to an existing semi-automatic catalog, we observe a full recovery of seismic events, and a significant improvement of new events identified and well-located by the ML-approach, with a marked increase in the quality and quantity of P- and S-wave arrivals. The refined seismic catalog not only provides a more detailed and accurate definition of the fault architecture but also offers new insights into the distribution of aftershocks, unrolling the complex pattern of faulting that normally remains masked during standard analyses. This work highlights the potential of ML methods in advancing our understanding of complex fault systems and seismic sequences.

AbstractProgressively denser mapping of ocean-floor magnetization has led to detailed reconstructions of past plate motions in the Cenozoic. These reconstructions often reveal rapid kinematic changes that provide crucial information for identifying geodynamic mechanisms that may have caused them, and for quantifying force budgets upon plates. In parallel to these advances, the notion of thin, low-viscosity asthenosphere beneath tectonic plates that facilitates their motions has emerged and consolidated. This weak, mobile layer promotes the formation of the pressure-driven Poiseuille flow that, in turn, generates basal shearing upon plates. In addition, it can be linked to dynamic topography variations due to pulsing plume activity. In this study, we use publicly available finite-rotation compilations of the North American plate (NA) to investigate its kinematic history since Oligocene time. After removing data that are possibly impacted by significant noise, we find that NA experienced a westward speedup near 27 Ma. Next, we explore the role that asthenospheric Poiseuille-type flow caused by increased Canary plume activity may have had in generating this kinematic change. Such plume activity is inferred from the combination of anomalously shallow residual bathymetry and records of past ocean-floor magmatism offshore northwestern Africa. We compare estimates of torque variation upon NA that are (i) required to explain the reconstructed kinematic change, and (ii) predicted by the Poiseuille-type flow associated with the Canary plume activity. Our results indicate that these two torque-variations estimates are in agreement with each other, both in terms of direction and magnitude. This inference suggests that the increased Canary plume activity is a geodynamically-plausible process to explain the Oligocene plate-motion change of NA.

SummarySeismic traveltime tomography represents a popular and useful tool for unravelling the structure of the subsurface across the scales. In this work we address the case where the forward model is represented by the eikonal equation and derive a formalism to solve the inverse problem where gradients are calculated efficiently using the discrete adjoint state method. Our approach provides gradients with respect to both velocity structure and source locations, allowing us to perform a consistent joint inversion. The forward problem is solved using a second-order fast-marching method, which provides a strategy to efficiently solve the adjoint problem. We allow for arbitrary positions of both sources and receivers and for a refined grid around the source region to reduce errors in computed traveltimes. We show how gradients computed using the discrete adjoint method can be employed to perform either deterministic inversion, i.e., solving an optimization problem, or for a probabilistic (Bayesian) approach, i.e., obtaining a posterior probability density function. We show applications of our methodology on a set of synthetic examples both in 2D and 3D using the L-BFGS algorithm for the deterministic case and the Hamiltonian Monte Carlo algorithm for the probabilistic case.

SummaryHigh-quality maps of subsurface temperature and the geothermal gradient are useful when assessing the geothermal potential of a region. However, determining geothermal potential is a challenge when direct measurements of in-situ temperature and thermal property information are sparse and indirect geophysical methods are sensitive to a range of parameters, not just temperature. Here, we produce subsurface temperature maps of Ireland using a joint geophysical-petrological inversion, where seismic and other geophysical and petrophysical data are inverted directly for temperature in 1D columns and are collated into a pseudo 3D temperature volume. Additionally, the inversion produces new models for Moho and LAB depth and for the average crustal radiogenic heat production.To assess the robustness of the resulting temperature model, an uncertainty analysis has been performed by inverting all of the 1D columns for a range of reasonable input parameters applicable to the Irish crust (rather than the ‘best’ input parameters). The resulting uncertainty model suggests temperature estimates at 2 km depth in our model could vary by ± 2 to 5°C with an average of 3.5°C in most locations. The uncertainty model can be used to assess confidence in different regions of the temperature model. In addition, 3D forward modelling was performed to assess the lateral heat flow variations when compared to the purely 1D inversion. The upper-crustal geothermal gradient ranges from 20 to 40°C/km indicating a higher geothermal gradient for Ireland than previously reported with subsurface temperatures at 2 km depth > 60°C everywhere, sufficient for residential and industrial heating purposes. The temperature gradient is typically higher in areas with thinner lithosphere. However, in some locations, the observed geotherms are elevated further due to high radiogenic heat production in granitic rocks. In Northern Ireland, a thin lithosphere, coupled with a weakly conductive basalt layer overlying warm crust, results in elevated temperatures. These are the first temperature maps for Ireland that include uncertainty estimates, providing ranges for the subsurface temperature values, and demonstrate that the maps are comparable to direct independent borehole temperature measurements, which are observed to fall within the model uncertainty. Our new methodology provides workflows for determining the geothermal potential in areas with limited direct temperature measurements. The final temperature model with uncertainty provides useful constraints for geothermal exploration and utilisation on the island of Ireland.

