Shear Wave Velocity Structure Research Articles

Surface-wave dispersion curves extraction method from ambient noise based on U-net++ and density clustering algorithm

It is crucial to accurately and rapidly extract dispersion curves of surface waves from ambient noise recordings for inverting the subsurface shear (S) wave velocity structures. Conventional manual picking methods are labor-intensive and affected by subjective factors related to seismologists. The existing deep learning methods can automatically and efficiently extract dispersion curves from dispersion spectrograms with a single branch. However, extracting dispersion curves from phase-velocity dispersion spectrograms with multiple branches remains challenging due to complex label-making processes and extensive label-preparation times. Notably, phase velocities typically display a positive correlation with periods and are slightly higher than group velocities. Given the significance of this empirical relationship, we present an intelligent surface-wave dispersion curves extraction method based on U-net++ and density clustering algorithm. Initially, guided by domain knowledge that dispersion curves are smooth, a global searching method is employed to automatically label group-velocity dispersion curves from group-velocity dispersion spectrograms. Subsequently, the group-velocity dispersion curves undergo transformation into probability images of the curves using a Gaussian function. The U-net++ then nonlinearly converts group-velocity dispersion spectrograms into probability images of group-velocity dispersion curves within a high-dimensional space. Following this, the Density-Based Spatial Clustering of Applications with Noise (DBSCAN) algorithm is applied to obtain multi-mode phase-velocity dispersion curves from phase-velocity dispersion spectrograms. We then remove invalid dispersion values in the multi-mode phase-velocity dispersion curves based on the empirical relationship between phase and group velocities, ultimately obtaining the phase-velocity dispersion curve corresponding to the group-velocity dispersion curve. Our proposed method has been tested using synthetic data and 3D real-world data collected near Lake Chao in Anhui Province. The test results show that our approach effectively extracts both group-velocity and phase-velocity dispersion curves. Inverted S-wave velocity structures based on extracted dispersion curves in real-world data can characterize the Tan-Lu fault zone, demonstrating the effectiveness of our proposed method.

Shear wave velocity structure in the middle segment of the Xiaojiang fault zone using ambient noise tomography

The middle segment of the Xiaojiang fault zone consists of strike-slip faults with frequent strong and medium–strong earthquakes under the joint influence of NE-trending faults and deep structural characteristics. The distribution characteristics of the deep subfaults of each branch need to be revealed. Two short-period dense array survey lines are deployed in the area. Shear wave velocity structures are obtained using ambient noise tomography. The results reveal that the faults in each branch dip nearly eastward, with strike-slip characteristics in the north‒south direction. Strong velocity lateral variation infer imply that the deformation characteristics of the upper crust may be brittle. NE faults are observed, which are characterized by typical compression. The high-velocity zone in the Songming Basin may be related to the deeper upwelling of high-velocity strata in the upper crustal flow.

High-Resolution Shear-Wave Velocity Structure of the 2019 Ms 6.0 Changning Earthquake Region and Its Implication for Induced Seismicity

AbstractFluid injection activities related to hydraulic fracturing (HF) and salt mining may induce moderate earthquakes. In the Changning area in southwest China, the Ms 6.0 earthquake on 17 June 2019 is the largest and the most damaging event ever recorded in this region. This earthquake occurred in the Changning anticline, which hosts multiple active faults and industrial production activities, raising an extensive controversy on the cause of the earthquake. Beyond seismogenic faults, a detailed 3D velocity structure of the source region is missing. Here, we applied an improved ambient noise tomography method to seismic data recorded by a portable dense seismic array to reveal the characteristic of 3D shear-wave velocity (VS) structure with high resolution in the Changning region. Our VS structure model provides some new observational evidence favoring that the Ms 6.0 Changning earthquake and the related seismicity in Shangluo shale gas field were, respectively, induced by fluid injection for salt mining and hydraulic fracturing. Moreover, it is suggested that the shallow segment of the pre-existing thrust faults were reactivated by fluid injection. This result provides some implications of VS structure of the induced-seismicity source region and warn us to pay more attention to the seismic risk assessment for such areas that have both similar industrial operation intensity and tectonic settings.

