Rayleigh Wave Phase Velocity Dispersion Research Articles

3-D Shear wave velocity and azimuthal anisotropy structure in the shallow crust of Binchuan Basin in Yunnan, SW China, from ambient noise tomography

The Binchuan Basin in northwest Yunnan, SW China, is a rift basin developed at the intersection of the Red River Fault and Chenghai Fault, where active faults developed and historical earthquakes occurred. Understanding the fine velocity structure of the shallow crust in this region is crucial for comprehending earthquake occurrences and mechanisms and for improving earthquake location accuracy. Using the continuous waveform data recorded by 381 dense array stations in 2017, we obtained 7915 Rayleigh-wave phase velocity dispersion curves in the period band of 0.2-6 s from ambient noise cross-correlation functions after rigorous data processing and quality control. We determined 3-D isotropic and azimuthally anisotropic shear wave velocity models at depths above 6 km in the shallow crust based on the direct surface wave azimuthal anisotropic tomography method. The isotropic model reveals a strong correspondence between the S-wave velocity structure at depths of 0-1 km and the regional topography and lithology. The Binchuan depocenter, Zhoucheng depocenter, Xiangyun Basin, and Xihai Rift Basin are primarily composed of Quaternary deposits, which show low-velocity anomalies, while the regions with the Paleozoic shale, limestone, and basalt exhibit high-velocity anomalies. The nearly N-S orientation of fast directions from azimuthal anisotropy models are mainly controlled by the active Binchuan Fault with N-S strike as well as the NNW oriented primary compressive stress.

Along‐Strike Variations of Alaska Subduction Zone Structure and Hydration Determined From Amphibious Seismic Data

AbstractWe develop a 3‐D isotropic shear velocity model for the Alaska subduction zone using data from seafloor and land‐based seismographs to investigate along‐strike variations in structure. By applying ambient noise and teleseismic Helmholtz tomography, we derive Rayleigh wave group and phase velocity dispersion maps, then invert them for shear velocity structure using a Bayesian Monte Carlo algorithm. For land‐based stations, we perform a joint inversion of receiver functions and dispersion curves. The forearc crust is relatively thick (35–42 km) and has reduced lower crustal velocities beneath the Kodiak and Semidi segments, which may promote higher seismic coupling. Bristol Bay Basin crust is relatively thin and has a high‐velocity lower layer, suggesting a dense mafic lower crust emplaced by the rifting processes. The incoming plate shows low uppermost mantle velocities, indicating serpentinization. This hydration is more pronounced in the Shumagin segment, with greater velocity reduction extending to 18 ± 3 km depth, compared to the Semidi segment, showing smaller reductions extending to 14 ± 3 km depth. Our estimates of percent serpentinization from VS reduction and VP/VS are larger than those determined using VP reduction in prior studies, likely due to water in cracks affecting VS more than VP. Revised estimates of serpentinization show that more water subducts than previous studies, and that twice as much mantle water is subducted in the Shumagin segment compared to the Semidi segment. Together with estimates from other subduction zones, the results indicate a wide variation in subducted mantle water between different subduction segments.

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.

Shallow seismic structure around the Vrancea Seismic Zone from joint inversion of ambient noise H/V ratios and surface wave dispersion

The South-East Carpathians in Romania host the Vrancea Seismic Zone (VSZ), an intraplate seismic nest of frequent high magnitude earthquakes that cause unusually high damage over extended areas. Estimating the amplification of seismic signals strongly depends on the depth to the geophysical bedrock and the complex near surface geological structures. To improve seismic hazard estimates and ultimately mitigate seismic risk, we determine the frequency of resonance at 81 broadband seismic stations in southeast Romania using the horizontal-to-vertical spectral ratio (HVSR) of ambient seismic noise. We jointly invert HVSR functions with Rayleigh wave phase velocity dispersion curves using a Markov-chain Monte Carlo algorithm under the assumption of a diffuse field. Our results show excellent correlation between the fundamental frequency of resonance and local surface geology: stations located on foreland sedimentary basins show HVSR peaks in the range 0–1 Hz, while those located in orogenic nappes reach values as high as 10 Hz. Seismic bedrock topography mirrors the crystalline bedrock depth inferred from gravity modeling, but is consistently ∼2000 m shallower, corresponding to the interface between Paleozoic-Triassic to Jurassic-Cretaceous sedimentary levels on the Moesian Platform constrained by borehole data. The near surface seismic model shows anomalously low velocities in the Focsani forearc basin, adjacent to the VSZ, consistent with extremely low resonance frequencies and deep sedimentary layer successions (>5 km). In the back-arc of the Carpathian Orogen, at the contact with the Transylvanian Basin, average resonance frequencies are <2 Hz and inferred seismic bedrock depths vary laterally, likely affected by buried volcanic structures and diapiric salt domes. Our study presents the largest endeavor to study anomalous intensity patterns and local site effects in and around the VSZ and has fundamental implications for seismic risk evaluation and future civil engineering design planning.

