Rayleigh Wave Group Velocity Dispersion Research Articles

Understanding the structure of crust and shallow upper mantle beneath western Tibet through the joint inversion of Rayleigh wave group velocity dispersion with interpolated receiver functions

We present the 3-D shear wave velocity variationin the crust and upper mantle structure of western Tibet using a joint inversion of Rayleigh wave group velocity dispersion and interpolated receiver function. The lateral sampling of the receiver function and the surface wave dispersion is equalized by the interpolation scheme. This new method of spatial interpolation eliminates difficulties caused by back azimuthal variation in the receiver function. We have jointly inverted these interpolated receiver functions with the Rayleigh wave group velocity dispersions obtained from the earthquake and ambient noise tomography to map the tectonic structure of western Tibet. The study reveals remarkable variation of the seismic characteristics in the crust and shallow upper mantle of western Tibet. The mapped Moho depth is ~74 km in the northern part of the plateau, whereas it is ~69 km in the southern part. A thick mid-crustal slow-velocity zone from ~10km up to ~40 km depth has been observed, which could be attributed to partial melt. A noteworthy finding is the variation in slower crustal velocity between the south and north of the Lhasa Block (LB). We interpret this as the Indian crust that has been underthrusted beneath the Tibetan plateau, and the northern limit of the Indian lithosphere extends to the southern part of the LB.

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.

Crustal imbrication and mountain building in the Eastern Iranian Ranges; Insights from receiver function and Rayleigh wave dispersion analysis

The evolution of the Eastern Iranian Ranges (EIR) dates back to the Late Cretaceous, coinciding with the closure of the Neotethys and the accretion of the Lut Block to Eurasia. However, controversy remains regarding its orogenic processes due to limited knowledge of the lithospheric structure. To gain a better understanding of the tectonic processes and mountain formation in eastern Iran, we analyzed seismic data from 29 stations in the northern part of the EIR. We initially perform receiver function (RF) analyses on 2160 high-quality RFs, emphasizing single-station analysis and Common Conversion Point (CCP) stacking to examine structural variations. Subsequently, we jointly invert the RFs with Rayleigh wave group velocity dispersion curves to derive a shear-wave velocity (Vs) model for each station. Our findings reveal a complex structure with west-dipping interfaces, indicating crustal layering and imbrication. We have identified a significant west-dipping layer, approximately 100 km in length, which could potentially indicate an intra-continental decoupling or shear zone. This decoupling may be responsible for the underthrusting of the Eurasian margin beneath the Lut Block, resulting in a deepening of the Moho depth to over 50 km beneath its footwall, compared to the adjacent Moho depth of around 40 km. Additionally, this process has caused an increase in the topography of approximately 1 km. These findings reflect the potential westward underthrusting of the Eurasian margin. In the deeper parts of the model, we observe a low-velocity anomaly below the crust, shedding light on a weakened lithosphere that facilitates crustal deformation in response to the compressional forces of convergence.

High-resolution seismic velocity structure using induced inter-event interferometry at the geothermal sites, Geysers, Eastern California

The possibility of retrieving Rayleigh wave empirical Green's functions (EGFs) using virtual seismometers provides an opportunity to calculate high-resolution velocity models without a set of dense stations and/or expensive seismic imaging/monitoring. The high-resolution tomographic map can give an overview of the fracturing within the saturated rock. Theoretically, the cross-correlations of earthquakes contain significant information about the EGFs between event-pairs. In this work, we developed an application of virtual seismometers on a very small scale in superficial layers where anthropogenic seismicity occurred. To test this method's capabilities, we applied it to investigate the NW-part of The Geysers (TG) geothermal field. We used 1276 micro-earthquakes, occurred in a cuboid with the horizontal side of ∼1 × ∼2 km and the vertical edge at a depth from 0.9 to 2.2 km, to obtain the high-resolution (100 × 100 m) velocity structure. A group of strict criteria was imposed separately for single event waveform and event pairs to stabilize the tomographic results. After retrieving the inter-event EGF signal and calculating the Rayleigh wave group velocity dispersion curves, we obtained the group velocity maps for each horizontal layer. The tomographic maps as a function of depth indicated a low-velocity anomaly related to existing inactive faults, topography variation of the top of the steam zone, and low-velocity anomalies possibly stimulated by fluid injection. The topography of the top of the steam varies from ∼1.0 km to 1.3 km of maps, and also anomalies connected with fluid migration appear at the depth of injection wells (1.5–1.9 km).

