Low-velocity Layer Research Articles

Solutions of Surface-Wave Dispersion and Attenuation in Stratified Viscoelastic Media Using a Spectral-Element Approach

ABSTRACT Efficient and accurate calculation for the dispersion and attenuation of the surface waves in viscoelastic media is numerically challenging because the eigen wavenumbers are located in the complex domain. In this study, we propose a semianalytical spectral-element method (SASEM), which can determine the complex eigen wavenumbers by solving linear eigenvalue problems. By simplifying the structure of the eigenvalue problem, we significantly improve the calculation efficiency. The implementation of the frequency-dependent automatic discretization, semi-infinite element, and mode filter guarantees the correctness and accuracy of the modal solutions. Because no root-searching schemes are required, the root-skipping problem is naturally avoided. The numerical tests show that the SASEM can provide sufficiently accurate solutions with much less computation cost than traditional Muller’s method. Meanwhile, SASEM exhibits high flexibility when applied to media the parameters for which vary continuously with depth. To demonstrate the effectiveness of SASEM for complicated dispersion features, the dispersion curves and eigen wavefields of the viscoelastic media with a low-velocity layer are also analyzed. The results of numerical tests indicate the versatility, efficiency, and accuracy of our method. With further study, the proposed SASEM has the potential to become a promising tool for the investigation and retrieval of viscoelastic subsurface structures.

Crustal structure beneath Central Kamchatka inferred from ambient noise tomography

In this study, we investigated the crustal structure beneath the area of Central Kamchatka (Kamchatka Peninsula, Russian Far East) limited by latitudes 52.8 and 54.1 degrees. This area comprises parts of the active Eastern Volcanic Front and two mountain ridges Sredinny and Ganalsky, as well as several backarc volcanoes. We used the continuous data from the permanent stations and the temporary network installed in 2019–2020 to perform ambient noise tomography. We implemented a two-step tomographic inversion algorithm that initially generated the two-dimensional maps of group velocities for periods ranging from 1 s to 30 s, then found the optimal 1D model of shear wave velocities and derived the three-dimensional S-wave velocity distribution. In the resulting model, we observe a prominent low-velocity anomaly below the active volcanoes of Avacha and Koryaksky, which is traced to depths of ∼40 km and may be associated with a zone of heated rocks suitable for petrothermal energy extraction. We propose that this anomaly represents a common magma pathway through the crust feeding both volcanoes. Beneath another active Zhupanovsky volcano, we observe three low-velocity layers at depths of 2 km, 8 km and 20 km indicative of a multilevel magma feeding system. We believe our findings will further the understanding of the complex processes occurring within these active volcanoes, which pose a tangible hazard to the relatively populated Petropavlovsk-Kamchatsky urban district. Beneath the Sredinny Ridge we reveal a high-velocity pattern at shallow depths, likely associated with consolidated magmatic structures. The low-velocity in the lower crust is interpreted as a high-temperature zone conserved here since the time when this area hosted an active volcanic arc. Upon further exploration, this high-temperature zone could prove to be a prospective source of geothermal energy. Below the Ganalsky Ridge, a prominent high-velocity anomaly extending to a depth of ∼15 km can be attributed to the Precambrian to Carboniferous metamorphic rocks of predominantly mafic composition.

N-S variations of crustal structure beneath the central Tarim Basin from joint inversion of receiver functions, ambient seismic noise surface wave dispersion, and magnetotelluric data