3-D shallow shear velocity structure of the Jakarta Basin from transdimensional ambient noise tomography.

Situated on the northern coast of the Indonesian island of Java, Jakarta and its metropolitan area (Greater Jakarta) are subject to significant earthquake hazards from a subduction zone south of Java and nearby active crustal faults. The seismic risk may be even higher because Greater Jakarta resides on a sedimentary basin filled with thick Pliocene-Pleistocene sediments. A comprehensive study of Jakarta Basin's properties and geometry is important for creating robust seismic hazard and risk assessments. The main objective of this study is to develop a 3-D model of Jakarta Basin's shallow shear-wave velocity (VS ) structure and improve on previous models that did not cover the basin edge due to the extent of data coverage. Between April and October 2018, we deployed a new temporary seismic network to extend the spatial coverage beyond that of a previous deployment in 2013, and sampled 143 locations through sequential deployments of 30 broad-band sensors covering Jakarta and its adjacent areas. We conducted a 2-stage transdimensional Bayesian inversion of Rayleigh wave phase velocity dispersion curves derived from seismic noise. To begin, we applied tomography and constructed 2-D phase velocity maps for periods 1-5s. Then, at each point in a regular grid defined on these maps, we invert each dispersion curve into 1-D depth profiles of VS . Finally, these profiles at gridpoints with ∼2km spacing are interpolated to form a pseudo-3-D VS model. Our results reveal the edge of the Pliocene-Pleistocene sediments along the south. Also, we resolve a basement offset across south Jakarta that we suggest may be related to the western extension of the Baribis Fault (alternatively, the West Java Backarc Thrust). We recommend using this 3-D model of the Jakarta Basin for scenario earthquake ground motion simulations. Such simulations would help establish how important it might be to re-assess seismic hazard and risk in Greater Jakarta so that basin resonance and amplification are considered.

Multi-objective particle swarm optimization for multimode surface wave analysis

Obtaining S-wave velocity profiles by inverting Rayleigh wave dispersion curves is crucial for the Rayleigh wave exploration method. Rayleigh wave inversion is a highly nonlinear and multi-extremum problem; therefore, global optimization algorithms are superior to local optimization methods for this inversion problem. Higher modes of dispersion curves have deeper penetration, which can improve the inversion quality. This study utilizes the multi-objective particle swarm optimization (MOPSO) algorithm, rather than a traditional single-objective optimization algorithm, to obtain S-velocity profiles from Rayleigh wave multimode phase velocity dispersion curves. The uncertainty of the inversion parameters is evaluated through the morphology of the Pareto front and the standard deviation of the Pareto optimal solution set. This is the first application of MOPSO in solving the multimode Rayleigh wave phase velocity dispersion curves joint inversion problem. It inverts three synthetic datasets, compares the inversion results with the synthetic data, and then conducts a comparative analysis with the particle swarm optimization (PSO) algorithm and the multi-objective grey wolf optimization (MOGWO) algorithm in one of these synthetic models to evaluate the accuracy and stability of MOPSO to the joint inversion. In addition, this research inverts observed data from an expressway roadbed in Henan, China, to examine the applicability of MOPSO for the joint inversion of multimode dispersion curves. The MOPSO inversion closely yields S-wave velocity profiles that match the borehole data. The synthetic and observed data inversion results indicated that the MOPSO algorithm accurately inverts multimode dispersion curves.