Shear wave velocity structure beneath the eastern Indian Ocean from Rayleigh wave dispersion measurements

Shear wave velocity structure beneath the eastern Indian Ocean from Rayleigh wave dispersion measurements

Apparent Low‐Velocity Belt in the Shallow Anninghe Fault Zone in SW China and Its Implications for Seismotectonics and Earthquake Hazard Assessment

AbstractThe Anninghe fault forms the eastern boundary of the Sichuan‐Yunnan block in Southwest China and has been identified as an earthquake gap zone. This study intends to construct the upper crustal shear wave velocity (Vs) structure beneath the Anninghe fault to understand its seismotectonics and potential large earthquake hazards. We deployed a dense seismic array along the southern central Anninghe fault valley. From the 3‐month continuous records, we calculated vertical‐component cross‐correlation functions (CCFs). However, the surface wave signals in the CCFs are intensely interfered by near zero‐time‐lag noise. We proposed a mode separation method based on the high‐resolution linear Radon transform, which suppressed the interfered noise and greatly enhanced the surface wave signals for ambient noise tomography of the Vs structure. The fine upper crustal structure reveals a distinct narrow low‐velocity belt within a depth of 3 km beneath the Anninghe fault zone. At deeper depths (4.5–8 km), the narrow low‐velocity belt shifts to the east and correlates with the distribution of local earthquakes. Combining previous results with our new findings, we presented a seismotectonic model of the southern central Anninghe fault, which interprets the narrow low‐velocity belt as a water‐contained fracture zone that forms a seismogenic zone at deeper depths under transpression. In addition, we demonstrated through scenario earthquake simulations that fine structures play a significant role in the assessment of earthquake hazards along the Anninghe fault. As such, this study provides a typical window into seismotectonics and large earthquake hazards in the active southeastern Tibetan Plateau.

Subcontinental lithospheric mantle discontinuities beneath the Eastern Himalayan Plate Boundary System, NE India

SUMMARY We use P-wave receiver function (P-RF) analysis and joint inversion with Rayleigh wave group velocity dispersion data to model the shear wave velocity (Vs) structure of subcontinental lithospheric mantle (SCLM) discontinuities beneath northeast (NE) India. The most prominent SCLM discontinuity is the Hales Discontinuity (H-D) observed beneath the Eastern Himalayan Foreland Basin (Brahmaputra Valley) and Shillong Plateau. The P-to-SV converted phase from the H-D (Phs) is a positive amplitude arrival at ∼10–12 s and has positive moveout with increasing ray-parameter. From joint inversion, the H-D is modelled at a depth range of 90–106 km, with 11–12 per cent Vs increase beneath the Brahmaputra Valley. Beneath the Shillong Plateau the H-D is at a depth range of 86–99 km, with 6–10 per cent Vs increase. An intralithospheric discontinuity (ILD) has been identified in the Shillong Plateau station P-RFs, as a positive amplitude PILDs phase, arriving at 8–8.5 s. This is modelled at a depth range of 66–75 km with Vs increase of 2–9 per cent. We construct 2-D profiles of depth-migrated common-conversion-point stack of P-RFs to distinguish the SCLM discontinuity arrivals from crustal phases. 3-D spline-interpolated surface of the H-D has been constructed to visualize its lateral variations. We use xenolith data from the Dharwar Craton, which has similar geological age, petrology and seismic structure as the Shillong Plateau, to petrologically model the SCLM H-D and ILD Vs structure in NE-India. From the calculated Vs structure we conjecture that the H-D is a petrological boundary between mantle peridotite and kyanite-eclogite, with its origin as metamorphosed paleosubducted oceanic slab, similar to other global observations. We further speculate that the shallower ILD could be formed as a contact between frozen asthenosphere-derived metasomatic melts within the SCLM.

Shear wave velocity structure at the Fukushima forearc region based on H/V analysis of ambient noise recordings by ocean bottom seismometers