Shear-wave velocity models in and around north-east India by modelling Rayleigh wave group velocity dispersion data

Shear-wave velocity models in and around north-east India by modelling Rayleigh wave group velocity dispersion data

High-resolution shallow crustal shear wave velocity structure of Anyuan mining area and its adjacent region in Jiangxi Province, China

High-resolution seismic image is critically important for mining minerals. In this study, we collected seismic data from a local dense seismic array consisting of 154 stations around the Anyuan mining area and its adjacent region of Pingxiang City, Jiangxi Province in South China, and applied the ambient noise tomography (ANT) method to image the shear wave velocity structure in the study area. Shallow crustal velocities at depths less than 3.3 km were determined by direct inversion of Rayleigh wave group velocity dispersion curves at the period range of 0.5–5.0 s. Overall, the S-wave velocity structure has a tight correlation with surface geological and tectonic features in the study area. The shear wave velocity structure in the shallow crust of the Anyuan Mine and its adjacent areas displayed distinct low-velocity anomalies, which can be attributed to the depression of sedimentary structures and coal mining activities in the Pingxiang-Leping region. The zones surrounding the Anyuan fault (AYF) and Wangkeng fault (WKF) zones exhibited low-velocity anomalies from the ground surface to ~ 3.3 km underground. And the low-velocity anomalies at depths less than 1.2 km could be related to the sedimentary environment of coal mine and the coal mining activities, while the low-velocity anomalies at depths below 1.2 km are caused by the presence of fracture medium, oil and gas in the fault zone. The shear wave velocity changes sharply across the AYF, and the characteristics of the velocity change interface indicate that the AYF is inclined toward the northwest, with its extension reaching depths of approximately 3 km underground.Graphical

Lithospheric architecture beneath the Amsterdam-St. Paul plateau, Southern Indian Ocean using the integrated gravity, magnetic and seismological study

Evolution of the Amsterdam-St. Paul (ASP) plateau is debated as a consequence of the Kerguelen mantle plume interaction with the South-East Indian Ridge (SEIR) or related with a weaker ASP plume origin. We performed an integrated geophysical approach using gravity and magnetic modeling along with the joint inversion of Ps receiver function and Rayleigh wave group velocity dispersion data to determine the crustal architecture, and upper mantle structure below the plateau. The gravity-magnetic model revealed that the plateau is associated with three crustal layers as basaltic layer, gabbroic layer and underplated material underlain by sedimentary strata. Our models further suggests that beneath the ASP plateau the Moho depth varies from ∼6.6 to 18.0 km. Joint inversion of Ps receiver function and Rayleigh wave group velocity dispersion curve suggested the Lithosphere-Asthenosphere Boundary at depth of 50 km below the Amsterdam Island. Further seismological result suggests evidence of a Low Velocity Zone below the Moho at depth of 20–36 km which may represent a magma chamber. Our model suggests that the evolution of the ASP plateau is guided by the Amsterdam and St. Paul fracture zones, which acted as a thermal boundary confining the horizontal movement of the ASP plume. We propose that the ASP plateau is not linked with the Kerguelen plume and emphasize origin of the ASP plateau by the interaction of SEIR with the ASP plume.

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.

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.