A deep-learning-based approach for seismic surface-wave dispersion inversion (SfNet) with application to the Chinese mainland

Surface-wave tomography is an important and widely used method for imaging the crust and upper mantle velocity structure of the Earth. In this study, we proposed a deep learning (DL) method based on convolutional neural network (CNN), named SfNet, to derive the vS model from the Rayleigh wave phase and group velocity dispersion curves. Training a network model usually requires large amount of training datasets, which is labor-intensive and expensive to acquire. Here we relied on synthetics generated automatically from various spline-based vS models instead of directly using the existing vS models of an area to build the training dataset, which enhances the generalization of the DL method. In addition, we used a random sampling strategy of the dispersion periods in the training dataset, which alleviates the problem that the real data used must be sampled strictly according to the periods of training dataset. Tests using synthetic data demonstrate that the proposed method is much faster, and the results for the vS model are more accurate and robust than those of conventional methods. We applied our method to a dataset for the Chinese mainland and obtained a new reference velocity model of the Chinese continent (ChinaVs-DL1.0), which has smaller dispersion misfits than those from the traditional method. The high accuracy and efficiency of our DL approach makes it an important method for vS model inversions from large amounts of surface-wave dispersion data.

Two Unconnected Low-Velocity Zones in the Eastern Boundary of the Sichuan-Yunnan Block Revealed with High-Resolution Ambient Noise Tomography

The Anninghe-Zemuhe-Xiaojiang fault zone (AZXFZ) is an important boundary fault zone on the southeastern margin of the Tibetan Plateau, with frequent strong earthquakes. Previous studies have imaged widespread low-velocity zones in this area. However, there are still many disputes on the connectivity and genesis of the low-velocity zones. In this study, we obtain the Rayleigh wave phase velocity dispersion curves at 4–25-s periods using observations from 378 broadband stations located near the AZXFZ. The new 3D S-wave velocity model has a lateral resolution of about 30 km in 0–35-km depth and is obtained by direct inversion of surface wave dispersion data. The new results clearly image two low-velocity zones and a high-velocity zone in the middle crust of the study region. The low-velocity zone on the western side of the Lijiang-Xiaojinhe fault is related to the eastward flow of crustal material and the movement of the left lateral strike-slip faults in the Tibetan Plateau, while the low-velocity anomaly distributed along the Daliangshan fault and Xiaojiang fault is the superimposition effect of shear heating of the faults and upwelling of mantle material. The uplift of Gongga Shan is a combination of the continuous accumulation of crustal material in the middle and lower crust of the southeastern margin of the Tibetan Plateau as well as the bending and compression of the Sichuan-Yunnan Block and the Xianshuihe-Anninghe fault zone.

3-D azimuthal anisotropy structure reveals different deformation modes of the crust and upper mantle in the southeastern Tibetan Plateau

The tectonic evolution and deformation process of the southeastern Tibetan Plateau has been one of the focuses of the geoscience community in recent decades. However, the crustal and mantle deformation mechanism in the southeastern Tibetan Plateau is still under debate. Seismic anisotropy inferred from surface wave tomography could provide critical insights into the deformation mechanism of the Earth’s tectonosphere. Here, we constructed a 3-D azimuthal anisotropy velocity model with Rayleigh wave phase velocity dispersion data from 132 permanent stations to analyze the deformation modes in the southeastern Tibetan Plateau. In the upper crust, the azimuthal anisotropy near the main strike-slip faults exhibits strong magnitude with fast axis subparallel to the fault strike, which is consistent with the rigid block extrusion pattern. In the mid-lower crust, two low-velocity anomalies appeared beneath the Sonpan-Ganzi Terrane and Xiaojiang Fault zone with strong azimuthal anisotropy, which may indicate ductile deformation of the weak mid-lower crust. However, the two low-velocity anomalies are separated by a high-velocity barrier with weak azimuthal anisotropy in the inner zone of the Emeishan large igneous province. In the upper mantle, the anisotropy pattern is relatively simple in the rigid Yangtze Craton and mainly represents fossil anisotropy. In the southern part, the lithosphere thinned beneath the Indochina and Cathaysia Blocks, and the anisotropy is plausibly caused by the upwelling and lateral flows of upwelling hot asthenospheric materials.

