Velocity Dispersion Data Research Articles

Technique for Combining Surface Wave Tomography with Receiver Function Results for Studying Upper Mantle Velocity Structure

Abstract—A technique is proposed for joint inversion of the surface-wave velocity dispersion data from long travelpaths and the results of the receiver function method. The technique is based on a modification of a surface-wave tomography procedure and allows for the use of the data not only from travelpaths but also obtained at individual isolated points. Initially, the data are reduced to a single type: either to surface wave velocity by calculating the dispersion curve from the vertical velocity section obtained by the receiver function method or to shear wave velocity by one-dimensional inversion of surface-wave dispersion curves from travelpaths. Thus, from the computational point of view, the problem reduces to the following one: to obtain a smoothed estimate of a two-dimensional function from the values ​​of this function at individual points and its average values along lines (travelpaths) which can be considered as straight. A direct application of a surface-wave tomography procedure under the assumption that the points are lines of zero length is impossible because the solution turns out to be singular at the points. To get rid of this obstacle, it is proposed to replace the points by circles of a small radius surrounding these points. Then, the problem of tomography is generalized by using data not only from linear travelpaths, but also from circular travelpaths. A numerical study has shown that an addition of point data results in an increase in the resolution.

High-resolution 3-D crustal shear-wave velocity model reveals structural and seismicity segmentation of the central-southern Tanlu Fault zone, eastern China

The Early-Mesozoic collision between the North and South China Cratons resulted in different tectonic and geologic structures including the Tanlu Fault zone. One way to understand the geodynamic processes beneath these structures is by studying the velocity structures and spatial distribution of earthquakes in the region. We performed ambient noise tomography for the region around the central-southern Tanlu Fault zone using the vertical component of continuous ambient noise data obtained in central East China. Empirical Green's functions were retrieved from ambient noise cross-correlation functions, from which we extracted the Rayleigh wave phase velocity dispersion curves and performed quality control on the dispersion data. The direct surface wave tomography method was used to invert all phase velocity dispersion data for the 3-D shear-wave velocity model in the crust and uppermost mantle. Our results show velocity segmentations with high-velocity structures existing beneath orogenic belts and uplifts within the first 10 km depths which correspond to the High-Pressure/Ultra-High Pressure Metamorphic rocks in the area. The existence of high-velocity structures beneath orogenic belts at shallow depths implies that the exhumation of eclogites is confined to the uppermost crust. Generally, fault zones in this area are characterized by a low velocity from 0 to 5 km and relatively high velocity in the lower part of the upper crust and the middle crust. The spatial distribution of earthquakes along Tanlu Fault reveals two seismicity zones: the southern segment and the Suqian-Tancheng segment with a low-velocity transitional zone at Suqian, which suggests that the two segments respond differently to stresses. Regions of dense earthquake distribution correspond to the transition zones of high to low shear-wave velocities, which could be as a result of instability in the upper mantle and variation in compositions of crustal materials.

Shear wave velocity structure of the upper-mantle beneath the northern Zagros collision zone revealed by nonlinear teleseismic tomography and Bayesian Monte-Carlo joint inversion of surface wave dispersion and teleseismic P-wave coda