Shallow subsurface imaging across the Himalayan Frontal Thrust (HFT) analysing ambient noise data from a riverine source

Modelling subsurface geology across the Himalayan Frontal Thrust (HFT) is an essential task as it plays a significant role in the transfer of stresses from infrequent major earthquakes, which influence site characterization, groundwater estimates, and environmental engineering. But it is challenging to carry out a controlled-source seismic survey across HFT due to the difficult terrain. Interestingly, the Himalayan rivers flow downhill across the HFT and generate seismic noise that can be useful in extracting information about the subsurface geology. In this study, we record the noise generated by one such river, the Dabka, near Pawalgarh in the Nainital district of Uttarakhand, India, to understand the shallow subsurface geology across this region of the HFT of the Siwalik Himalayas. We use a network of 23 geophones of 5 Hz natural frequency, deployed for 46 days, to record the noise datasets. We cross-correlate these time series for the station pairs and extract the noise correlation functions (NCFs). Dispersion analysis of the NCFs provides the Rayleigh wave group velocity dispersion curves for the station pairs, using which we created 2D Rayleigh wave group velocity and shear wave velocity maps at different depths. These velocity-depth models demarcate the location of the HFT where there is a jump in the Rayleigh and shear wave velocities, with average standard deviations of ∼50 m/s and ∼ 93 m/s, respectively, across the HFT. The obtained shallow subsurface velocity models provide crucial information for geotechnical assessment and site characterization of this earthquake-prone region and thus can be useful for future hazard assessment.

3D-ambient noise surface wave tomography of Fogo volcano, Cape Verde

Fogo volcano belongs to the Cape Verde Archipelago, and it is one of the most active volcanoes in the Atlantic Ocean, which most recent eruption occurred from November 2014 to February 2015. We analyzed ambient seismic noise recordings of three different networks deployed in the island, totalizing 14 seismic stations, to derive a crustal 3D shear-wave crustal velocity model of the volcano. Through the phase cross-correlation technique followed by a time-domain phase weighted stack, we were able to measure Rayleigh wave group-velocity dispersion measurements in the period range from 1.0 to 10 s. These dispersion measurements were used to invert for 2D group velocity maps at selected periods, and then inverted to produce a 3D shear-wave velocity model of the island. The tomographic model shows three velocity domains. First, an asymmetric upper layer, above 5–6 km of depth, with lower velocities concentrated in the northeastern sector of the island and a clear higher-velocity horizontal body at 3–4 km of depth in the southwestern sector of the island; the spatial correlation between these two velocity zones and the Galinheiros normal fault suggests a genetic link between the high velocities and long-term surface deformation, which we related to sill intrusions between 3 and 4.5 km depth, beneath the southwestern sector of the island. Second, a marked higher-velocity horizontal layer in between 5 and 6 km and 8–9 km, interpreted as the seismic expression of pervasive sill and laccolith intrusions, now cooled, beneath the volcanic edifice and within the underlying oceanic crust. Third, a lower velocity layer below 8–9 km of depth, more pronounced beneath the northeastern sector, which could be explained by a hotter and possibly melt-rich zone beneath the volcano or a significantly altered/serpentinized crust. Finally, our study also confirms that Fogo lacks any sizable magma chambers (ancient or recent) within the volcanic edifice, in agreement with other geophysical and petrological studies. These observations demonstrate that 3D-ambient noise Rayleigh wave tomography is a powerful tool to image the crustal and upper mantle structure beneath volcanic islands, as shown here for Fogo volcano.

Crustal and uppermost mantle structure of Cape Verde from ambient noise tomography