The Urban Underground Space beneath the Karst Basin of Guilin, China, Revealed by Ambient Seismic Noise Tomography

Abstract Detection of hidden faults and sedimentary layers in the urban subsoil is significant for the utilization of the underground space, earthquake hazard mitigation, and so forth. Guilin, located in southwest China, is well known for the development of the most typical karst landform in the world and has become an international tourist city that needs scientific planning and knowledge of the urban underground space. After collecting waveform data recorded continuously over a period of about 1.5 months by a dense array of 114 short-period seismic stations installed in and around Guilin, we adopt ambient seismic noise cross-correlation method to extract Rayleigh-wave phase velocity dispersion curves within the period range from 0.5 to 5 s and to obtain a high-resolution S-wave velocity (VS) model of the shallow crust above 9 km using surface-wave tomography. The vertical VS gradient image indicates that the sediment thickness of the Guilin Karst basin is about 1–3 km. Sedimentary layers are relatively thick between Yanshan Mountain and Haiyang Mountain, and along the Yi river valley, where karst groundwater may be abundant and used as an important option for urban water supply. Both the absolute VS velocity image and the relative VS anomaly image clearly reveal the occurrence, location, and deep extension characteristics of major faults. Longsheng–Yongfu, Nanning–Guilin, Yaoshan–Yanshan, Baishi, and Guanyang–Hengyang may be regional deep faults cutting through the upper crust at least. The evolution of the basin is mainly controlled by the steep dipping Longsheng–Yongfu and Baishi faults, and partially controlled by the gently dipping Nanning–Guilin fault in the interior of the basin. The Nanning–Guilin fault is an active and partially buried fault obliquely crossing Guilin city. Urban seismic imaging, such as the new VS tomography presented here, can play an important role in understanding tectonic and tectonic-subsidence earthquake hazards associated with these buried faults.

Crustal structure beneath the northeastern margin of the Tibetan plateau and its surrounding regions revealed by direct surface wave radial anisotropy tomography

We assemble the continuous waveform data recorded by 571 broadband temporary seismic stations from 2014 to 2015 in the northeastern (NE) margin of the Tibetan plateau and its surrounding regions. Rayleigh and Love wave phase velocity dispersion curves (T = 5–50 s) are obtained by the image transformation technique. Then the phase velocity dispersion data are used to jointly invert the 3-D shear wave velocity (Vs) and radial anisotropy (ξ) models in the study region with the direct surface wave radial anisotropy tomography (DRadiSurfTomo) method. The ξ is negative and positive in the upper crust and mid-to-lower crust, respectively, indicating that the crustal deformation pattern is depth-dependent. The middle crust beneath the NE margin of the Tibetan plateau generally exhibits positive ξ. Two low-velocity zones (LVZs) are detected beneath the Songpan-Garzê block and the western Qinling orogen as well as the northern Qilian orogen. High-velocity zone (HVZ) and positive ξ are characterized in the mid-to-lower crust beneath both the Alxa block and the Ordos basin. Combining with the previous results, we propose these two blocks, as a whole belonging to a part of the North China Craton (NCC), effectively restrict the northeastward growth of the Tibetan plateau. The uneven crustal deformation along the boundary between the NE margin of the Tibetan plateau and its surrounding regions is likely to be affected by the far-field stress originating from the collision between the Indian and Eurasian plates.

Lithospheric Structure of the South India Precambrian Terrains From Surface Wave Tomography

AbstractWe have developed a 3‐D shear velocity (Vsv) model of Precambrian terrains of South India with a lateral resolution of 55 km to a depth of 250 km by inversion of fundamental mode Rayleigh wave phase velocity dispersion data in the period of 30–140s, combined with a well‐constrained crustal velocity model from an earlier study. The dispersion data were computed from 748 earthquakes recorded at 85 homogeneously distributed seismographs. Our velocity model shows 150–200 km thick lithosphere in most of the Archean Dharwar craton where Vsv is &gt;4.7 km/s in the depth of 50–100 km and progressively decreases to 4.6 km/s at 120–150 km followed by a constant velocity of 4.5 km/s beyond 150–200 km depth. It correlates well with the results of the petrological studies of kimberlite xenoliths. An extraordinary high shear velocity (up to 4.8 km/s) and thick lithosphere (150 km) are observed beneath the Proterozoic Carbonatite complex, located at the south‐eastern edge of the Dharwar craton. We infer compositional modification of lower lithosphere at the south‐western margin of the Dharwar craton and lithospheric erosion at the Granulite terrain both possibly due to interaction with the Marion mantle plume at ∼90 Ma. The region is underlain by a 2%–3% lower velocity channel at ∼180–220 km depth in the asthenosphere, uncorrelated with the overlying lithosphere, possibly due to relative motion between them. It may be attributed to deep Earth‐processes such as asthenospheric upwelling.