The continental collision between the Arabian and Eurasian plates plays an important role in the geodynamical evolution of the Zagros and Iranian plateau. In order to investigate the upper mantle structure across the collision zone, we calculated the absolute and relative shear-wave velocity structure along a dense and long temporary seismic profile in NW Iran. The probabilistic 1-D absolute shear-wave velocity (Vs) depth profiles beneath 23 seismic stations were calculated using a trans-dimensional joint inversion approach of P-wave coda together with the Rayleigh wave phase and group velocity dispersion data. These 1-D models are then interpolated to obtain a 2-D Vs model along the profile. In addition, we used a nonlinear teleseismic tomography method to determine shear wave velocity variations beneath 44 seismic stations. Both the absolute and relative velocity models, obtained by different data sets and methods, show a thick (>150 km) high-velocity anomaly beneath the northern part of the Zagros, which could be the thickened Zagros lithosphere associated with continental shortening. A relatively thin lithosphere (~80 km) and lower S-wave velocity beneath Central Iran may suggest a weakened Iranian mantle lithosphere by upwelling of the hot asthenosphere following slab break-off. The presence of a thickened lithosphere beneath the Zagros implies that the Arabian-Eurasian convergence is accommodated mainly as shortening in the mantle lithosphere beneath the Zagros, rather than underthrusting beneath Central Iran. A narrow low-velocity corridor between the Zagros lithosphere and overriding plate (Central Iran), south of the Sanandaj-Sirjan Zone (SSZ), probably due to delamination of the subducting Arabian lithosphere from the Zagros lower crust, likely prohibits transferring stress from the Zagros lithosphere to Central Iran, causing less deformation in the SSZ which is the edge of the overriding plate.

3-D shear wave velocity structure in the shallow crust of the Tanlu fault zone in Lujiang, Anhui, and adjacent areas, and its tectonic implications

The Tan-Lu fault zone is a large NNE-trending fault zone in eastern China. Investigations of the structures of the fault zone and its surrounding areas have attracted much attention. In this study, we used dense-array ambient noise tomography to construct a three-dimensional shear wave velocity model of shallow crust in an area about 80km × 70km in Lujiang, Anhui Province, eastern China. For approximately one month we collected continuous ambient noise signals recorded by 90 short-period seismographs in the region, and obtained the short-period Rayleigh wave empirical Green's functions between stations by the cross-correlation method; we also extracted 0.5–8 s fundamental mode Rayleigh wave group velocity and phase velocity dispersion curves. Based on the direct surface wave tomography method, we jointly inverted the group velocity and phase velocity dispersion data of all paths and obtained the 3-D shear wave velocity structure in the depth range of 0–5 km. The results revealed important geological structural features of the study area. In the north region, the sedimentary center of the Hefei Basin — the southwestern part of the Chaohu Lake — shows a significant low-velocity anomaly to a depth of at least 5 km. The southwestern and southeastern regions of the array are the eastern margin of the Dabie orogenic belt and the intrusion area of Luzong volcanic rocks, respectively, and both show obvious high-speed anomalies; the sedimentary area within the Tan-Lu fault zone (about 10 km wide) shows low-velocity anomalies. However, the volcanic rock intrusion area in the fault zone is shown as high velocity. Our shallow crustal imaging results reflect the characteristics of different structures in the study area, especially the high-speed intrusive rocks in the Tan-Lu fault zone, which were probably partially derived from the magmatic activity of Luzong volcanic basin. From the Late Cretaceous to Early Tertiary, the Tan-Lu fault zone was in a period of extensional activity; the special stress environment and the fractured fault zone morphology provided conditions for magma in the Luzong volcanic basin to intrude into the Tan-Lu fault zone in the west. Our 3-D model can also provide important information for deep resource exploration and earthquake strong ground motion simulation.

Reliable workflow for inversion of seismic receiver function and surface wave dispersion data: a \u201c13 BB Star\u201d case study

Non-linear inverse problems arising in seismology are usually addressed either by linearization or by Monte Carlo methods. Neither approach is flawless. The former needs an accurate starting model; the latter is computationally intensive. Both require careful tuning of inversion parameters. An additional challenge is posed by joint inversion of data of different sensitivities and noise levels such as receiver functions and surface wave dispersion curves. We propose a generic workflow that combines advantages of both methods by endowing the linearized approach with an ensemble of homogeneous starting models. It successfully addresses several fundamental issues inherent in a wide range of inverse problems, such as trapping by local minima, exploitation of a priori knowledge, choice of a model depth, proper weighting of data sets characterized by different uncertainties, and credibility of final models. Some of them are tackled with the aid of novel 1D checkerboard tests—an intuitive and feasible addition to the resolution matrix. We applied our workflow to study the south-western margin of the East European Craton. Rayleigh wave phase velocity dispersion and P-wave receiver function data were gathered in the passive seismic experiment “13 BB Star” (2013–2016) in the area of the crust recognized by previous borehole and refraction surveys. Final models of S-wave velocity down to 300 km depth beneath the array are characterized by proximity in the parameter space and very good data fit. The maximum value in the mantle is higher by 0.1–0.2 km/s than reported for other cratons.