SUMMARYThis study presents the shear wave velocity (VS) structures of sedimentary sequences and a section of the upper crustal layer in the Fukushima forearc region of the Japan Trench subduction zone, which were obtained by analysing the horizontal-to-vertical (H/V) spectral ratios of ambient vibration records. The H/V curves were derived using 31 d of continuous seismic data from 3 broad-band and 16 short-period ocean bottom seismometer (OBS) stations. Using the broad-band data, H/V ratios from 0.01 to 10 Hz were derived, but the ratios below 0.1 Hz frequencies were unusually large and temporally unstable. Characterization of seismic noise energy from ∼1 yr of seismic data of three broad-band OBSs revealed variable and elevated energy conditions below 0.1 Hz due to typical long-period oceanic noise; we link these observations with the unstable H/V ratios below this frequency. Therefore, H/V analysis was performed in the frequency range of 0.1–10 Hz for both broad-band and short-period OBSs to obtain subsurface VS profiles. For the forward calculation of the H/V ratios in the inversion process, we used the recently developed ‘hvgeneralized’ method, which is based on the diffuse field assumption, and accounts for the water layer on top of stratified media. Moreover, available prior geological and geophysical information was utilized during the inversion of the H/V curves. We found that subsurface VS ranged from approximately 30 m s−1 at the seabed to approximately 4900 m s−1 at 7000 m below the sea floor (mbsf). Starting with the best model candidate at each OBS location, the effect of the water layer on the H/V curve in the deep ocean was investigated by comparing synthetic H/V curves with and without the water layer. The synthetic H/V analysis revealed that the water layer had a significant effect on H/V amplitudes at higher frequencies (>1 Hz), whereas comparatively little effect was observed at lower frequencies (<1 Hz). This study provides an empirical basis for H/V analysis using OBS data to determine VS down to several kilometres of sedimentary sequences to the upper crust with high-resolution.

Rapid construction of Rayleigh wave dispersion curve based on deep learning

Introduction:The dispersion curve of the Rayleigh-wave phase velocity (VR) is widely utilized to determine site shear-wave velocity (Vs) structures from a distance of a few metres to hundreds of metres, even on a ten-kilometre crustal scale. However, the traditional theoretical-analytical methods for calculating VRs of a wide frequency range are time-consuming because numerous extensive matrix multiplications, transfer matrix iterations and the root searching of the secular dispersion equation are involved. It is very difficult to model site structures with many layers and apply them to a population-based inversion algorithm for which many populations of multilayers forward modelling and many generations of iterations are essential.Method:In this study, we propose a deep learning method for constructing the VR dispersion curve in a horizontally layered site with great efficiency. A deep neural network (DNN) based on the fully connected dense neural network is designed and trained to directly learn the relationships between Vs structures and dispersion curves. First, the training and validation sets are generated randomly according to a truncated Gaussian distribution, in which the mean and variance of the Vs models are statistically analysed from different regions’ empirical relationships between soil Vs and its depth. To be the supervised dataset, the corresponding VRs are calculated by the generalized reflection-transmission (R/T) coefficient method. Then, the Bayesian optimization (BO) is designed and trained to seek the optimal architecture of the deep neural network, such as the number of neurons and hidden layers and their combinations. Once the network is trained, the dispersion curve of VR can be constructed instantaneously without building and solving the secular equation.Results and Discussion:The results show that the DNN-BO achieves a coefficient of determination (R2) and MAE for the training and validation sets of 0.98 and 8.30 and 0.97 and 8.94, respectively, which suggests that the rapid method has satisfactory generalizability and stability. The DNN-BO method accelerates the dispersion curve calculation by at least 400 times, and there is almost no increase in computation expense with an increase in soil layers.

Shear wave velocity structure and crustal lithology beneath the ultraslow spreading Southwest Indian Ridge at 50° E

SUMMARY Shear wave velocities provide an important constraint on crustal lithology. Limited crustal shear wave data are available from the ultraslow spreading mid-ocean ridges. We combine observations of both compressional (P) and shear (S) waves in ocean–bottom seismometer data from the Southwest Indian Ridge to determine crustal P-wave velocity (Vp), S-wave velocity (Vs), Vp/Vs and Poisson's ratio variations along the ridge at 49°17′E–50°49′E. Similar layered crustal structures were revealed beneath both the magmatically robust segment centres (Vp/Vs of 1.76–1.94, Poisson's ratio of 0.26–0.32) and the non-transform discontinuity (NTD) between them (Vp/Vs of 1.76–2.03 and Poisson's ratio of 0.26–0.34). Because laboratory measurements show an overlap in Poisson's ratio between mafic igneous and ultramafic rocks, particularly at Vp values typical of oceanic Layer 3, it can be difficult to distinguish crustal composition using this parameter only. However, our observed Vp gradients of 0.1 ± 0.1 s−1 suggest that in this area, oceanic Layer 3 consists primarily of mafic igneous rocks both at segment centres and at the NTD. Oceanic crustal Layers 2A and 2B above are likely also to consist of mafic igneous rocks, with some evidence for increased fracturing at the NTD.