To understand the complex crustal structure beneath the central Tarim Basin, we perform joint inversions of P-wave receiver functions, ambient seismic noise Rayleigh wave dispersion, and magnetotelluric data along a nearly N-S oriented linear array. In this contribution, we present a multistep flowchart to enhance the efficiency and reliability of the joint inversion. Synthetic examples indicate that this method is effective and appropriate for realistic conditions. By using these three complementary datasets, we can construct accurate 1-D models, particularly in the presence of thick overlying low-velocity layers. We apply this approach to data recorded at 45 stations within the Tarim Basin and generate quasi-2-D images that reveal distinct lateral variations along the N-S profile. In the near-surface region, we observe that the conductive and low-velocity sedimentary layer is thinner beneath uplift units and thicker beneath structural depressions, which correlates well with surface geology and previous studies. Additionally, we interpret several major faults, concealed crustal detachment layers, and discuss the geological implications. A conspicuous feature of the integrated images is the identification of a localized and deep-rooted doming structure beneath the Bachu uplift, characterized by high-velocity and high-resistivity anomalies. This structure also exhibits elevated density, intense magnetism and the thinnest overlying sedimentary layer. Consequently, we infer that these anomalies are probably linked to a hybrid basement consisting of plume-related intrusions and mafic dyke swarms.

Deformation microstructures of blueschists in Alpine Corsica, France, and implications for seismic anisotropy and the low-velocity layer in subducting oceanic crust

Understanding the deformation microstructures and seismic properties of blueschists is important for interpreting the seismic anisotropy at the top of subducting oceanic crust. The deformation microstructures and seismic properties of epidote blueschist, epidote-lawsonite blueschist, and lawsonite blueschist from Alpine Corsica, France, were studied using electron backscatter diffraction mapping and transmission electron microscopy. The crystal-preferred orientations (CPOs) of glaucophane show a strong maximum of the [001] axes aligned sub-parallel to the lineation. The maximum of the (010) poles of epidote is aligned sub-parallel to the lineation. The [001] axes of lawsonite are strongly aligned sub-normal to the foliation. Intracrystalline misorientation, dislocation, and occasional fragmentation of glaucophane indicate that glaucophane CPO was developed mainly by dislocation creep. The intracrystalline deformation features of epidote, such as subgrain boundaries and dislocations, indicate that the CPO of the epidote was formed by dislocation creep. In contrast, our data indicate that lawsonite CPO was developed by a combination of rigid-body rotation and dislocation creep with a minor effect of twinning. The P-wave anisotropy (AVp) and maximum S-wave anisotropy (max.AVs) of epidote blueschists (AVp = 12.3–16.9% and max.AVs = 6.53–11.75%) are higher than those of lawsonite blueschists. The seismic velocities of the blueschists are lower than those of mantle peridotites. The coexistence of epidote and lawsonite in the blueschist results in variations in seismic anisotropy and P-wave velocity in the whole rock, depending on the modal content of epidote and lawsonite. A high content ratio of epidote to lawsonite in blueschist produces strong P- and S-wave anisotropies of blueschist when the glaucophane content is more than ∼50%. The P-wave velocity of blueschist decreases proportionally with increasing epidote content. Our results indicate that the seismic properties of deformed blueschists contribute to the seismic anisotropy and the low-velocity layer in subducting oceanic crusts in subduction zones.

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.

First-break prediction in 3-D land seismic data using the dynamic time warping algorithm

SUMMARY This paper presents a new methodology to assist geophysicists in determining the first-break event in a 3-D seismic data set using the well-known technique called dynamic time warping algorithm (DTW), which is usually used to find the optimal alignment between two time-series. We used the optimal path from the cost matrix to identify the first break in the seismogram using a few picks (seeds) made by an interpreter as a reference to perform this task. Furthermore, the data were pre-conditioned by the topographic and linear moveout to improve the method’s accuracy. To demonstrate the technique’s robustness, first, we applied the methodology in a synthetic seismic data. After demonstrating the efficiency of the algorithm, we applied the aforementioned methodology in the Polo-Miranga 3-D seismic cube located in the Recôncavo sedimentary basin, Bahia-Brazil, and in the seismic data acquired from the Blackfoot field in Alberta, Canada. The high-quality results showed consistency in determining the first break in all ranges of offsets, demonstrating an alternative way to accelerate this seismic processing step. Furthermore, we compared the results obtained by the proposed methodology with an algorithm based on comparing the short-time averages with long-time averages. Finally, we performed the static correction calculation to ensure that the time distortion resulting from the terrain and the low-velocity layer was mitigated in shoot gathers and in the stacked section.

