P-wave Velocity Model Research Articles

Crustal deformation shown by high-resolution P-Wave velocity structures in the Ordos Block and northeastern Qinghai–Xizang Plateau

Summary The interior of the Ordos block, located northeast of the Qinghai–Xizang Plateau, is not entirely stable or homogeneous and may have been subjected to tectonic deformation. However, there has been no definite conclusion regarding the extent and mechanism of this deformation. Here, we obtained a high-resolution P-wave velocity (Vp) model for the northeastern Qinghai–Xizang (Tibetan) Plateau, Ordos block, and surrounding areas using travel time data involving over 1200 stations from a newly deployed dense temporary array and permanent seismic network. Our results showed that the northern part of Ordos has a relatively high Vp with minimal lateral change. In contrast, the Vp structure in the southern Ordos block varied in the lateral direction, with low-velocity anomalies at a depth of 15 km. The lower crust with Vp from 6.8 to 7.3 km/s is thicker in the west and gradually thins towards the east. The northern part of the Ordos block is relatively stable, whereas the southern part has undergone crustal deformation. This deformation may be related to the eastward compressive forces from the Longxi block, which could be associated with the change in the Haiyuan fault from thrust to strike-slip. In the Bayan Har block, a thick low-velocity anomaly exists in the middle and upper crusts, whereas the lower crust is relatively thin. The crust in this area likely experienced crustal shortening and delamination. Beneath the Longxi and Alxa blocks, a low-velocity layer appears in the middle crust, which may be related to ductile shear in the crustal brittle-ductile transition zone caused by plateau expansion. The range of this low-velocity layer indicates that the influence of the plateau expansion exceeded that of the Haiyuan–Tianjingshan fault zone.

Magnetotelluric image of the Patagonian slab window: Constraints on upper mantle physical properties and sources of intraplate magmatism

The Patagonian slab window (PSW) is a region of the Southamerican subduction zone where the absence of subducted slabs is interpreted, due to the subduction of the Chile Mid-Ocean Ridge at the Chile Triple Junction. Here we report the results of a long-period magnetotelluric (MT) study conducted in two 300 km-long trench-parallel transects crossing the northern boundary of the PSW in the proximal backarc. We modeled the MT data using 3-D inversion, obtaining an electrical resistivity model of the continental crust and upper mantle up to a depth of ∼150 km. Our model shows a heterogeneous resistivity structure in the uppermost mantle, dominated by resistivities >300 Ωm below the array of sites even within the PSW, and some low-resistivity zones (LRZs, <10 Ωm) mainly at the edge of the array. Using petrophysical models, we estimated the mantle temperature, water content, melt fraction, and viscosity based on obtained resistivity values and a preexistent model of P-wave velocity (Vp) at 50 km and 100 km depth. These estimates suggest that the uppermost mantle within the PSW region is heterogeneous and dominated by high-viscosity blocks, compatible with the continental mantle lithosphere or even subducted slabs. Based on relatively hot and low-viscosity zones estimated in the periphery of LRZs, we interpret the presence of asthenospheric mantle in areas where LRZs coincide with relatively low Vp. According to this interpretation, asthenospheric upwelling in the study area at depths ≤150 km would be localized rather than ubiquitous over the interpreted extent for the PSW. Such localized asthenosphere upwelling processes in the past could explain the scattered distribution of Neogene basaltic lavas in the southern Patagonia backarc. The continental crust exhibits LRZs in the upper and lower crust. Remarkably, ensembles of LRZs at different crustal depths within the presumable area of the PSW were found below the General Carrera Lake, and towards the North Patagonian Icefield, likely indicating the presence of hidden intraplate magmatic and/or hydrothermal systems.

Seismic lithospheric model across Ukrainian Shield from the Carpathians to the Dnieper-Donets Basin and its tectonic interpretation