Moho depth variation and shear wave velocity structure in northern Algeria from joint inversion of P-wave receiver functions and Rayleigh wave dispersion data

SUMMARY In this work, the Moho depth and the velocity structure of the crust and upper mantle beneath broad-band seismic stations of the Algerian broad-band seismic network are investigated. Teleseismic P-wave receiver functions jointly inverted with Rayleigh wave dispersion curves obtained from local earthquakes have been used. The seismic stations are located in different geological settings including the Tell Atlas, High Plateaus and the Saharan Atlas. The crustal thickness and the Vp/Vs ratio are first derived by the H–κ stacking method of receiver functions. The inversion results show the variation in Moho depth in the different geological contexts. The shallowest depths of the Moho (∼20–30 km) are estimated along the Algerian continental margin and Tell Atlas. In the High Plateaus region, the Moho depths vary from 30–36 km, whereas the deepest Moho depths are found in the Saharan Atlas (36–44 km). Two-layer crust is observed in the whole study area. In the upper crust, ∼8–14 km thick, the average shear wave velocity is ∼3.0 km s−1. The lower crust of about 12–30 km thick has an average shear wave velocity that ranges between 3.4 and 3.8 km s−1. The lower crust is thicker than the upper crust particularly in the Saharan Atlas. The upper mantle shear wave velocity varies from 4.1 to 4.5 km s−1 maximum and is stable, generally, below ∼60 km depth. Two low-velocity zones are clearly observed particularly in the eastern part of the Tell Atlas and the High Plateaus. The first one about 10 km thick is in the lower part of the lower crust and the other one is in the upper mantle between 40 and 60 km depth. The obtained results are in accordance with the previous results found in the region, particularly those using land gravity and seismic data. As the first estimate of the Moho depth from earthquake data in northern Algeria, using the receiver function method, this study sheds new light on the crustal structure and the Moho depth in this region of the world.

Enhancing the Frequency–Bessel Spectrogram of Ambient Noise Cross-Correlation Functions

ABSTRACT The frequency–Bessel (F–J) spectrogram has been used for the extraction of multimodal dispersion curves to constrain the fine crustal shear-wave velocity structure. The original F–J spectrogram was contaminated with curved as well as straight crossed artifacts, which hindered obtaining the dispersion curves, while introducing a considerable error in the inversion result. Curved crossed artifacts in the multicomponent F–J spectrogram are typically removed using the modified F–J transform formulas; to remove straight crossed artifacts, we used the so-called k-filtering method. Based on a synthetic test and field data from the central Asian orogenic belt, we show that our proposed methods can enhance the multicomponent F–J spectrograms by efficiently removing the two types of crossed artifacts, while identifying more higher modes dispersion curves, and the accuracy of picking can also be improved.

Direct surface-wave tomography under Northeast China: New insights into 3-D crustal S-wave velocity structure and dynamics of intraplate volcanism

A new three-dimensional shear-wave velocity structure of the crust beneath Northeast China is constructed by inversion of phase velocity dispersion curves between 5 and 40 s using the direct surface-wave ambient noise tomography method. In addition to 127 NECESSArray (NorthEast China Extended SeiSmic Array) seismic stations, 19 SCVDSZ (Study of Changbai Volcanoes and Deep Subduction Zone) plus 3 CDSN (China Digital Seismograph Network) stations are added in the data collection. The addition of these data greatly improves the ray path coverage and imaging results around the Zhangguangcai range and the Changbaishan mountain. Our results show an obvious low-velocity anomaly extending from the surface down to uppermost mantle under the Changbaishan, suggesting that the volcano formation could be related to mantle upwelling. One more striking feature is a widely distributed mid-lower crustal low-velocity layer under the entire Northeast China, which is connected to those in the upper crust under the volcanoes, indicating that these volcanoes could have exchanges of material and energy in the mid-lower crust. In addition, under the Shuangliao and Erke volcanoes, low-velocity anomalies are visible in the upper and middle crust but high-velocity anomalies in the lower crust, whereas under the Abaga and Halaha volcanic groups, the low-velocity anomalies extend from the crust down to uppermost mantle, suggesting their continuous mantle magma provision. Integrating with previous global and regional body-wave tomographic models, our results suggest that the volcanoes in Northeast China could be related to hot and wet mantle upwelling in the big mantle wedge caused by the deep subduction of the subducted Pacific slab in the mantle transition zone.