Direct Inversion for Three‐Dimensional Shear Wave Speed Azimuthal Anisotropy Based on Surface Wave Ray Tracing: Methodology and Application to Yunnan, Southwest China

AbstractAzimuthal anisotropy retrieved from surface waves is important for constraining depth‐varying deformation patterns in the crust and upper mantle. We present a direct inversion technique for the three‐dimensional shear wave speed azimuthal anisotropy based on mixed‐path surface wave traveltime data. This new method includes two steps: (1) inversion for the 3‐D isotropic Vsv model directly from Rayleigh wave traveltimes and (2) joint inversion for both 3‐D Vsv azimuthal anisotropy and additional 3‐D isotropic Vsv perturbation. The joint inversion can significantly mitigate the trade‐off between strong heterogeneity and anisotropy. With frequency‐dependent ray tracing based on 2‐D isotropic phase speed maps, the new method takes into account the ray‐bending effect on surface wave propagation. We apply the new method to a regional array in Yunnan, southwestern China. Using Rayleigh wave phase velocity dispersion data in the period band of 5–40 s extracted from ambient noise interferometry, we obtain a 3‐D model of shear wave speed and azimuthal anisotropy in the crust and uppermost mantle in Yunnan. This model reveals that two midcrust low‐velocity zones are possible weak channels, and the azimuthal anisotropy at a depth of 5 to 30 km is mainly controlled by nearby strike‐slip faults, some of which also approximately coincide with the lateral boundaries of the crustal low‐velocity zones. Approximately south of 26°N, the upper crustal azimuthal anisotropy from our model is significantly different from the upper mantle anisotropy inferred by shear wave splitting, indicating different deformation styles between the crust and upper mantle in southern Yunnan.

Mapping Crustal Shear Wave Velocity Structure and Radial Anisotropy Beneath West Antarctica Using Seismic Ambient Noise

AbstractUsing 8‐ to 25‐s‐period Rayleigh and Love wave phase velocity dispersion data extracted from seismic ambient noise, we (i) model the 3‐D shear wave velocity structure of the West Antarctic crust and (ii) map variations in crustal radial anisotropy. Enhanced regional resolution is offered by the UK Antarctic Seismic Network. In the West Antarctic Rift System (WARS), a ridge of crust ∼26–30 km thick extending south from Marie Byrd Land separates domains of more extended crust (∼22 km thick) in the Ross and Amundsen Sea Embayments, suggesting along‐strike variability in the Cenozoic evolution of the WARS. The southern margin of the WARS is defined along the southern Transantarctic Mountains and Haag‐Ellsworth Whitmore Mountains (HEW) block by a sharp crustal thickness gradient. Crust ∼35–40 km is modeled beneath the Haag Nunataks‐Ellsworth Mountains, decreasing to ∼30–32 km thick beneath the Whitmore Mountains, reflecting distinct structural domains within the composite HEW block. Our analysis suggests that the lower crust and potentially the middle crust is positively radially anisotropic (VSH>VSV) across West Antarctica. The strongest anisotropic signature is observed in the HEW block, emphasizing its unique provenance among West Antarctica's crustal units, and conceivably reflects a ∼13‐km‐thick metasedimentary succession atop Precambrian metamorphic basement. Positive radial anisotropy in the WARS crust is consistent with observations in extensional settings and likely reflects the lattice‐preferred orientation of minerals such as mica and amphibole by extensional deformation. Our observations support a contention that anisotropy may be ubiquitous in the continental crust.