The SW-NE directed wide-angle reflection-refraction (WARR) SHIELD’21 profile crosses entire width of Ukraine. It targeted the structure of the crust and uppermost mantle in the southwestern part of the East European Craton (EEC), between Carpathians and Voronezh Massif, across Ukrainian Shield and Dnieper-Donets Basin. The ∼660 km-long profile is an extension of the RomUkrSeis profile in Romania and Ukraine. SHIELD’21 experiment, using TEXAN and DATA-CUBE short-period seismic stations, provided high-quality seismic sections from 10 shot points. This allowed construction of a ray-tracing P-wave velocity model, supplemented by Vp/Vs ratio, for the upper part of the lithosphere in the Sarmatia segment of the EEC and constraining geodynamic processes that led to the formation of the Ukrainian Shield and its margins since Archean times.The velocity distribution in the crystalline crust indicates a rather uniform structure, with velocity changing from 6.0 km/s near the surface to 6.8 km/s at the Moho. The entire section shows the lack of a high-velocity lower crust (Vp > 6.8 km/s), presumably resulting from delamination of the primitive mafic lower crust during early evolution of a juvenile continental crust. The seismic boundaries in the upper crust reflect a Paleoproterozoic extensional detachment system below the SW flank of Dnieper-Donets rift basin, initiated in the Devonian. At larger depths, a wide dome of the lower crust with velocities of 6.5–6.8 km/s in the SW-central segment of the profile, probably represents an enormous Palaeoproterozoic(?) granitoid batholith. In the southwesternmost part of the profile, the crystalline crust shows exclusively low velocities. The prominent Moho is undulating and varies in depth between 32 and 50 km. It is underlain by high-velocity bodies (Vp: 8.36–8.40 km/s), against the background of 8.16–8.25 km/s in the mantle. The velocity model corresponds with the anomalies of potential fields and zones of high electric conductivity.

Crustal variations and tectonic implications across the South Yellow Sea based on active-source wide-angle seismic analyses

The South Yellow Sea and its environs are pivotal for unraveling the complexities of crustal dynamics and continental collision processes. A holistic assessment of deep structural variations from northern China to the Korean Peninsula is essential for a comprehensive and accurate determination of the tectonic affinity of the Korean Peninsula.Thus, we deployed a pioneering active-source seismic profile (Line2016) spanning the South Yellow Sea and the eastern onshore region of the Korean Peninsula, provides crucial insights into the collision dynamics between the Sino-Korean Block and the Yangtze Block. Our innovative approach, incorporating forward modeling, tomography, and finite-difference wavefield modeling, yielded a high-resolution crustal P-wave velocity model, addressing a significant knowledge gap in understanding the geological intricacies between northern China and the Korean Peninsula. The results confirmed and precisely located the West Marginal Fault of the Korean Peninsula, a significant crustal-scale tectonic structure, likely representing the eastern boundary between the Yangtze Block and the Sino-Korean Block. The study advocates for classifying the Korean Peninsula as part of the Sino-Korean Block, presenting evidence for the one-part affinity hypothesis. This collision resulted in the creation of two distinct suture zones—an orogenic belt in the northern part and a significant strike-slip fault zone in the eastern part of the South Yellow Sea. The study emphasizes the pivotal role of block morphology in regulating plate convergence, providing valuable insights for understanding similar phenomena in other collision zones.

Ground Motion Amplification and 3D P-Wave Velocity Model of the Crati Valley Graben (Italy)

Ground Motion Amplification and 3D P-Wave Velocity Model of the Crati Valley Graben (Italy)

Seismic Imaging of the Arctic Subsea Permafrost Using a Least-Squares Reverse Time Migration Method

High-resolution seismic imaging allows for the better interpretation of subsurface geological structures. In this study, we employ least-squares reverse time migration (LSRTM) as a seismic imaging method to delineate the subsurface geological structures from the field dataset for understanding the status of Arctic subsea permafrost structures, which is pertinent to global warming issues. The subsea permafrost structures in the Arctic continental shelf, located just below the seafloor at a shallow water depth, have an abnormally high P-wave velocity. These structural conditions create internal multiples and noise in seismic data, making it challenging to perform seismic imaging and construct a seismic P-wave velocity model using conventional methods. LSRTM offers a promising approach by addressing these challenges through linearized inverse problems, aiming to achieve high-resolution, subsurface imaging by optimizing the misfit between the predicted and the observed seismic data. Synthetic experiments, encompassing various subsea permafrost structures and seismic survey configurations, were conducted to investigate the feasibility of LSRTM for imaging the Arctic subsea permafrost from the acquired seismic field dataset, and the possibility of the seismic imaging of the subsea permafrost was confirmed through these synthetic numerical experiments. Furthermore, we applied the LSRTM method to the seismic data acquired in the Canadian Beaufort Sea (CBS) and generated a seismic image depicting the subsea permafrost structures in the Arctic region.