Topography effect on ambient noise tomography: a case study for the Longmen Shan area, eastern Tibetan Plateau

SUMMARY Ambient noise tomography (ANT) is a widely used method to obtain shear wave velocity structure in the crust and upper mantle. Usually, the topography is assumed to have negligible effect on the resulting models. This, however, might not be proper in regions with large topographic variation, such as plateau edges, submarine slopes and volcanic islands. In this study, we use synthetics from waveform-based numerical simulation to quantify the topography effect on ANT in the Longmen Shan area, eastern Tibetan Plateau margin. Three kinds of models are used in forward simulation to obtain theoretical waveforms, including Case1: the layered model, Case2: the layered model with topographic variation and Case3: the flattened model of Case2. The final inversion results show that the bias of ANT is negligible in the blocks with relatively flat topography, such as the interior regions of the Tibetan Plateau and the Sichuan Basin. However, for the Longmen Shan boundary zone with significant topographic variation (∼4 km), the shear wave velocity image has an obvious negative bias that can reach up to −4 per cent. The maximum depth of bias is ∼5 km, which is mirrored with the maximum topographic elevation difference of the region, and the average bias disappears as the depth decreases to the surface (0 km) or increases to three times of the maximum influence depth (∼15 km). The horizontal distribution of the tomographic bias is almost linearly related to the topographic elevation difference with a slope of −1.04 and a correlation coefficient of 0.90 at maximum influence depth. According to this first-order correction formula and the decreasing trend of average bias with depth, the topography effect on ANT can be suppressed to a certain extent.

Unraveling an enigmatic boundary along the Sunda-Banda volcanic arc

This study utilizes a range of complementary geological, geophysical, and geochemical data to analyze the structural, deformational, and compositional variations along the Sunda-Banda volcanic arc transitional zone from oceanic to continental subduction. The core component is a synthesis of three station-based anisotropy inferences and two 3-D shear wave velocity models in the crust and upper mantle, all of which are derived based on ∼5 years of broadband seismic data collected across eastern Indonesia. These seismic results are based on receiver functions, shear wave splitting, ambient noise tomography, and a new teleseismic surface wave tomography model. We observe distinct along-arc spatial variations in the shear wave velocity and anisotropic structure within the crust and mantle wedge above the subducting slab. An intriguing arc-perpendicular orientation is observed in crustal fabrics and mantle anisotropy consistently in central Flores, contrasting with the arc-parallel orientations on the western and eastern sides. The orientations correlate with the change in strike of the highest topography as well as the presence of ∼N-S aligned cinder cones in central Flores. The 3-D shear wave velocity model in the crust and uppermost mantle also presents a sharp structural change in central Flores that indicates a relatively thinner crust. At deeper depths, the newly imaged upper mantle structure exhibits along-arc variations, but the change across central Flores appears to be distributed more broadly and outlines a relatively slow mantle wedge beneath eastern Flores. These new observations roughly correlate with previously identified changes in volcanic geochemistry characteristics across central Flores. Our study links the surface topography, geology, and volcanism with subsurface crust and mantle structures, unraveling an enigmatic structural and/or compositional boundary along the Banda volcanic arc.

Shallow shear wave velocity structure of the Dongshan sag area using surface wave data in a deep reflection profile of the Yuanmou area of Yunnan province, China

Deep reflection seismic data contain abundant surface wave information for which the velocity structure in the near-surface shallow underground can be obtained using multichannel analysis of surface waves (MASW). This paper uses the MASW method to process deep reflection data in the Yuanmou area in Yunnan province, China. The MASW method uses the phase velocity spectral transformation of Rayleigh waves to obtain its dispersion curve. The initial model for different lithology classifications is established to invert the underground velocity structure. This study obtains a two-dimensional shear wave velocity structure profile to investigate the shear wave velocity structure. Generally, the study results have shown that the shear wave velocity structure obtained by the MASW method corresponds well with the surface conditions and accurately reflects faults' changes, folds, and lithology. Moreover, the formation sand content calculation shows a large difference in Dongshan sag's formation sand content. The sand contents of Cretaceous and Middle Jurassic strata are over 30% similar, and the sandstone content of underlying Early Jurassic strata is mostly 30% lower. The stratum buried depth of Early Jurassic strata is between 0– 800 m. The dip angle of LZJF at this location is 90°. The west side of the LZJF is based on Proterozoic uplift, and the Quaternary sedimentary thickness exceeds 300 m above the uplift stratum. Whereas on the east side of the LZJF, the stratum of the Dongshan sag section is mainly a synclinal structure.