Measuring the phase of ambient noise cross correlations: anisotropic Rayleigh and Love wave tomography across the Oman Mountains

SUMMARY Ambient seismic noise tomography has, over the last two decades, developed into a well-established tool for imaging seismic properties of the Earth’s crust. Fundamental mode Rayleigh and Love wave phase velocity dispersion curves can be measured from ambient noise cross-correlation functions (CCF) either using a high-frequency approximation theory, or by fitting the spectrum of the CCF to a Bessel function. Here, we advance the latter approach and present an automated algorithm that fits the phase of the Hankel function to the phase of the causal symmetric part of the CCF in order to determine phase velocity curves as continuous functions of frequency. Synthetic tests verify the reliability of the proposed method in the presence of low signal-to-noise ratio (SNR). Moreover, usage of the phase allows for robust phase velocity measurements at longer periods than when using the zero crossings of the Bessel function only and is, therefore, particularly useful at short inter-station distances. In the frequency domain, acceptable bandwidths of smooth phase velocity curves are obtained in an automated procedure using a set of fine-tuned quality criteria. We apply the method to 2.5 yr of continuous waveform data recorded by 58 temporary and permanent broad-band seismic stations in northern Oman. We obtain 1072 and 670 phase velocity curves for Rayleigh and Love waves, respectively, in the period range of 2–40 s. The data are inverted for isotropic and azimuthally anisotropic period-dependent phase velocity maps. Synthetic reconstruction tests show that the phase velocity maps have a lateral resolution of ∼30 km. The results suggest distinctly different middle to lower crustal architecture between the northern and eastern Oman Mountains. Azimuthal anisotropy shows contrasting fast propagation orientations in the shallow and deep crust, which we attribute to stress-induced and structural anisotropy in the upper crust and to lattice-preferred orientation in the lower crust.

S-Wave Velocity Structure of the Crust and Upper Mantle beneath the North China Craton Determined by Joint Inversion of Rayleigh-Wave Phase Velocity and Z/H Ratio

Abstract The North China craton (NCC) is one of the oldest craton in the world. Since the Cenozoic, the NCC has undergone severe lithospheric thinning, accompanied by extensive crustal deformations and volcanic activities. To better understand the mechanisms of the crustal and lithospheric deformations and intraplate volcanisms, we construct a high-resolution 3D S-wave velocity model for the NCC by jointly inverting Rayleigh-wave phase velocity dispersion and Z/H ratio measurements. Across the NCC, our model reveals significant lateral variations. In the shallow crust, prominent low-velocity anomalies associated with thick sediments are resolved in the Bohai Bay basin, and the Ordos basin and its surrounding grabens. Meanwhile, our model also shows that sediments are thin or even missing in the southeast of the Ordos basin and other orogens in the study area. The sedimentary structures of the Bohai Bay basin and the surrounding grabens of the Ordos basin may be the superficial response to the subduction of the Pacific plate and the northeastward push of the Tibetan plateau, respectively. Under the Datong volcano, our model reveals an integrated low-velocity anomalies from the mid-to-lower crust to the mantle, which verifies that the Datong volcano has a deep origin. Besides, the low-velocity zone beneath the eastern NCC (ENCC) connects to the low-velocity anomalies under the Datong volcano in the upper mantle. Combining the previous studies, we further propose that the asthenospheric upwelling of the Datong volcano and the subduction of the Pacific plate may jointly contribute to the lithospheric thinning of the ENCC.