Shear-Wave Velocity Model from Site Amplification Using Microtremors on Jeju Island

This study examines shear-wave velocity structures in the Jeju region utilizing horizontal-to-vertical spectral ratios (HVSRs) of environmental noise, focusing on identifying significant low-velocity layers (LVLs). Although conventional methodologies predominantly involve borehole and active seismic exploration, recent advancements in the diffuse-field theory of seismic waves have offered a theoretical foundation for this approach. In the volcanic region of Jeju Island characterized by unique geological features, a pervasive LVL composed of quaternary marine sediments and the Seoguipo sedimentary layer has been observed. These components are crucial for site amplification and attenuation in seismic microzonation. The present study introduces a novel discovery of a distinct LVL, specifically at the UDO site, suggesting that its origin may be attributable to a local magmatic intrusion event. Advanced algorithms and HVSR curve analysis have enabled reliable inversion processes, enhancing the comprehension of the subsurface geology of Jeju. These insights are essential for seismic microzonation practices and contribute significantly to the development of seismic design standards in the Jeju region.

The structure of the mantle transition zone and its implications for mantle upwelling beneath the central Tibetan plateau from triplicated P-waveforms modeling

The Tibetan plateau is considered as an ideal place for studying the continental collisions, tectonic movements and regional geodynamics. Although multiple episodes of igneous activities in Tibetan plateau have been suggested derived from mantle upwelling, their origins and corresponding geodynamic models remain vigorously debate. This study presents a high-resolution upper mantle velocity model of P-wave and its lateral variation beneath the central Tibetan plateau based on triplicated waveform modeling. The results show that, compared with IASP91 model, the 660-km discontinuity is deepened approximately 17–28 km from the Qiangtang terrane to the Beishan block. The lower mantle transition zone (MTZ) is characterized by high P-wave velocity anomalies (HVA) in range of 0.3%–0.9%. We infer that they are mainly related to the hydrous materials with low temperature in the lower MTZ. Besides, the depression of the 410-km discontinuity is about 6–15 km. A remarkable low velocity layer (LVL) is detected atop the 410-km discontinuity, exhibiting the thickness of 31–40 km and low P-wave velocity anomaly of −2.5% to −1.6%. This is against completely temperature-dominated origin of the LVL, and suggests that the material composition has also contributes. Additionally, we propose a new geodynamic model to demonstrate the asthenospheric upwelling beneath the Qiangtang and Songpan-Ganzi terranes, which suggests that the passively ascending water-enriched materials from the lower MTZ and their dehydration are responsible for the observed LVL atop the 410-km discontinuity, as well as the origin of the Cenozoic magmatic intrusion and volcanic eruption of the northern Tibet volcano in Pleistocene.

Parallel-swath-array analysis of passive source surface waves based on beamforming