Reference 1D Seismic Velocity Models for Volcano Monitoring and Imaging: Methods, Models, and Applications

Abstract Seismic velocity models of the crust are an integral part of earthquake monitoring systems at volcanoes. 1D models that vary only in depth are typically used for real-time hypocenter determination and serve as critical reference models for detailed 3D imaging studies and geomechanical modeling. Such models are usually computed using seismic tomographic methods that rely on P- and S-wave arrival-time picks from numerous earthquakes recorded at receivers around the volcano. Traditional linearized tomographic methods that jointly invert for source locations, velocity structure, and station corrections depend critically on having reasonable starting values for the unknown parameters, are susceptible to local misfit minima and divergence, and often do not provide adequate uncertainty information. These issues are often exacerbated by sparse seismic networks, inadequate distributions of seismicity, and/or poor data quality common at volcanoes. In contrast, modern probabilistic global search methods avoid these issues only at the cost of increased computation time. In this article, we review both approaches and present example applications and comparisons at several volcanoes in the United States, including Mount Hood (Oregon), Mount St. Helens (Washington), the Island of Hawai’i, and Mount Cleveland (Alaska). We provide guidance on the proper usage of these methods as relevant to challenges specific to volcano monitoring and imaging. Finally, we survey-published 1D P-wave velocity models from around the world and use them to derive a generic stratovolcano velocity model, which serves as a useful reference model for comparison and when local velocity information is sparse.

Insight into the Evolution of the Eastern Margin of the Wyoming Craton from Complex, Laterally Variable Shear Wave Splitting

Abstract Dense seismic arrays such as EarthScope’s Transportable Array (TA) have enabled high-resolution seismic observations that show the structure of cratonic lithosphere is more heterogeneous and complex than previously assumed. In this study, we pair TA data with data from the Bighorn Arch Seismic Experiment and the Crust and lithosphere Investigation of the Easternmost expression of the Laramide Orogeny (CIELO) to provide unprecedented detail on the seismic anisotropic structure of the eastern margin of the Wyoming Craton, where several orogens emerged from nominally strong cratonic lithosphere during the Laramide Orogeny. In this study, we use the splitting of teleseismic shear waves to characterize fabrics associated with deformation in the Earth’s crust and mantle. We constrain distinct anisotropic domains in the study area, and forward modeling shows that each of these domains can be explained by a single layer of anisotropy. Most significantly, we find a fast direction in the southern part of the Powder River Basin, which we refer to as the Thunder Basin Block (TBB), that deviates from absolute plate motion (APM). This change in splitting behavior coincides with changes in other modeled geophysical observations, such as active source P-wave velocity models, potential field modeling, and seismic attenuation analysis, which all show a significant change moving from the Bighorn Mountains to the TBB. We argue that these results correspond to structure predating the Laramide Orogeny, and most likely indicate a Neoarchean boundary preserved within the lithosphere.

Acoustic full waveform inversion with elastic wave equation forward modeling

Full-waveform inversion (FWI) is a high-resolution velocity inversion method. Currently, acoustic full waveform inversion (AFWI) has achieved promising results in marine data inversion, but it still faces significant challenges in land data inversion. One of the reasons for the failure of AFWI in land data inversion is that the acoustic wave equation does not account for the elastic effects of seismic waves, such as surface waves and converted waves. This results in errors between the synthesized acoustic waveform and the observed elastic waveform, particularly in the amplitude and phase of the P-wave. To reduce the error caused by the elastic effect, we have improved the inversion process of AFWI. During the forward modeling stage, we replace the original acoustic wave equation with the elastic wave equation to simulate the wavefield. This allows us to obtain synthetic waveforms that incorporate elasticity, thereby reducing the corresponding error. When computing model gradients, we still base on the acoustic wave equation and only update the P-wave velocity model. This approach helps maintain a lower computational cost. We tested the method on the Marmousi model and field land data, and the results demonstrate that the method successfully reduces the error caused by the elastic effect, thereby improving the clarity of the inversion results.