UGC 1378 – a Milky Way sized galaxy embedded in a giant low surface brightness disc

ABSTRACT The dominant physical processes responsible for the formation and longevity of giant gaseous and stellar discs in galaxies remain controversial. Although they are rare (less than 10 confirmed as of now), giant low-surface brightness (gLSB) discy galaxies provide interesting insights given their extreme nature. We describe observations of UGC 1378 including deep spectroscopy with the Russian 6-m telescope and multiband imaging with Binospec at the MMT. Galaxy UGC 1378 has both high surface brightness and an extended low surface brightness discs. Our stellar velocity dispersion data for the high surface brightness, Milky Way sized, disc appears inconsistent with a recent major merger, a widely discussed formation scenario for the very extended low surface brightness disc. We estimate the star formation rates (SFRs) from archival Wide-Field Infrared Survey Explorer data. The SFR surface density in the LSB disc is low relative to its gas density, consistent with recent gas accretion. We argue that the unusually large size of UGC 1378’s disc may be the product of a rich gas reservoir (e.g. a cosmic filament) and an isolated environment that has preserved the giant disc.

High-resolution 3D crustal S-wave velocity structure of the Middle-Lower Yangtze River Metallogenic Belt and implications for its deep geodynamic setting

The Middle-Lower Yangtze River Metallogenic Belt (MLYMB) is an important mineral resource region in China. High-resolution crustal models can provide crucial constraints to understand the ore-forming processes and geodynamic setting in this region. Using ambient seismic noise from 107 permanent and 82 portable stations in the MLYMB and the adjacent area, we present a new high-resolution 3D S-wave velocity model of this region. We first extract 5–50 s Rayleigh wave phase velocity dispersion data by calculating ambient noise cross-correlation functions (CFs) and then use the surface wave direct inversion method to invert the mixed path travel times for the 3D S-wave velocity structure. Checkerboard tests show that the horizontal resolution of the 3D S-wave velocity model is approximately 0.5°–1.0° and that the vertical resolution decreases with increasing noise and depth. Our high-resolution 3D S-wave velocity model reveals: (1) A V-shaped high-velocity zone (HVZ) is located in the lower crust and the uppermost mantle in the study region. The western branch of the HVZ passes through the Jianghan Basin, the Qinling-Dabie orogenic belt and the Nanxiang Basin. The eastern branch, which almost completely covers the main body of the MLYMB, is located near the Tanlu Fault. The low-velocity anomalies between the western and eastern branches are located in the area of the Qinling-Dabie orogenic belt. (2) High-velocity uplifts (HVUs) are common in the crust of the MLYMB, especially in the areas near the Tanlu Fault, the Changjiang Fault and the Yangxin-Changzhou Fault. The intensities of the HVUs gradually weaken from west to east. The V-shaped HVZ in the lower crust and uppermost mantle and the HVUs in the middle and lower crust likely represent cooled mantle intrusive rocks. During the Yanshanian period, fault systems formed in the MLYMB due to the convergence between the South China Plate and the North China Plate, the multiple-direction drifting of the Paleo-Pacific Plate and its subduction beneath the Eurasian Plate. The dehydration of the cold oceanic crust led to partial melting in the upper mantle. Temperature differences caused strong convection of the upper mantle material that underplated the lower crust and rose to near the surface along the deep fault systems. After mixing with the crustal materials, mineralization processes, such as assimilation and fractional crystallization, occurred in the MLYMB.