The detection of mined-out coal area remains a pivotal subject in geophysical prospecting and presents an urgent issue requiring resolution. A refined and effective approach to tackle this challenge involves conducting dispersion analysis on passive surface waves, which are derived from ambient noises. While classical passive surface wave methods, such as the Spatial Autocorrelation Method (SPAC), rely on spatial and time domain stationary assumptions, these can be difficult to fulfill. This is especially true when using ambient noises with frequencies >1 Hz, leading to a growing disparity between basic assumptions and actual scenario. To reduce this discrepancy, it's crucial to account for the non-stationary features of noise sources. In this study, we introduce a pioneering approach for parallel-swath-array analysis of passive source surface waves based on beamforming, making full use of non-stationary ambient noises. The parallel-swath array allows for efficient data collection and high-density spatial observation. The method's core concept involves: 1) The collection of noise data by a 2-D array arranged in three parallel lines; 2) The use of a plane-wave basis function to directly scan the original noise records in a time-segment mode within the time domain. These are conducive to unveil the azimuths and time-variant characteristics of the noise sources, resulting in accurate surface wave dispersion information. To ascertain the feasibility and effectiveness of this innovative method, we simulated passive surface wave records for a geological model, accommodating both time-invariant and time-variant distribution of noise sources. The results demonstrate that the method successfully reveals the time-invariant or time-variant characteristics of noise sources and the associated surface wave dispersion information. Simultaneously, we employ the preprocessed steepest-descent (PSD) algorithm to invert the dispersion curves, and the low-velocity layer in the numerical tests can be clearly revealed. In conclusion, we deployed this method in a coal mine situated in Northern China, which encompasses abandoned mined-out areas. This procedure effectively disclosed the intricate azimuthal properties of the noise sources, yielding high-resolution dispersion images. The inversion outcomes depicted a 2-D S-wave velocity section plunging to a depth of 60 m. By interpreting the low-velocity features of the mined-out area, we can accurately delineate its boundaries. The results of the interpretation align coherently with the findings from the drilling data, to verify the feasibility and effectiveness of parallel-swath-array analysis of passive source surface waves based on beamforming in detecting coal mined-out area under non-stationary ambient noises.

Revealing the Secrets of the Western Mediterranean: A Deep Earthquake and the Overturned Slab

Abstract The Rif–Betics–Alboran region has been vital in the tectonic evolution of the western Mediterranean. Seismic images support the idea of continuous slab rollback being a prominent force in this region. However, the detailed slab structure and the physical mechanisms generating local deep (> 600 km) earthquakes remain unclear. Here, we analyze waveforms recorded from dense seismic networks above the deep earthquake beneath Granada in 2010 to study the slab structure. We discover a thin low-velocity layer (LVL) at the base of the slab to explain both the long codas observed in Morocco and the secondary arrivals observed in Spain. This LVL indicates the presence of hydrous magnesium silicates extending to ∼600 km depth, which suggests that dehydration embrittlement promotes the occurrence of deep-focus earthquakes. Our findings contradict the traditional slab model with the LVL sitting on the top of the slab, suggesting that the Alboran slab has been overturned.

The effects of low-velocity layer and basin topography in near-field ground motion amplification

The effects of low-velocity layer and basin topography in near-field ground motion amplification

Upper mantle melt caused by a subducted slab in the Indian-Eurasian continental subduction zone

A low-velocity layer atop the mantle transition zone has been extensively observed worldwide. In subduction zones, this layer is widely explained as partial melting triggered by slab subduction on a regional or global scale. However, direct observational evidence is still absent, and the response of the layer to slab subduction is not well known. Here, we image the seismic velocity around the mantle transition zone by matching synthetic and observed triplicated seismic P and sP waveforms in the Indian–Eurasian continental subduction zone. Our observations reveal a laterally varied low-velocity layer atop the mantle transition zone beneath the Hindu Kush, where a subducted slab extends to the mantle transition zone. The geometric morphology of the low-velocity layer documents that it is a partially molten layer induced by the subducted slab on a regional scale. Interestingly, our observations also support that the layer has a low viscosity. The decreased viscosity may facilitate slab motion in the deep mantle, contributing to slab stretching, tearing and break-off and its resultant rare recurring large intermediate-depth earthquakes in an intracontinental setting.