Determination of near-surface shear-velocity structure based on the joint inversion of Rayleigh-wave dispersion and ellipticity from multistation active-seismic records

The dispersive characteristics of high-frequency (≥2 Hz) Rayleigh waves have been widely applied to determine subsurface shear velocity ([Formula: see text]). However, the [Formula: see text] of the high-velocity surface layers (e.g., rigid roads in urban areas) and underlying layers is poorly constrained in the Rayleigh-wave dispersion inversion. Compared to the phase velocity, the Rayleigh-wave horizontal-to-vertical (H/V) ratios of particle motion (i.e., ellipticity) are particularly sensitive to shallow [Formula: see text] structures. Therefore, we have proposed a robust method for obtaining near-surface [Formula: see text] structures by inverting Rayleigh-wave dispersion and H/V data. Initially, we separate the fundamental-mode Rayleigh waves from the active-seismic records using a high-resolution linear Radon transformation method. Then, we extract the high-frequency H/V and interstation phase traveltimes from the separated fundamental mode using deconvolution interferometry and the frequency-time analysis method. Based on the phase traveltimes, we calculate the phase velocity dispersion using ray-based tomography. Finally, we perform the joint inversion of dispersion and H/V data to obtain the [Formula: see text] structure. According to the inversion results of synthetic and field data, the accuracy of the [Formula: see text] in the rigid surface layer and underlying layers is considerably improved. We find that the joint inversion of Rayleigh-wave phase velocity dispersion and H/V data from surface-wave surveys is a useful tool for accurately determining the complex shallow [Formula: see text] structures.

Lithospheric Structure underneath the Archean Tanzania Craton and Adjacent Regions from a Joint Inversion of Receiver Functions and Rayleigh-Wave Phase Velocity Dispersion

Abstract Lithospheric structure beneath the Archean Tanzania craton and adjacent regions, including segments of the East African rift system (EARS) and the Proterozoic–early Paleozoic orogenic belts between the EARS and the craton, is imaged by a joint inversion of receiver functions and Rayleigh wave dispersion measurements derived from ambient seismic noise for shorter periods and teleseismic data for longer periods. Our resulting crustal thickness, crustal VP/VS measurements and 3D shear-wave velocity model for the upper 120 km show a clear spatial correspondence with major surficial geological features. The new results suggest the presence of a mafic layer in the bottom of the crust of the entire Archean craton, which is previously only identified beneath the southern portion of the craton. High crustal VP/VS values measured in the Rungwe Volcanic Province and most areas of the Kenya and Tanganyika rift segments can be attributed to a combined result of basaltic sediments atop the crust, magmatic intrusion, and crustal partial melting. The Kivu Volcanic Province and parts of the Kenya rift segment are characterized by localized lower-than-normal crustal VP/VS values and shear velocities in the lower crust and uppermost mantle, which, given the presence of large volume of CO2 from surficial observations, can be best interpreted by CO2-filled fractures or conduits. Lower-than-normal shear velocities in the uppermost mantle are revealed beneath almost the entire study region with the lowest values found in all the three volcanic provinces. The low velocities are indicative of an underplated layer formed by mantle-derived magmatic materials trapped below the Moho. The relatively low velocities beneath the volcanic provinces might be caused by a higher degree of partial melting in the uppermost mantle.

Crust and upper mantle structure of East Asia from ambient noise and earthquake surface wave tomography

The complex tectonic background of East Asia makes it an ideal region for investigating the evolution of the continental lithosphere, for which high-resolution lithospheric structural models are essential. In this study, we measured Rayleigh-wave phase-velocity dispersion curves at periods of 10–120 s and group velocity dispersion curves at periods of 10–140 s using event records from more than 1,000 seismic stations in and around China. By jointly inverting new and previously published dispersion data from ambient noise and earthquakes, we developed a high-resolution shear-wave velocity model down to a depth of ∼300 km beneath East Asia. Our model revealed heterogeneous lithospheric structures beneath East Asia, and allowed us to investigate the velocity structure of the entire lithosphere. We also derived crustal and lithospheric thickness models from the three-dimensional (3D) shear-wave model, revealing strong spatial heterogeneity and a general thinning trend of lithospheric thickness from west to east across the study region. Overall, our models reveal important lithospheric features beneath East Asia and provide a valuable baseline dataset for understanding continental-scale dynamics and evolution.