Surface wave tomography of the arctic from seismic Rayleigh and Love wave group velocity dispersion data

The results of studying the deep structure of the Earth’s crust and upper mantle of the Arctic from surface wave data are presented. For this purpose, based on the frequency-time analysis procedure, a representative dataset of group velocity dispersion curves of seismic Rayleigh and Love waves (1555 and 1265 paths, respectively) in the period range from 10 to 250 s is obtained. With the use of a two-dimensional tomography technique for a spherical surface, group velocity distributions are calculated at separate periods. Overall, 18 maps for each type of surface waves are constructed and the horizontal resolution of the mapping is estimated. For four tectonically different regions of the Arctic, the dispersion curves calculated from the tomography results are inverted for the velocity sections of the SV- and SH-waves. Based on the obtained distributions, the main large-scale features are analyzed in the deep structure of the Earth’s crust and upper mantle of the Arctic, and the revealed velocity irregularities are correlated to various geological structures. The results of the study are of considerable interest for further constructing the three-dimensional model of the shear wave velocity distributions and for studying the anisotropic properties of the upper mantle of the Arctic, as well as for building the geodynamical models of the region.

Surface Wave Tomography of the Arctic from Rayleigh and Love Wave Group Velocity Dispersion Data

The results of studying the deep structure of the Earth’s crust and upper mantle of the Arctic from surface wave data are presented. For this purpose, based on the frequency-time analysis procedure, a representative dataset of group velocity dispersion curves of seismic Rayleigh and Love waves (1555 and 1265 paths, respectively) in the period range from 10 to 250 s is obtained. With the use of a two-dimensional tomography technique for a spherical surface, group velocity distributions are calculated at separate periods. Overall, 18 maps for each type of surface waves are constructed and the horizontal resolution of the mapping is estimated. For four tectonically different regions of the Arctic, the dispersion curves calculated from the tomography results are inverted for the velocity sections of the SV- and SH-waves. Based on the obtained distributions, the main large-scale features are analyzed in the deep structure of the Earth’s crust and upper mantle of the Arctic, and the revealed velocity irregularities are correlated to various geological structures. The results of the study are of considerable interest for further constructing the three-dimensional model of the shear wave velocity distributions and for studying the anisotropic properties of the upper mantle of the Arctic, as well as for building the geodynamical models of the region.

Transdimensional ambient noise tomography of Bass Strait, southeast Australia, reveals the sedimentary basin and deep crustal structure beneath a failed continental rift

Debate is ongoing as to which tectonic model is most consistent with the known geology of southeast Australia, formerly part of the eastern margin of Gondwana. In particular, numerous tectonic models have been proposed to explain the enigmatic geological relationship between Tasmania and the mainland, which is separated by Bass Strait. One of the primary reasons for the lack of certainty is the limited exposure of basement rocks, which are masked by the sea and thick Mesozoic–Cenozoic sedimentary and volcanic cover sequences. We use ambient noise tomography recorded across Bass Strait to generate a new shear wave velocity model in order to investigate crustal structure. Fundamental mode Rayleigh wave phase velocity dispersion data extracted from long-term cross-correlation of ambient noise data are inverted using a transdimensional, hierarchical, Bayesian inversion scheme to produce phase velocity maps in the period range 2–30 s. Subsequent inversion for depth-dependent shear wave velocity structure across a dense grid of points allows a composite 3-D shear wave velocity model to be produced. Benefits of the transdimensional scheme include a data-driven parametrization that allows the number and distribution of velocity unknowns to vary, and the data noise to also be treated as an unknown in the inversion. The new shear wave velocity model clearly reveals the primary sedimentary basins in Bass Strait as slow shear velocity zones which extend down to 14 km in depth. These failed rift basins, which formed during the early stages of Australia–Antarctica break-up, appear to be overlying thinned crust, where high velocities of 3.8–4.0 km s−1 occur at depths greater than 20 km. Along the northern margin of Bass Strait, our new model is consistent with major tectonic boundaries mapped at the surface. In particular, we identify an east dipping velocity transition zone in the vicinity of the Moyston Fault, a major tectonic boundary between the Lachlan and Delamerian orogens, which are part of the Phanerozoic accretionary terrane that makes up eastern Australia. A pronounced lineament of high shear wave velocities (∼3.7–3.8 km s−1) in the lower crust of our new model may represent the signature of relict intrusive magmatism from failed rifting in the early stages of Australia–Antarctica break-up along a crustal scale discontinuity in the Selwyn Block microcontinent which joins Tasmania and Victoria.