Detection of Low-Velocity Layer Using Passive Surface-Wave Data and Deep Learning: An Application in The Urban Area of Hangzhou, China

Passive surface-wave methods using dense seismic arrays have gained growing attention in near-surface high-resolution imaging in urban environments. Deep learning (DL) can release a tremendous workload brought by dense seismic arrays. We presented a case study of shear-wave velocity (Vs) structure imaging in the Hangzhou urban area (eastern China) using DL inversion. Noise data were recorded by dense linear arrays with approximately 5 m spacing deployed along two crossing roads for investigating the top 80 m of the subsurface. Phase-velocity dispersion curves are extracted from virtual shot gathers using multichannel analysis of surface waves. We divided the area where the low-velocity layer (LVL) may exist into three layers with a thickness of 5 m. We gave the four layers weak constraints to generate training dataset and adopted a convolutional neural network to directly invert fundamental-mode Rayleigh-wave phase velocity for 1D Vs models. To improve the accuracy, we further applied the sensitivities to weight the loss function in DL inversion. The obtained pseudo-2D Vs profiles correspond to the velocities estimated from logging data and previous survey. The well-trained neural network successfully identified that the LVL is located at 50-60 m deep. And this network was also achieved accurately the inversion of a dense seismic network nearby. The results of this survey demonstrate the accuracy and efficiency of delineating near-surface structures from traffic-induced noise using the DL technique, which has great potential for monitoring subsurface changes in urban areas.

High-resolution lithospheric structure of continental China from joint inversion of surface wave and gravity data

The lithospheric structure of continental China has been previously determined by seismic travel time tomography, surface wave tomography, and joint inversion of body wave and surface wave data. However, due to the inherent limitations of seismic data, the lithospheric structure of continental China is still not well resolved in the shallow part and in some regions where the station coverage is relatively sparse. In this study, we aim at improving the lithospheric structure by joint inversion of seismic surface wave data and satellite gravity data to take advantage of the uniform distribution and complementary strength of the gravity data. The empirical relationship between velocity and density is used as a bridge for joint inversion of surface wave and gravity data. The joint inversion shear-wave velocity Vs (density) model, named as USTClitho1.0g, can fit both surface wave and gravity data well. This high-resolution Vs model can better fit the active airgun source seismic arrival times and better delineate some features in continental China, such as the velocity contrast across the north–south gravity lineament (NSGL), the lithosphere thinning in eastern China, the crustal footprint of the Hainan mantle plume, the likely magma chamber beneath volcanos in northeast China, the middle-lower crust low velocity layer beneath the Tibetan plateau, and the tearing of subducted Indian mantle lithosphere. Our joint inversion Vs model can provide a reference model for geosciences in continental China and surrounding areas.

Seismic structure of the Eastern European crust and upper mantle from probabilistic ambient noise tomography

The Eastern European lithosphere is a natural laboratory to study continental formation and evolution through time, comprising Archean continental remnants, Proterozoic rifts and belts, and younger accreted terranes. We investigate the seismic structure of the East European Craton (EEC) crust and uppermost mantle, and the transition from Precambrian to Phanerozoic Europe across the Trans European Suture Zone (TESZ) using probabilistic transdimensional ambient noise tomography. We cross-correlate noise recorded at broadband seismic stations from Eastern, Northern, and Central Europe, remove earthquake signals using continuous wavelet transform, and extract Rayleigh wave phase velocity dispersion curves. We invert these for the highest resolution shear wave velocity model of the Eastern European lithosphere to date, using Markov chain Monte Carlo Bayesian inversion. Our shear wave velocity model exhibits spatial correlation with major tectonic units and bears similarities with active seismic survey profiles in terms of seismic velocity patterns and main discontinuities. The crust thickens across the TESZ boundary and the mantle is seismically faster than beneath younger terranes, consistent with a less dense Precambrian lithosphere in the EEC. The crust and lithosphere beneath the Pannonian region is hyper-extended but the adjacent Transylvanian basin crust shows significant heterogeneity. The Precambrian building blocks of the EEC exhibit contrasting seismic fabrics. The Baltic orogens of Fennoscandia are underlain by uniform crust with a flat Moho, while Sarmatia shows alternating high and low velocity layers and a regional south-dipping crustal boundary from beneath the Ukrainian Shield towards the Crimean Peninsula. The last observation supports a geodynamic style driven by horizontal rather than vertical tectonics, with fundamental implications for the formation and evolution of early continents.