SUMMARY We present a seismic ambient noise tomography of the Cape Verde archipelago, located in the Atlantic Ocean, approximately 600 km west of Senegal. We used 38 seismic broad-band stations that continuously recorded for 10 months, in order to construct the first 3-D model of Sv-wave velocities for the crust and uppermost mantle beneath the Cape Verde region. We started by computing phase cross-correlations for vertical component recordings using all possible inter-island station pairs. Next, a time–frequency phase-weighted stack was applied to obtain robust Rayleigh-wave group-velocity dispersion curves in the period band between 10 and 24 s. Group-velocity maps at different periods are obtained by inverting the dispersion curves. We then inverted the group-velocity maps to obtain the 3-D shear wave velocity structure of the crust and uppermost mantle beneath Cape Verde. The final 3-D model extends from 8 km down to 23 km and has a lateral resolution of about 50 km. The crust in the southwestern sector, encompassing Fogo, presents lower S-wave velocities that may be caused by the presence of melt pockets and/or hydrothermal fluids circulation. The uppermost mantle beneath the northwestern sector is characterized by higher S-wave velocities in agreement with previous results obtained from Ps and Sp receiver functions. Those high-velocity anomalies can reflect non-altered crust or remnants of magma chambers or solidified basaltic intrusions, which fed the volcanism in these islands. Our maps revealed the presence of crustal underplating across the entire archipelago, yet stronger beneath the groups Santo Antão—São Vicente—São Nicolau and Fogo—Santiago—Maio.

Shear-wave velocity structure of upper mantle along the Zagros collision zone

The lithosphere beneath the Zagros and central Iran has sustained long-lasting Neo-Tethyan subduction and subsequent collisional orogenesis, leading to the complicated crustal and lithospheric structure beneath the Iranian plateau. To better understand the geotectonic and geodynamic evolution of the Iranian plateau, we estimate the shear-wave velocity structure along profiles perpendicular to the Zagros using a self-parameterized Bayesian inversion of the Rayleigh wave group velocity dispersion curves in the period range of 5 to 120 s. Our velocity models, in line with a segmented slab model, show that different geodynamic processes have dominated along the Zagros. In the northern Zagros, the thick and high-velocity lithosphere beneath the Sanandaj-Sirjan Zone (SSZ) and Urumieh-Dokhtar Magmatic Arc (UDMA) is abruptly separated from the low-velocity lithosphere beneath central Iran, while the high-velocity anomaly underlies the UDMA and central Iran in the central and southern Zagros. We propose that the Arabian lithosphere near the suture has probably experienced a distributed thickening in the northern Zagros, and it underthrusts beneath central Iran in the central and southern Zagros. In the central and southern Zagros, a high-velocity anomaly elongates at a depth of 80–120 km between the low-velocity lithospheres of the UDMA and Lut block. This suggests that flat-slab subduction has not extended to the east of the Iranian plateau, therefore, the Eocene-Oligocene flare-up magmatism in the Lut block may be directly independent of the Neo-Tethys subduction. This study suggests that lateral tearing might be responsible for the slab detachment beneath the central and southern Zagros. The slab lithosphere may have started to tear from beneath the central UDMA and have propagated to the edges since ~10 Ma. Furthermore, the southeasternmost Zagros in the vicinity of the Makran zone, as a corner continental collision, has probably experienced slab tearing and break off during the Middle-Late Miocene.

Ambient seismic noise monitoring and imaging at the Theistareykir geothermal field (Iceland)

In autumn 2017 a network of 14 broadband seismic stations was deployed at the Theistareykir high temperature geothermal field (NE Iceland). This experiment was conducted as part of the current efforts to characterize the field's main structures, and possible short and long term stress variations due to the ongoing fluid injection and extraction operations which started in autumn 2017. In this work, we use two years of continuous seismic records (October 2017–October 2019) to compute a 3D shear wave velocity model of the geothermal field and to detect possible crustal stress changes related to the injection and production activities. From phase cross-correlations of the vertical component recordings, we measure the Rayleigh wave group velocity dispersion curves to obtain 2D group velocity maps between 1 and 5 s. Subsequently, we use a neighborhood algorithm to retrieve the 3D shear wave velocity model of Theistareykir. Mainly, two sets of elongated high and low velocity anomalies can be observed oriented in a NW/WNW direction, parallel to the lineaments of the active Tjörnes fracture zone. Velocity reductions west of Ketilfjall and at Baerjafjall could indicate the location of upflow zones of the magmatic reservoir or hydrothermal system. We analyzed the temporal evolution of phase and amplitude of phase auto-correlations using the stretching technique and discuss their behavior in relation to the geothermal field operations. We notice a slightly stronger long-term velocity decrease in the reservoir region compared to outer regions. This could be related to the mass depletion in that area (higher fluid extraction compared to the water reinjection). In summary, our findings show how a monitoring network can be set up to enable a detailed imaging and monitoring of reservoir behavior in general.