Seismic Characterization of Subsurface Structures at Rock Valley, Nevada

ABSTRACT The Source Physics Experiment (SPE) aims at improving explosion monitoring techniques by investigating source characteristics of chemical explosions in geologic formations. One of the critical tasks in Rock Valley Direct Comparison (RV/DC), SPE phase III, is to prepare for the main experiment by characterizing the subsurface structures at the test site. Based on the seismic data acquired during an accelerated-weight-drop (AWD) seismic survey at Rock Valley, we first pick the P-wave first-arrival travel times and derive a P-wave velocity model using the adjoint-state first-arrival travel-time tomography. We then apply reverse-time migration to the processed seismic data and obtain high-resolution images of the subsurface structures along the two main survey lines. Our migration results show several reflectors corresponding to major geologic formation boundaries. We employ a multitask machine learning model to enhance the reverse-time migration images and identify faults from these images. We find that our automatically picked faults correlate well with the locations of known faults in the region in addition to many geologically undetected faults. Our subsurface characterization results refine our understanding of the geology in this region and provide valuable velocity and structural information for RV/DC geologic model building and fault identification.

Three-dimensional density model of the mantle beneath the Ukrainian shield

Purpose. Mantle density models are key tools for understanding the fundamental geological and physical processes occurring within the Earth and are essential to our scientific and applied understanding of the planet. Methodology. The tasks were solved by a complex research method, including analysis and generalization of literary and patent sources, analytical, experimental studies, using computer and mathematical modelling methods. Findings. One-dimensional models simplify the mantle density distribution by assuming that it is uniform only in the vertical direction. This limitation does not allow for horizontal variations in mantle density, which may be important on a regional scale. 3D models are more complex and require more data and computational resources, so their use may be limited. In this study, we present a quasi-three-dimensional model of mantle density beneath the Ukrainian Shield. This 3D model is obtained using a basic set of one-dimensional seismic tomographic velocity models calculated for 21 mantle domains in the depth range from 50 to 2,600 km. The process of converting the P-wave velocity model into a density model includes the following stages: 1) determining seismic boundaries in the mantle based on P-wave velocity curves for each mantle domain; 2) creating a synthetic mantle model beneath the Ukrainian Shield for the P,S-wave velocity curves; 3) solving the Adams-Williamson equation for each domain, considering polynomial corrections to extract heterogeneities during its solution; 4) analysing existing models by comparing the calculated gravitational potential at the central point of the Ukrainian Shield as the standard reference for selecting one of 5 reference models. Here, we focus on the final stages of constructing the mantle density model by: 1) balancing the mass of the upper and lower mantle for each domain when determining density using the Adams-Williamson equation and introducing polynomial corrections; 2) calculating densities for each of the 21 mantle domains and their 3D integration. Originality. The obtained mantle-density model of the Ukrainian Shield aligns well with the division of the mantle into three main layers: lithosphere, upper mantle, and lower mantle. Each of the mantle’s structural layers has its representation pattern in density heterogeneities. Anomalies of decreased density in the lithosphere of the Ukrainian Shield correlate with thermal anomalies, whereas anomalies of increased density correspond to tectonic zones dividing its megablocks. Practical value. Regions of increased density gradient are associated with mantle thrust faults, which in some cases can be boundaries between different petrological formations and serve as channels for magma ascent into the Earth’s crust at certain stages of geological development of the Ukrainian shield and, in turn, be sources of minerals.

Fast and semi-automatic S-wave and P-wave velocity estimations from landstreamer data: a field case from the Middle East

Seismic surface and body wave analyses are powerful tools for the geotechnical characterization of sites. The use of landstreamers facilitates the acquisition of dense data sets over large areas. However, efficient processing workflows are needed to estimate 3D velocity models from these massive data sets. For surface wave analysis, the manual picking of dispersion curves (DCs) of large data sets is very time-consuming, whereas the accuracy can be biased by operator choices. We apply a semi-automatic workflow for the analysis, processing, and interpretation of a large-scale landstreamer data set acquired for engineering purposes in the Middle East. The workflow involves the application of a validated automatic DC picking algorithm, and the transformation of the DCs into S- and P-wave velocity models through the wavelength-depth technique. The method has a high level of automation, is data driven and does not require extensive data inversion. Another remarkable benefit is that the auto-picking is more than 1,000 times more efficient than standard manual picking and the estimated velocities are in good agreement with available geotechnical and geophysical information. We conclude that the semi-automatic approach may represent a fast and straightforward method suitable for both research and industrial projects, thus enhancing further collaborations and developments.