Seismic structure beneath the Reykjanes Peninsula, southwest Iceland, inferred from array-derived Rayleigh wave dispersion

The aim is to obtain a site-specific S-wave structure beneath the Reykjanes Peninsula, southwest Iceland. Nine broadband stations of the Reykjanet network are used to find Rayleigh-wave phase velocity dispersion in a relatively wide range of periods (from 3 to 50 s). The records analyzed were made in the years 2013 to 2015 and concern fourteen selected earthquakes whose epicentral distances range from tens of kilometers to almost ten thousand kilometers. Our approach to retrieving Rayleigh phase velocity dispersion involves two partly independent methods allowing for array apertures larger than a wavelength: 1) the zero-crossing point method, and 2) the phase-plane method. The two methods used here work with seismograms decomposed into quasi-harmonic components and implicitly assume a single plane wave propagation. The good match between the dispersion curves obtained by means of the two methods indicates that the assumption has been reasonably fulfilled. The Rayleigh wave phase velocity dispersion data are inverted into a horizontally layered isotropic S-wave velocity model of the Earth's crust and uppermost mantle by a modified method of the single-parameter variation. At shallow depths, the derived model is rather similar to some previous models that were derived predominantly from the arrival times of body waves. At depths exceeding about 20 km, the dispersion data require low S-wave velocities, indicating a noticeable low-velocity zone.

Seismic Imprints of Plume‐Lithosphere Interaction Beneath the Northwestern Deccan Volcanic Province

AbstractThe interaction of the Indian continental lithosphere with the Reunion plume resulted in massive outpouring of basalts manifested in the Deccan Volcanic Province. Although it is postulated that preexisting weak rift zones facilitated the eruption, deeper signatures of the plume remain contentious. In this study, we investigate the shear wave velocity (Vs) structure of the crust and upper mantle by inverting regionalized group velocity dispersion data in the period range of 6 to 100 s, down to 220 km depth, using the genetic algorithm technique. We used 1,286 dispersion curves derived from waveforms of 77 regional earthquakes recorded at 38 broadband stations. Our results reveal distinct intracrustal layers, Moho, mantle lid, and asthenospheric low‐velocity layer. Signatures of magmatic underplating are recognized in terms of high Vs and a thick crust beneath the Kutch seismic zone and Western Ghats. The lithospheric thickness varies from 80 to 124 km, being thinnest at the junction of Cambay and Narmada rifts, which could be the source zone of volcanic eruption. A thin lithosphere has also been observed beneath the Kutch seismic zone. A low‐velocity zone at depths ~80 km can be related with upwarping of the asthenosphere and/or presence of partial melts. A predominant low‐velocity zone beneath the Cambay, Saurashtra, and adjoining regions may be due to the effect of a residual thermal anomaly. A thin lithosphere beneath northwestern Deccan Volcanic Province could be the result of weakening due to plume‐lithosphere interaction, which facilitated volcanism through preexisting rift zones.