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.

Shear velocity structure beneath Northern Algeria from Rayleigh-wave analysis

The few scientific studies of the crust and upper mantle in Northern Algeria are based mainly on regional Western Mediterranean studies. This area has a certain degree of deformation due to its location between the African and Eurasian tectonic plates. This study continues previous studies reporting possible traces of subduction zones or hot spots throughout the Mediterranean basin. After installing 17 permanent broadband seismic stations in this area, we used teleseismic events recorded between 2007 and 2016 to define the shear wave velocity (Vs) structure as a function of depth by inverting the Rayleigh wave group velocity dispersion curves using the single-station method. The group velocity measurements for periods of 5–200 s were selected through the inversion of the regionalized dispersion curve to compute average 1-D shear-velocity models at area around station, which helped generate the relative shear velocity variation (Δ Vid) maps at different depths for the study area. From the variation of Vs, we deduced Moho depth ranges of 20–50 km for the study area. From the Δ Vid maps, we noticed that for layers deeper than 60 km, the failed subduction slab traces could be observed in the eastern part. The central-western part appears to be congruent with the direction of convergence between the two tectonic plates. This finding is consistent with the results obtained from the GPS study.

Surface‐Wave Tomography of the Northern Canadian Cordillera Using Earthquake Rayleigh Wave Group Velocities

AbstractWe measure fundamental‐mode Rayleigh wave group velocity dispersion from regional earthquakes and employ transdimensional Bayesian tomography to invert for azimuthally anisotropic group velocity maps at 10–60 s periods. Seismic azimuthal anisotropy within the crust and uppermost mantle shows fast‐axis alignment with principal northern Canadian Cordillera (NCC) tectonic structures suggesting large‐scale and coherent deformation across the crust and upper mantle. We extract group velocity dispersion curves at each grid point of the isotropic group velocity maps and invert them independently for VS as a function of depth in a probabilistic framework. At mid‐to‐lower crustal depths, our pseudo 3‐D VS model reveals an anomalously low VS structure (<3.8 km/s) across the NCC, which likely reflects elevated crustal temperatures that buoyantly support regional high elevations. Two high‐velocity regions (≥4.5 km/s) at uppermost mantle depths underpin the edges of the arcuate Mackenzie fold and thrust belt. We observe variations in the dip direction of the high‐velocity craton edge in the upper mantle, from subvertical in the northern NCC to westward in the southern NCC. We interpret a low‐velocity region (≤4.3 km/s) between the Tintina and Denali faults at uppermost mantle depths to reflect upwelling asthenosphere, which likely postdates 430 km of previously estimated Eocene lithosphere‐scale horizontal displacement along the Tintina fault. Our surface‐wave model resolves a high‐velocity region (≥4.4 km/s) dipping north‐northeast below the Wrangell volcanic field, which we interpret as a down‐going segment of the Pacific slab and may contribute to the observed volcanism in the area.

Mapping lithospheric seismic structure beneath the Shillong plateau (India) and adjoining regions by jointly fitting receiver functions and surface wave dispersion