SummaryThe role of pre-existing lithospheric heterogeneities in rifting processes remains unclear. The Eastern and Main Ethiopian rifts lie within the same geodynamic province and are kinematically connected through the Turkana Depression, but they transect heterogeneous lithosphere: Pan-African accreted terranes, failed Mesozoic-Paleogene rift systems, zones of Eocene-Oligocene flood magmatism. Rifting in these pre-extension heterogeneities offers the opportunity to evaluate their relative importance in Oligo-Miocene to Recent stretching and magmatism. We use 3D Rayleigh shear-wavespeed (Vs) models inverted from ambient noise signals recorded on a temporary seismic network to image heterogeneities in lithospheric structure, and to evaluate their influence on syn-rift faulting and magmatism. Crustal feeder zones for Eocene-Oligocene flood magmatism in southwestern Ethiopia are marked by ≤ 50 km-wide, 10-15 km-thick mid-lower crustal fast wavespeed (Vs ≥ 3.8 km/s) anomalies that are localized rather than widespread. Evidence for active magma intrusions only occurs beneath aligned chains of Quaternary eruptive centers in Lake Turkana and ≤ 1 Ma shield volcanoes east of the Turkana rift having localized low Vs (≤3.4 km/s) at 0-20 km depth. Evidence for widespread lower crustal intrusions, however, is lacking. Pan-African oceanic accreted terranes in southern Ethiopia have high Vs anomalies of 3.6 km/s throughout the crust and overlay previously imaged high wavespeed lithospheric mantle that has been interpreted as cold and strong Proterozoic accreted terrane. The integrated strength of this lithospheric-scale pre-existing mechanical heterogeneity resisted Oligocene-Miocene stretching and subsequently contributed to the unusual breadth of this East African rift sector lying north of the Turkana Depression.

SummaryThe Eastern Continental Margin of India (ECMI) is a classic passive margin formed during the Mesozoic breakup of the supercontinent Gondwanaland. Since its formation, the margin has undergone complex post-rift thermal subsidence, magmatic activity, and interactions with adjacent tectonic plates. Extensive shipborne magnetic data have been acquired over the years, providing substantial coverage of the area. However, knowledge about the regional thermal structure and magnetic nature of the upper mantle of the ECMI and adjacent deep offshore Bay of Bengal is not understood uniformly throughout the region. In this study, we estimate Curie depth from shipborne magnetic data using a power spectrum inversion technique within a Bayesian framework, incorporating fractal source distribution and a priori sediment thickness to constrain the top depth of magnetic slab. The Curie depth is a proxy for the 580°C isotherm, providing insight into the regional thermal structure and crustal rheology that controls post-rift thermal evolution and mantle magnetization. The obtained Curie depths range from 16 km to 28 km with corresponding surface heat flow values varying between 55 and 85 mW/m². A shallower Curie depth and higher heat flow are observed in northern part of the offshore Krishna-Godavari (KG) basin and the southern part of Mahanadi basin, linked to rift-related magmatic intrusions and mantle plume activity. Conversely, deeper Curie depths and lower heat flow characterize the Cauvery basin and the southern ECMI. Our results show, across much of the region from the Continent-Ocean Transition (COT) zone to deep offshore areas, the Curie depth lies below the Moho, suggesting that magnetic sources extend into the upper mantle. This suggests the presence of serpentinised upper mantle and exhumed mantle peridotite which provides the secondary magnetization. The obtained thermal lithosphere thickness varies from 50 to 90 km, shallower in the Krishna-Godavari and Mahanadi basins and deeper towards the Cauvery and central basins. Geotherms intersects mantle adiabat at 50 km depth in the KG and Mahanadi basins, signifying these are thermal overprint basins linked to magmatic activity. The obtained thermal lithosphere deepens from north to south, mirroring trends in the Lithosphere-Asthenosphere Boundary (LAB). Finally, a positive correlation between Curie depth and effective elastic thickness (Te) reflects the regional variation in crustal strength and tectono-magmatic processes controlling the margin evolution.