Was the Milky Way a chain galaxy? Using the IGIMF theory to constrain the thin-disc star formation history and mass

The observed present-day stellar mass function (PDMF) of the solar neighborhood is a mixture of stellar populations born in star-forming events that occurred over the life-time of the thin disk of the Galaxy. Assuming stars form in embedded clusters which have stellar initial mass functions (IMFs) which depend on the metallicity and density of the star-forming gas clumps, the integrated galaxy-wide IMF (IGIMF) can be calculated. The shape of the IGIMF thus depends on the SFR and metallicity. Here, the shape of the PDMF for stars more massive than $1\,M_\odot$ in combination with the mass density in low-mass stars is used to constrain the current star-formation rate (SFR), the star formation history (SFH) and the current stellar plus remnant mass ($M_*$) in the Galactic thin disk. This yields the current SFR, $\dot{M}_*= 4.1^{+3.1}_{-2.8}~M_\odot$yr$^{-1}$, a declining SFH and $M_*=2.1^{+3.0}_{-1.5}\times 10^{11}M_\odot$, respectively, with a V-band stellar mass-to-light ratio of $M_*/L_V=2.79^{+0.48}_{-0.38}$.These values are consistent with independent measurements. We also quantify the surface density of black holes and neutron stars in the Galactic thin disk. The invariant canonical IMF can reproduce the PDMF of the Galaxy as well as the IGIMF, but in the universal IMF framework it is not possible to constrain any of the above Galactic properties. Assuming the IGMF theory is the correct framework and in combination with the vertical velocity dispersion data of stars, it follows that the Milky Way would have appeared as a chain galaxy at high redshift.

A possible origin of intraplate earthquakes in the Kachchh rift zone, India, since the 2001 Mw7.7 Bhuj earthquake

We herein use the joint inversion of P-receiver functions and fundamental mode group velocity dispersion data (9–70 s) of Rayleigh and Love waves to estimate crustal and lithospheric thicknesses at twenty three-component mobile broadband stations in Kachchh, Gujarat. Modeled Moho depths range from 35 to 43 km, while lithospheric thicknesses vary from 64 to 106 km. The main result of our modelling is the delineation of a marked crustal (∼2–4 km) as well as lithospheric (∼10–20 km) thinning and a 2–6% drop in Vs across the lithosphere-asthenosphere-boundary (LAB), within the Samkhiali graben (associated with gravity high) underlying the central Kachchh rift zone (KRZ), where 95% of the continued aftershock activity took place since 2001. Such a large drop in Vs could be attributed to the presence of carbonatite melts in the upper mantle. Our modeling reveals a 4 km crustal thinning below the central KRZ and a 4 km crustal thickening below the surrounding riftless regions. This kind of crustal structure is inferred to induce large flexural deviatoric stresses (∼50–100 MPa) in the upper crust, thereby, these stresses in the presence of regional plate tectonic stresses can bring the Samkhiali graben below the central KRZ near to the critical stress level. While stress-transfer, meteoric water (in the upper crust), and metamorphic fluid as well volatile CO2 flows (in the lower crust) provide the required triggering effect to the critically stressed graben structure (down to 35 km depth) for generating continued aftershock activity in the Kachchh rift zone, since 2001. We also propose that the deeper circulation of volatile CO2 through the inferred conduit (related to the 65 Ma Deccan Plume activity) extending from lower crust down to asthenosphere plays a key role in the generation of uninterrupted occurrence of earthquakes in the Kachchh rift zone, Gujarat, India.

A Probabilistic Shear Wave Velocity Model of the Crust in the Central West Australian Craton Constrained by Transdimensional Inversion of Ambient Noise Dispersion

AbstractThe Capricorn Orogen in central Western Australia played important roles in initializing and finalizing the West Australian craton. Surface geological mapping and isotopic studies show that the crust has recorded over a billion years of tectonic history spanning from its crustal formation in the Archean to episodes of tectonothermal events during the Proterozoic cratonization processes. The region therefore provides us with an ideal laboratory to characterize the seismic signature associated with tectonic processes. We constructed a crustal shear wave velocity model of the core region of the orogen, the Glenburgh Terrane and its north boundary, by inverting the array group velocity dispersion data measured from a high‐density temporary array. A modified Bayesian transdimensional tomography technique, which incorporates a smooth‐varying regional reference velocity model and Moho topography, was used to invert for the crustal velocity variations. The inverted velocity model adds great detail to the intracrustal structure and provides complementary seismic velocity information to refine the regional tectonic processes. Distinct patterns in the velocity structure support that the Glenburgh Terrane is an Archean microcontinent and favor the role of Paleoproterozoic subductions/accretions during the assembly of the West Australian Craton.