Comparison of surface-wave techniques to estimate S- and P-wave velocity models from active seismic data

Abstract. The acquisition of seismic exploration data in remote locations presents several logistical and economic criticalities. The irregular distribution of sources and/or receivers facilitates seismic acquisition operations in these areas. A convenient approach is to deploy nodal receivers on a regular grid and to use sources only in accessible locations, creating an irregular source–receiver layout. It is essential to evaluate, adapt, and verify processing workflows, specifically for near-surface velocity model estimation using surface-wave analysis, when working with these types of datasets. In this study, we applied three surface-wave techniques (i.e., wavelength–depth (W/D) method, laterally constrained inversion (LCI), and surface-wave tomography (SWT)) to a large-scale 3D dataset obtained from a hard-rock site using the irregular source–receiver acquisition method. The methods were fine-tuned for the data obtained from hard-rock sites, which typically exhibit a low signal-to-noise ratio. The wavelength–depth method is a data transformation method that is based on a relationship between skin depth and surface-wave wavelength and provides both S- and P-wave velocity (Vs and Vp) models. We used Poisson's ratios estimated through the wavelength–depth method to constrain the laterally constrained inversion and surface-wave tomography and to retrieve both Vs and Vp also from these methods. The pseudo-3D Vs and Vp models were obtained down to 140 m depth over an area of approximately 900 × 1500 m2. The estimated models from the methods matched the geological information available for the site. A difference of less than 6 % was observed between the estimated Vs models from the three methods, whereas this value was 7.1 % for the retrieved Vp models. The methods were critically compared in terms of resolution and efficiency, which provides valuable insights into the potential of surface-wave analysis for estimating near-surface models at hard-rock sites.

Joint wave-equation inversion of Rayleigh and Love dispersion curves

Wave-equation dispersion (WD) inversion techniques for surface waves have proven to be a robust way to invert the S-wave velocity model. Unlike 1D dispersion curve inversion, WD method obviates the need for a layered model assumption and reduces the susceptibility to cycle-skipping issues in surface wave full-waveform inversion. Previous WD inversion experiments conducted on Rayleigh and Love waves have highlighted that inverting Love waves yields better stability due to their independence from the P-wave velocity model. Nevertheless, Rayleigh waves possess the advantage of greater penetration depth compared with Love waves with similar wavelengths. Therefore, combining the two types of surface waves is a feasible way to improve the accuracy of S velocity tomograms. In light of this, we develop a novel approach: a joint WD inversion encompassing Rayleigh and Love waves. This innovative technique adjusts the weighting of individual WD gradients using the sensitivity factor of an equivalent layered model, offering a significant advancement in subsurface characterization. Synthetic model tests demonstrate that the joint WD inversion method can generate a more accurate S-wave velocity model, particularly in the presence of complex low-velocity layers or high-velocity layers, when compared with individual wave WD inversion techniques. Simultaneously, the results of field tests validate the effectiveness of the proposed joint WD inversion strategy in producing a more dependable S-wave velocity distribution that aligns closely with the actual geologic structure.

DEEP REFLECTION SEISMIC DATA FOR IMPROVED IMAGING OF CRUST STRUCTURE: 2D CASE STUDY OF THE SOUTHWEST SUB-BASIN IN THE SOUTH CHINA SEA

Seismic imaging of crustal structures becomes difficult in the presence of rough basements or complex bathymetry. Here, we present a 900 km deep seismic reflection profile collected across the Southwest Sub-basin (SWSB) of the South China Sea. By analyzing the types and distinctions of noise and effective signals, we employed deep structure migration techniques to improve crustal structure imaging, wide-line processing to predict 3D space multiples, and F-K domain time-space variable adaptive de-ghosting and different offset stacking to enhance the weak signals reflected from deep strucutures. The imaged continental crustal structure in the Penxi Bank exhibits moderate thinning, down to 15 km, and is intersected by continental-ward low-angle normal faults. Within the limitations of the OBS P-wave velocity model, we detected sub-horizontal lower crustal reflections that may be indicative of a weak lower crust. Two small-scale rollover structures along detachment faults rooted and rafted to the top of these weak lower crust. Based on the presence of narrow continent-ocean transitions(COTs), continental-ward detachment faults, and high lithosphere heat flow, we deduced that the mantle lithosphere breakup occurred earlier than the crust in the SWSB. Moreover, the continent-ocean transitions and oceanic crust domains demonstrate rough basements with numerous faults and approximately 20% diffuse or weak Moho reflections. From the southern COT to the initial oceanic domain, the thickness of the crust gradually reduces to only 3-5 km. This suggests a relatively low magmatic budget and protracted tectonic extension from the continental breakup to the onset of seafloor spreading. Within the oceanic crust domain, the crust thickness ranges from approximately 4-6 km, indicating a thinner oceanic crust than normal crust. Lower crustal reflections with a ridge-ward dipping pattern terminate at the Moho reflections and are partly connected to syn-spreading faults, hinting at their possible generation through syn-spreading faulting.