3D shear-wave velocity structure for Oran city, northwestern Algeria, from inversion of ambient vibration single-station and array measurements

The city of Oran experienced several earthquakes. Therefore, the characterization of its subsurface is crucial for a better evaluation of the seismic hazard. Using ambient vibration single-station and array measurements at 193 sites, the HVSR curves showed a variation of the fundamental frequency peak between 0.3 and 7.4 Hz, growing towards the west, with maximum amplitude of ∼6. The shallow structure has been explored by means of F–K analysis. Joint inversion of the HVSR and Rayleigh wave dispersion curves showed 3 layers of soft and stiff sediments of varying thicknesses overlying the bedrock. The shear-wave velocity (Vs) of the sediments varies between 280 and 1300 m/s and that of the bedrock can reach 2500 m/s. Cross sections show a decrease in sediments thickness from east to west where bedrock outcrops. The maximum depth of bedrock is 1050 m northeast of the city. The obtained Vs models support a soil classification ranging from stiff soil to hard rock.

Empirical H/V spectral ratios at the InSight landing site and implications for the martian subsurface structure

SUMMARY The horizontal-to-vertical (H/V) spectral ratio inversion is a traditional technique for deriving the local subsurface structure on Earth. We calculated the H/V from the ambient vibrations at different wind levels at the InSight landing site, on Mars, and also computed the H/V from the S-wave coda of the martian seismic events (marsquakes). Different H/V curves were obtained for different wind periods and from the marsquakes. From the ambient vibrations, the recordings during low-wind periods are close to the instrument self-noise level. During high-wind periods, the seismic recordings are highly contaminated by the interaction of the lander with the wind and the martian ground. Therefore, these recordings are less favourable for traditional H/V analysis. Instead, the recordings of the S-wave coda of marsquakes were preferred to derive the characteristic H/V curve of this site between 0.4 and 10 Hz. The final H/V curve presents a characteristic trough at 2.4 Hz and a strong peak at 8 Hz. Using a full diffuse wavefield approach as the forward computation and the Neighbourhood Algorithm as the sampling technique, we invert for the 1-D shear wave velocity structure at the InSight landing site. Based on our inversion results, we propose a strong site effect at the InSight site to be due to the presence of a shallow high-velocity layer (SHVL) over low-velocity units. The SHVL is likely placed below a layer of coarse blocky ejecta and can be associated with Early Amazonian basaltic lava flows. The units below the SHVL have lower velocities, possibly related to a Late Hesperian or Early Amazonian epoch with a different magmatic regime and/or a greater impact rate and more extensive weathering. An extremely weak buried low velocity layer (bLVL) between these lava flows explains the data around the 2.4 Hz trough, whereas a more competent bLVL would not generate this latter feature. These subsurface models are in good agreement with results from hammering experiment and compliance measurements at the InSight landing site. Finally, this site effect is revealed only by seismic events data and explains the larger horizontal than vertical ground motion recorded for certain type of marsquakes.

Ambient Noise Tomography of Jinan: The Migration of Groundwater and the Formation of Geothermal Water