SummaryThe stress and load path dependencies of elastic properties and their evolution under varying damage states is of critical interest to a multitude of communities, such as geophysicists understanding rock properties for subsurface engineering as well as both civil and geological engineers interested in fundamental damage mechanics of materials. Here, we perform a set of laboratory experiments on a Dakota Mahogany granite to understand the dependence of stress path, orientation, and magnitude on static and dynamic properties as well as dynamic evolution under varying states of damage. Localized strain and ultrasonic velocity, axial and radially aligned with respect to the sample, are recorded along four distinct load paths with varying ratios of mean and differential stress. Differential stress is found to be the predominant factor for variations in static Youngs modulus, while undamaged axial dynamic Youngs modulus is primarily a factor of increasing mean stress. Radial dynamic Young’s modulus demonstrates an overall positive correlation with increasing mean stress and negative correlation with differential stress. A novel relationship is constructed to predict phase velocity and orientation/polarization as a function of stress and load path. The effect of damage within the material is analyzed by subjecting the sample to increasing stresses along a single load path, after which the multipath testing is repeated. Ultrasonic velocity and thus dynamic moduli become less sensitive to increases in differential stress for wave propagation parallel with the maximum principle stress. For P-wave velocity aligned parallel, the contribution of differential stress decreases from nearly that of confining pressure (0.88) to below half at the highest damage state tested. Similar decreases also occur in the contribution of differential stress to the remaining three wave polarizations and orientations. This shows that the degradation of physical properties brought about by microcracking and subsequent decrease in velocity overcomes any increase resulting from consolidation with increasing stress. The results provide a way to anticipate changes in elastic response and subsurface acoustic velocity brought about by increased damage and changing stress state through the use of a new empirical model. Additional methods to establish the distribution of microcracks and their orientations within a damaged material through differences in velocity from loading to unloading are presented which provide useful tools for non-destructively assessing damage state.

SummaryIncorporating anisotropy in seismic imaging is important to produce correct locations and amplitudes of subsurface reflectors. The pure quasi-P-wave equation has a good accuracy to describe wave propagation in the anisotropic media, but it requires complicated computation strategies. To mitigate this issue, we present a novel pure quasi-P-wave equation in the vertical transverse isotropic (VTI) media with a nonlinear scalar operator, which is determined by the anisotropic parameters and the phase-velocity vector. Inaccurately calculating the directions of wave propagation results in incorrect phase-velocity vector and accumulated simulation error. Here, we utilize the optical flow to accurately calculate the direction of wave propagation while maintaining computational efficiency. Then, we optimize the wavefield simulation workflow and accelerate the calculation of optical flow. Numerical experiments show that the proposed wavefield simulation method can accurately describe wave propagation in the VTI media with good computational efficiency. Finally, we apply the proposed method to reverse-time migration to correct the anisotropic effects in seismic imaging. Numerical tests for benchmark models and a land survey demonstrate the feasibility and adaptability of the proposed method.

AbstractGeophysicists using the spontaneous potential method measure differences in electrical potential without providing an active source of current. Most spontaneous potential surveys have been carried out on land or in marine environments. In the present paper, I evaluate the use of the spontaneous potential method in surface fresh water for small-scale environmental and engineering applications. In one survey reported here, the electrical potential between an electrode at the river edge and one suspended from a bridge was used to measure a high resolution profile across a river. In another, electrical potentials were measured between sets of electrodes mounted on a canoe. In both surveys, significant and consistent anomalies were detected particularly near bridge structures, and simple modeling in terms of point sources and line sources was undertaken to better characterize the causes of the anomalies. The possibility of an induction-induced voltage difference across the river caused by Earth’s magnetic field and flow in the river was also investigated. The absence of this potential is attributed to significant electrical conduction through the riverbed. The present work demonstrates the utility of spontaneous potential as a technique for detecting and characterizing anomalies of environmental and engineering interest in fresh water environments.