Crustal Velocity Structure of the Sichuan–Yunnan Block Revealed by High-Quality Crustal Phase Travel Time

Abstract The Sichuan–Yunnan block is located at the southeastern margin of the Tibetan Plateau, which is the key area as a transition belt from the active plate extrusion zone to the stable Yangtze Craton. Using a semiautomatic measuring method based on a graphical interface, we pick 81,585 precise travel times from 449 local earthquake records and finally obtain a crustal 3D P-wave velocity model of the Sichuan–Yunnan block. The model reveals an unexpected velocity contrast between the shallower and deeper crusts. It is summarized as weakly perturbed low-velocity belts encircling a high-velocity zone in the upper crust and strongly perturbed low-velocity anomalies in the mid-lower crust, respectively. The weak low-velocity anomalies are revealed along the major strike-slip faults, and their small perturbations may imply a slip-driven mechanism. The strong low-velocity anomalies are distributed extensively in the Sichuan–Yunnan block, and their great perturbations may be related to the partial melting of weak material extruded from Tibet. Besides, our result shows noticeable high-velocity anomalies in the core zone of the Emeishan Large Igneous Province (ELIP), which may be an indication of magma solidification from the ancient mantle plume. The result further exhibits an interesting pattern that the strong low-velocity anomalies are partially separated by the high-velocity anomalies in the ELIP. Such a specific pattern probably reflects that the stable zone in the ELIP leads to the bifurcation of weak Tibetan material.

High-resolution geophysical investigations in the central Apennines seismic belt (Italy): Results from the Campo Felice tectonic basin

The Campo Felice basin, in the central Apennines seismic belt (Italy), developed in the hangingwall of a 30 km-long system of NW-trending normal faults with Holocene paleoseismic activity and potential sources of M 6–7 earthquakes. We provide the first subsurface images of a key portion of the basin bounded by the Mt. Cefalone fault along two intersecting profiles trending NNE-SSW (CF-Dip, 1195 m-long) and WNW-ESE (CF-Strike, 1315-m long). We combined high-resolution depth-migrated reflection sections with P-wave velocity and electrical resistivity tomography models. CF-Dip profile displays a wedge-like syn-tectonic sedimentary sequence of alluvial and glacial deposits with Vp ∼ 2500–3000 m/s and resistivity > 500 Ωm in the hangingwall of Mt. Cefalone fault, overlying a high-Vp (>4000 m/s) limestone bedrock ∼ 300 m deep. The whole sequence displays reflectors truncated by the Mt. Cefalone fault zone and subsidiary antithetic faults. CF-Strike profile, tied to three 80–110 m-deep boreholes, shows a thick fluvio-lacustrine sequence with low-Vp (<2000 m/s) and low resistivity (<100 Ωm), and a bedrock that deepens to the southeast (>450 m). Single-station ambient noise measurements display Horizontal to Vertical Spectral Ratios with peaks at ∼1 Hz, decreasing to ∼0.8 Hz to the southeast in agreement with the bedrock deepening indicated by seismic profiling. According to our results, the Campo Felice basin is a deep asymmetric half-graben controlled by faulting whose activity likely started before the Middle Pleistocene. Our minimum displacement estimate accrued in the past 0.5 Ma by the Mt. Cefalone fault is in the range of ∼100–250 m.

Integrating earthquake-based passive seismic methods in mineral exploration: Case study from the Gerolekas bauxite mining area, Greece

As the global need for aluminum constantly rises, bauxite is considered to be a critical mineral, and the mining industry is in search of new and effective exploration solutions. In this context, we design and implement a purely earthquake-based passive seismic survey at the Gerolekas bauxite mining site in Greece. It is a very difficult exploration setting, characterized by rough topography, limited accessibility, and a very complex geotectonic regime. We gather a passive seismic data set consisting of four months of continuous recordings (May to August 2018) from 129 stand-alone 3C seismological stations. We then analyze this data set and extract 848 microearthquakes that will serve as sources for the application of local earthquake tomography (LET) and transient-source seismic interferometry (TSI) by autocorrelation. We apply LET to estimate the 3D P- and S-wave velocity models of the subsurface below the study area and TSI by autocorrelation to retrieve the zero-offset virtual reflection responses below each of the recording stations. The velocity models provide a relatively coarse image of a previously completely unexplored part of the mining concession, whereas the higher-resolution virtual reflection imaging illuminates in detail the different interfaces. We also reprocess three lines of legacy active seismic data that were shot in 2003, using the LET P-wave velocity model for depth migration, and confirm the improvement of seismic imaging. Finally, we evaluate the obtained results using well data and jointly interpret them, extracting useful information on the expected target depths and indicating that earthquake-based passive seismic techniques can be an innovative and environmentally friendly option for mineral exploration.