AbstractJinan is an important city in eastern China, with rich groundwater in the region. There are four famous springs in the urban area and an abundance of geothermal water in the northern part, which makes the migration of groundwater in this area a very important issue. To study the shallow shear wave velocity structure and groundwater migration in Jinan, we utilized almost a month of continuous waveform data from 175 short period seismometers deployed by the Chinese Academy of Geological Sciences, in order to calculate the cross‐correlation function. We picked 7749 group dispersion curves and 6117 phase dispersion curves with a period range of 0.2–2 s. Through inversion, we obtained the fine three‐dimensional shear wave velocity and azimuthal anisotropy structure (0–2.4 km). Combining the results with local geological and hydrological data, the following conclusions were reached. (1) There are widespread high velocity anomalies in the region between the Qianfoshan and Wenhuaqiao faults, as well as to the east of the Wenhuaqiao Fault, which may be related to the intrusive gabbro known as the Jinan Intrusive Rock. (2) The two distinct high velocity anomalies in our model (referred to as west and east Jinan Intrusive Rock in this paper) may indicate that the Jinan Intrusive Rock was broken through crustal movement. (3) There is an obvious low velocity layer under the intrusive rock, which could be the channel of groundwater migration. The precipitation in the southern mountain region seeps down into the ground, then is blocked by the Jinan Intrusive Rock and can only progress downwards to a deeper part, where the groundwater is heated by the geothermal gradient. The heated water finally arrives at the northern part and forms geothermal water. (4) The depth of the low velocity layer beneath the Jinan Intrusive Rock varies laterally, which may indicate that the depth of the groundwater migration is different beneath the west and east Jinan Intrusive Rock. (5) There is strong azimuthal anisotropy in southern Jinan, with nearly E‐W fast orientation, which may be related to the tilt limestone layering structure.

Plume-lithosphere interaction beneath southwestern Africa – Insights from multi-mode Rayleigh wave tomography

A stationary mantle plume heats the overlying lithosphere from below, generating melts and causing lithospheric erosion. When the lithosphere moves away from the plume, it can be re-established by melt residues that have been since attached, partly due to cooling, at the base of the lithosphere. The Etendeka flood basalt region in northwest Namibia used to overlie the Tristan da Cunha mantle plume in the Early Cretaceous and thus provides an ideal place to examine plume-lithosphere interaction. Here we determine the upper mantle shear wave velocity structure of southern Africa down to 400 km depth by waveform inversion of multi-mode Rayleigh waves in order to find the imprints left by the Tristan da Cunha mantle plume. Thick lithosphere with high shear wave velocities is observed beneath the Congo and Kalahari Cratons, extending down to 200 km depth with the largest thickness beneath the Limpopo Belt. Along the landfall of the Walvis Ridge and Damara Belt, our model reveals a thick lithosphere down to 100–200 km depths. The thick lithosphere seems to constitute the Congo Craton, but the thick Congo cratonic lithosphere is extended farther south than observed in most of the previous models. We propose that the lithosphere along the landward extension of the Walvis Ridge was affected by magmatic processes related to the plume, but has been partially reconstructed since then by melt depletion and successive cooling. A high velocity anomaly beneath the northwest coast of Namibia down to a depth of ∼80 km coincides with distribution of the Etendeka basalt at the surface and the seaward-dipping reflectors at the continental margin. At sub-lithospheric depths, a low-velocity anomaly (at 100–200 km depth) and a high-velocity anomaly (at 250–350 km depth) can be detected. The origin of these anomalies could be related to an ongoing edge-driven convection process.

Determination of Shear Wave Velocity Structure by Using Inversion of Horizontal/Vertical Spectral Ratios Obtained from Earthquake Records: Example from the East of Lake Van

Van Gölü doğusu farklı özellikteki aktif fayların varlığı sebebiyle deprem üretme potansiyeli yüksek olan bir bölgedir. Depremlere bağlı oluşan hasarların değerlendirilmesinde yeraltının fiziksel özelliklerinin ve deprem sırasındaki davranışının iyi bilinmesi gerekmektedir. Kayma dalgası hız değişimi ve anakaya derinliğinin belirlenmesi bu açıdan son derece önemlidir. Bu çalışmada 2011-2021 yılları arasında Van Gölü doğusunda meydana gelen ve farklı tipteki faylarda oluşmuş dokuz deprem verisinden yararlanılmıştır. 6 istasyonda kaydedilen depremler yatay-düşey spektral oran yöntemi ve Monte-Carlo ters çözüm algoritması ile analiz edilerek, kayma dalgası hız yapısı ve anakaya derinlikleri belirlenmiştir. İstasyonlar altında alüvyon birimlerinin kalınlığına bağlı olarak nispeten düşük frekans değerleri elde edilmişken, farklı kaya birimlerin varlığı baskın frekans değerlerini yükseltmiştir. Spektral oran eğrilerindeki farklı frekanslardaki pikler, jeolojik yapının özelliklerine bağlı olarak değişkenlik göstermiştir. İstasyon altı anakaya derinliği 10-350 m arasındadır. Artan anakaya derinlik seviyeleri yıkıcı depremlerin hasar oranını arttıran bir faktördür.