SUMMARYThe northeastern Indian region is characterized by complex lithospheric structure that developed due to collision between the Indian and Eurasian tectonic plates, in the north, and to subduction beneath the Burmese arc, in the east. We report results from joint modelling of Ps and Sp receiver functions and Rayleigh wave group velocity dispersion curves in which a broad search for acceptable models is performed via simulated annealing. We identify three tectonic domains, the Shillong plateau, Brahmaputra valley and Indo-Burma convergence zone (IBCZ), sampled by teleseismic earthquake data recorded by nine broad-band seismic stations. Our results reveal that the region's thinnest crust lies beneath the Shillong plateau, where it increases slightly from the plateau's eastern edge to its centre and reaches a maximum at the western edge of the plateau. Crustal Vp/Vs ratios range between 1.69 and 1.75 for the Shillong plateau, which is consistent with a felsic composition. Deeper Moho depths beneath the Brahmaputra valley, adjacent to the northern front of the Shillong plateau, may be due to the flexure of Indian lithosphere subducting beneath Asia. Low velocity zones are indicated at ∼5–10 km depth beneath the Brahmaputra valley, which may have been developed by NE–SW trending compressional stresses from the collision at the Himalayan arc and subduction at the Burmese arc. The crust is thickest in Kohima, beneath the Naga thrust in the IBCZ, where a high velocity zone is observed for both Vp and Vs at a depth of 25–40 km. This anomaly may be associated with a high velocity slab, trending N–NE to S–SW, that comprises the subducting Indian lithosphere in the IBCZ.

Transversely isotropic lower crust of Variscan central Europe imaged by ambient noise tomography of the Bohemian Massif

Abstract. The recent development of ambient noise tomography, in combination with the increasing number of permanent seismic stations and dense networks of temporary stations operated during passive seismic experiments, provides a unique opportunity to build the first high-resolution 3-D shear wave velocity (vS) model of the entire crust of the Bohemian Massif (BM). This paper provides a regional-scale model of velocity distribution in the BM crust. The velocity model with a cell size of 22 km is built using a conventional two-step inversion approach from Rayleigh wave group velocity dispersion curves measured at more than 400 stations. The shear velocities within the upper crust of the BM are ∼0.2 km s−1 higher than those in its surroundings. The highest crustal velocities appear in its southern part, the Moldanubian unit. The Cadomian part of the region has a thinner crust, whereas the crust assembled, or tectonically transformed in the Variscan period, is thicker. The sharp Moho discontinuity preserves traces of its dynamic development expressed in remnants of Variscan subductions imprinted in bands of crustal thickening. A significant feature of the presented model is the velocity-drop interface (VDI) modelled in the lower part of the crust. We explain this feature by the anisotropic fabric of the lower crust, which is characterised as vertical transverse isotropy with the low velocity being the symmetry axis. The VDI is often interrupted around the boundaries of the crustal units, usually above locally increased velocities in the lowermost crust. Due to the north-west–south-east shortening of the crust and the late-Variscan strike-slip movements along the north-east–south-west oriented sutures preserved in the BM lithosphere, the anisotropic fabric of the lower crust was partly or fully erased along the boundaries of original microplates. These weakened zones accompanied by a velocity increase above the Moho (which indicate an emplacement of mantle rocks into the lower crust) can represent channels through which portions of subducted and later molten rocks have percolated upwards providing magma to subsequently form granitoid plutons.

The transition zone between the Eastern Alps and the Pannonian basin imaged by ambient noise tomography

The aim of this study was to establish a 3D shear-wave velocity model of the transition zone between the Eastern Alps and the Pannonian basin by means of ambient noise tomography. Datasets from permanent networks as well as past and recently deployed temporary networks of broadband seismic stations have been processed. Empirical Green's functions were estimated via cross-correlation of vertical component ambient seismic noise and further processed to obtain Rayleigh-wave group-velocity dispersion curves. They were directly inverted for shear velocities through a wavelet-based sparsity-constrained tomography method avoiding the intermediate step of constructing 2D group velocity maps. A horizontal resolution of 0.2° in the upper crust and 0.3°–0.5° in the lower crust was achieved. Our upper crustal shear velocity model revealed low velocities in the sedimentary depocenters and high velocities below the surrounding mountains. However, at greater depths (24–29 km), high shear velocities beneath the basins suggest crustal thinning accompanied by mantle updoming. An extremely deep low velocity anomaly was mapped beneath the Vienna basin, which we argue is caused by either sediment transfer to the lower crust; ductile deformation suggested by seismic anisotropy; or the presence of fluids.