Crustal structure of the Tian Shan Orogen and its adjacent areas inferred from EIGEN-6C4 gravity field data

The study of crustal structure plays a crucial role in understanding the tectonic development and seismicity of orogens. In this contribution, we analyzed the gravitational responses caused by the subsurface structure of the Tian Shan orogen using global EIGEN-6C4 gravity field data. The quality of the EIGEN-6C4 gravity data in the eastern Tian Shan area was evaluated, and we employed the wavelet multi-scale analysis method in combination with an existing P-wave velocity model to reveal the structures at different scales. The gravity Moho of the study area that best fits the seismological Moho was obtained from the filtered third-order wavelet approach component after several tests using different reference depths and density contrasts. The results reveal significant lateral discontinuities in the lower crust and upper mantle structures across the eastern and western Tian Shan orogens. The Moho beneath the western Tian Shan is about 9 km deeper compared to the eastern Tian Shan. Towards the Tarim and Junggar basins, the Moho decreases to about 45 km in depth. The analysis of the isostatic anomaly suggests that the eastern and western Tian Shan possess varying capabilities for isostatic adjustment. Furthermore, it indicates a gradual weakening of mantle upwelling in the Tian Shan orogen from west to east, implying that different stages of mantle dynamics may have occurred in the western and eastern Tian Shan. The north Tarim fault, Nikolaev-North Nalati fault, and Kazakh fault serve as channels that control magma upwelling in the Tian Shan orogen. The distribution of earthquakes differs between the eastern and western Tian Shan, with the western Tian Shan exhibiting greater gravitational potential energy. These variations between the eastern and western Tian Shan may be attributed to the clockwise rotation of the Tarim craton.

Velocity structure (1D) and earthquakes relocation in the flat-slab to normal plate transition zone in the Argentinean transarc

The Argentinean Andean foreland region between latitudes 31°S to 33.5°S presents an intense crustal and upper mantle seismic activity that was recorded by a local seismological network called CHARSME (Chile Argentina SeisMic Experiment), and was used to obtain new velocity determinations. In a first moment, from 4 P-wave velocity (VP) models proposed by other authors, the detail in the layers was increased, resulting in better models used as initial model in the investment. Finally, by carefully selecting a set of earthquakes, a simultaneous velocity inversion, hypocentre determination and station correction were performed using the VELEST code. In order to obtain better velocity models (VP and VS) and VP/VS ratio, different tests were carried out in all processes.The final VP model presented in this paper incorporates considerations made in other seismological studies. This model accurately determined velocities between 10 and 65 km depth. The upper mantle velocity (VP) was defined in 8.05 km/s. From the Wadatti diagram, which represents a line whose slope is related to the wave velocities and the Poisson's coefficient, a VP/VS value of 1.732 was obtained. Alternative VP/VS values were also considered to invert VP, VS and VP/VS simultaneously. The minimum detailed model of VP, VS and VP/VS decreased the RMS by 25% relative to the initial models, implying an improvement in the uncertainty of locations. The new and precise locations of crustal earthquakes allowed defining clusters within Cuyania, a tectonic terrain that collided to Gondwana in the Early Paleozoic. Although it is known that foreland seismicity is mainly concentrated in this terrain, this work shows that there are well-defined zones within Cuyania that are more seismically active. Earthquakes of intermediate depth (upper mantle) are strongly agglomerated in the flat-slab zone of the subducting Nazca Plate.The discontinuities present in our model suggest upper, middle and lower crustal structuring, as well as a marked contrast between the sediments and the underlying basement. It was possible to interpret an upper crust consisting of sediments and basement to a depth of 25 km, a middle crust between 25 and 45 km and a lower crust between 45 and 55 km. We also defined the Mohorovičić (Moho) discontinuity at 55 km depth. The crustal velocities are in agreement with previous seismological studies.