Semi‐Supervised Surface Wave Tomography With Wasserstein Cycle‐Consistent GAN: Method and Application to Southern California Plate Boundary Region

AbstractMachine learning algorithm has been applied to shear wave velocity (Vs) inversion in surface wave tomography, where a set of starting 1‐D Vs profiles and their corresponding synthetic dispersion curves are used in network training. Previous studies showed that the performance of such trained network is dependent on the diversity of the training data set, which limits its application to previously poorly understood regions. Here, we present an improved semi‐supervised algorithm‐based network that takes both model‐generated and observed surface wave dispersion data in the training process. The algorithm is termed Wasserstein cycle‐consistent generative adversarial networks (Wasserstein Cycle‐GAN [Wcycle‐GAN]). Different from conventional supervised approaches, the GAN architecture enables the inclusion of unlabeled data (the observed surface wave dispersion) in the training process that can complement the model‐generated data set. The cycle‐consistency and Wasserstein metric significantly improve the training stability of the proposed algorithm. We benchmark the Wcycle‐GAN method using 4,076 pairs of fundamental mode Rayleigh wave phase and group velocity dispersion curves derived in periods from 3 to 16 s in Southern California. The final 3‐D Vs model given by the best trained network shows large‐scale features consistent with the surface geology. The resulting Vs model has reasonable data misfits and provides sharper images of structures near faults in the top 15 km compared with those from conventional machine learning methods.

Seismic anisotropy evidence for modified lithosphere below the Bohai Sea region, eastern North China Craton

The Bohai Sea is proposed as the center of deformation associated with destruction of the North China Craton, and its tectonic history is essential for understanding the response of craton evolution to oceanic subduction. However, interior deformation of the craton remains unclear due to a lack of high-resolution imaging in this region. Here, we present high-resolution 3-D azimuthally-anisotropic shear-wave velocity structure of the crust and uppermost mantle using direct inversion of Rayleigh-wave phase-velocity dispersion from ambient noise. The anisotropic velocity structures reveal significant consistency between the regional strain field and seismic activity in the shallow crust. The N-S fast direction and relatively high velocity anomalies observed in the uppermost mantle beneath the Tanlu fault zone, as well as prominent Moho uplift, support the conclusion that the Tanlu fault zone has facilitated lithospheric modification of the eastern North China Craton since late Mesozoic. Anisotropy images also indicate that the northern and southern Bohai Bay Basin have undergone different deformation histories. The strong and consistent azimuthal anisotropy within the uppermost mantle beneath the southern Bohai Bay Basin may originate primarily from re-orientation arrangement of the olivine induced by new lithosphere formed from rapid cooling of upwelling asthenosphere after lithosphere delamination due to the subduction of the (Paleo-) Pacific Plate.

Crustal and Upper Mantle Structure Beneath the Southeastern United States From Joint Inversion of Receiver Functions and Rayleigh Wave Dispersion

AbstractUsing data from 186 stations belonging to the USArray Transportable Array, a three‐dimensional shear wave velocity model for the southeastern United States is constructed for the top 180 km by a joint inversion of receiver functions and Rayleigh wave phase velocity dispersion computed from ambient noise and teleseismic earthquake data. The resulting shear wave velocity model and the crustal thickness and Vp/Vs () measurements show a clear spatial correspondence with major surficial geological features. The distinct low velocities observed in the depth range of 0–25 km beneath the eastern Gulf Coastal Plain reflect the thick layer of unconsolidated or poorly consolidated sediments atop the crystalline crust. The low (1.70–1.74) and slow lowermost crustal velocities observed beneath the eastern Southern Appalachian Mountains (including the Carolina Terrane and Inner Piedmont) relative to the adjacent Blue Ridge Mountains and Valley and Ridge can be interpreted by lower crustal delamination followed by relamination. The Osceola intrusive complex in the central Suwannee Terrane has similar crustal characteristics as the eastern Southern Appalachian Mountains and thus can similarly be attributed to crustal delamination/relamination processes. The Grenville Province and adjacent areas possess relatively high values which can be attributed to mafic intrusion associated with crustal extension in a recently recognized segments of the eastern arm of the Proterozoic Midcontinent Rift.