Seismic Characterization of a Landslide Complex: A Case History from Majes, Peru

Seismic characterization of landslides offers the potential for developing high-resolution models on subsurface shear-wave velocity profile. However, seismic methods based on reflection processing are challenging to apply in such scenarios as a consequence of the disturbance to the often well-defined structural and stratigraphic layering by the landslide process itself. We evaluate the use of alternative seismic characterization methods based on elastic full waveform inversion (E-FWI) to probe the subsurface of a landslide complex in Majes, southern Peru, where recent agricultural development and irrigation activities have altered the hydrology and groundwater table and are thought to have contributed to increased regional landslide activities that present continuing sustainability community development challenges. We apply E-FWI to a 2D near-surface seismic data set for the purpose of better understanding the subsurface in the vicinity of a recent landslide location. We use seismic first-arrival travel-time tomography to generate the inputs required for E-FWI to generate the final high-resolution 2D compressional- and shear-wave (P- and S-wave) velocity models. At distances greater than 140 m from the cliff, the inverted models show a predominantly vertically stratified velocity structure with a low-velocity near-surface layer between 5–15 m depth. At distances closer than 140 m from the cliff, though, the models exhibit significantly reduced shear-wave velocities, stronger heterogeneity, and localized shorter wavelength structure in the top 20 m. These observations are consistent with those expected for a recent landslide complex; however, follow-on geotechnical analysis is required to confirm these assertions. Overall, the E-FWI seismic approach may be helpful for future landslide characterization projects and, when augmented with additional geophysical and geotechnical analyses, may allow for improved understanding of the hydrogeophysical properties associated with suspected ground-water-driven landslide activity.

The 2022 Ms 5.8 and 6.0 Maerkang Earthquakes: Two Strike-Slip Events Occurred on V-Shaped Faults

ABSTRACT Within four hours on 10 June 2022, three consecutive earthquakes of Ms≥5.0 with a maximum magnitude up to Ms 6.0 struck Maerkang, Sichuan, where is supposedly less prone to earthquakes. This article uses seismic observations recorded by the Sichuan Seismic Network to relocate the earthquake sequences and refine velocity models in Maerkang using a double-difference seismic tomography method. The results show that the aftershocks align along northwest and north-northwest directions with a V-shaped pattern, and the Ms 5.8 and 6.0 earthquakes are located in the center of the sequences. The seismogenic faults of the Maerkang earthquake are the two intersecting secondary faults on the east side of the Songgang fault that dip to the northeast and southwest, respectively, with dip angles of about 80°. Analysis of the seismicity parameters and Coulomb stress changes revealed that the Ms 5.8 and 6.0 earthquakes occurred on two separate faults, and the Ms 5.8 earthquake triggered the Ms 6.0 earthquake, while the Ms 6.0 earthquake prevented the rupture of the Ms 5.8 earthquake. The apparent high-velocity anomaly in the upper crust and the low-velocity and high-conductivity layer in the middle-lower crust of the source region suggest that drag forces exerted by the ductile middle-lower crust may have caused the brittle upper crust to move under the eastward extrusion of the Tibetan plateau, leading to the stresses accumulate and release on the causative faults. The Maerkang earthquake highlighted the lateral movement of deep materials and energy redistribution in the intrablock deformation of the Bayan Har block.

Characterisation of geological thin layer by borehole radar

To investigate a thin layer, we conducted borehole radar measurements at the GPR facility of Tohoku University, Japan. Both zero-offset profiling and tomography measurements were used to obtain water content distributions. Analyzing first arrival times with amplitudes, we found that a borehole radar can detect velocity changes caused by a high-water content, which acts as an electromagnetic waveguide. We conducted a 3D FDTD simulation and confirmed that the EM wave propagates as a guided wave in a thin layer related to antenna frequency. We determined a thin geological layer compared to an antenna frequency that cannot be detected by reflection measurement due to the poor radar resolution. By comparing the simulated data with measurement data, we could determine the geological boundary between host layers and a thin layer with high water content. In contrast, both the amplitude and the travel time of direct waves are affected by antenna positions and electrical parameters, including permittivity and conductivity. A vertical velocity profile was obtained by combining complementary zero-offset profiling and tomography data, which can be used to investigate water content distribution. The low-velocity thin layer was characterised by amplitude attenuation and late arrival time.

Magnetotelluric imaging reveals fragmented crustal blocks beneath the Konkan plains adjoining the Western Ghats, Deccan Volcanic Province, India

The Deccan plateau and the Konkan plain of the western continental margin of India are significant morphotectonic features. Seismic activity along with regional-scale geophysical and geological studies suggests that the plateau is tectonically disturbed and the traps together with the sub-basaltic terrain have been experiencing structural deformation along pre-existing linear zones. The recent Palghar swarm activity has drawn significant attention on the crustal architecture of the Konkan plain and the adjoining Western Ghats region. Here, we present the results of a magnetotelluric study carried out along a 100 km long E-W profile traversing major tectonics features between the Konkan plain and the Bhatsa swarm activity region. Two-dimensional modelling of the distortion corrected impedance tensors reveals high resistivity crustal blocks down to 35–40 km dissected by faults/fractures. These crustal blocks are underlain by a moderately conductive mantle lithosphere which is also characterized as a seismic low velocity and low density layer. The persistent elevated topography of the Western Ghats could be explained by a buoyant, low-velocity/low resistivity upper mantle coupled with reactivation of deep crustal features during the Quaternary period. The model yields some westward dipping conductive and resistive zones coinciding with the exhumed topography highs. The west coast fault in the western and the Ghod fault in the eastern part of the profile are imaged as crustal-scale steeply dipping fault zones segmenting the Eastern Dharwar craton (EDC) crust into the EDC1 and the EDC2. The model in conjunction with earlier results from the Palghar swarm activity region suggests southward continuation of the major conductivity anomalies obtained at Palghar. This study proposes that NW extension of the Kurudwadi rift and its interaction with local structural features is possibly affecting the stress state of the region leading to local seismicity.

The Crustal and Uppermost Mantle Vs Structure of the Middle and Lower Reaches of the Yangtze River Metallogenic Belt: Implications for Metallogenic Process

AbstractThe Middle and Lower Reaches of the Yangtze River metallogenic belt (MLYMB) is one of the most important Fe‐Cu polymetallic belts in China. However, the mechanism and deep geodynamical process for the formation of this belt are still controversial. Here, we obtain the crustal and the uppermost mantle structures using ambient noise data from a dense seismic profile. A low velocity zone is revealed beneath the Moho of MLYMB, interpreted as the source of the deep mineralization materials. In addition, a low velocity layer (LVL) and a high velocity layer (HVL) are observed in the crust of the southern segment of the profile. The LVL is interpreted as a tectonic detachment layer between the upper and the lower crust, and the HVL is interpreted as the aggregation zone for mineralizing melts or crystallized magma chambers. Based on the observed velocity features, we propose a three‐stage model for the formation of ore deposits in MLYMB. Our model suggests that an upwelling of asthenosphere triggered by the delamination of a previously thickened lithosphere leads to the partial melting of upper mantle rocks, which eventually ponders under the Moho. The magma then infiltrates through the ductile lower crust and reaches a depth of ∼7–13 km, forming a minerals‐enriched magma chamber. Minerals‐rich hot fluids originating from the magma chamber continue to move upward along the pre‐existent faults and the minerals finally precipitate in dense veinlets when reaching shallow depths, forming the ore deposits in and around the MLYMB.