Dynamical Constraints on the Dark Matter Distribution of the Sculptor Dwarf Spheroidal from Stellar Proper Motions

Abstract We compare the transverse velocity dispersions recently measured within the Sculptor dwarf spheroidal galaxy to the predictions of our previously published dynamical model. This was designed to fit the observed number count and velocity dispersion profiles of both metal-rich and metal-poor stars, both in cored and in cusped potentials. At the projected radius where the proper motions (PMs) were measured, this model (with no change in parameters) predicts transverse dispersions in the range of 6–9.5 km s−1, with the tangential dispersion about 1 km s−1 larger than the (projected) radial dispersion. Both dispersions are predicted to be about 1 km s−1 larger for metal-poor than for metal-rich stars. At this projected radius, cored and cusped potentials predict almost identical transverse dispersions. The measured tangential dispersion (8.5 ± 3.2 km s−1) agrees remarkably well with these predictions, while the measured radial dispersion (11.5 ± 4.3 km s−1) differs only at about the 1σ level. Thus, the PM data are in excellent agreement with previous data, but do not help to distinguish between cored and cusped potentials. This will require velocity dispersion data (either from PMs or from radial velocities) with uncertainties well below 1 km s−1 over a range of projected radii.

High-resolution surface wave tomography of the European crust and uppermost mantle from ambient seismic noise

Taking advantage of the large number of seismic stations installed in Europe, in particular in the greater Alpine region with the AlpArray experiment, we derive a new high-resolution 3-D shear wave velocity model of the European crust and uppermost mantle from ambient-noise tomography. The correlation of up to 4 yr of continuous vertical-component seismic recordings from 1293 broad-band stations (10 • W-35 • E, 30 • N-75 • N) provides Rayleigh wave group velocity dispersion data in the period band 5-150 s at more than 0.8 million virtual source-receiver pairs. 2-D Rayleigh wave group velocity maps are estimated using adaptive parametrization to accommodate the strong heterogeneity of path coverage. A probabilistic 3-D shear wave velocity model, including probability densities for the depth of layer boundaries and S-wave velocity values, is obtained by nonlinear Bayesian inversion. A weighted average of the probabilistic model is then used as starting model for the linear inversion step, providing the final V s model. The resulting S-wave velocity model and Moho depth are validated by comparison with previous geophysical studies. Although surface wave tomography is weakly sensitive to layer boundaries, vertical cross-sections through our V s model and the associated probability of the presence of interfaces display striking similarities with reference controlled-source seismology (CSS) and receiver function sections across the Alpine belt. Our model even provides new structural information such as an ∼8 km Moho jump along the CSS ECORS-CROP profile that was not imaged by the reflection data due to poor penetration across a heterogeneous upper crust. Our probabilistic and final shear wave velocity models have the potential to become new reference models of the European crust, both for crustal structure probing and geophysical studies including waveform modelling or full-waveform inversion.

Crustal Structure beneath the Kashmir Basin Adjoining the Western Himalayan Syntaxis

We present a crustal shear‐wave velocity model for the intermontane Kashmir valley derived from eight broadband seismic stations located on hard‐rock sites surrounding and within the valley. Receiver functions at these sites were calculated using the iterative time‐domain deconvolution method of Ligorria and Ammon (1999) jointly inverted with fundamental‐mode Rayleigh‐wave group velocity dispersion data to estimate the underlying shear‐wave velocity structure. The inverted Moho depths were further constrained within ±2 km by forward modeling and conform to those independently estimated using H‐κ (crustal thickness vs. VP/VS⁠) stacks. The Moho descends steeply (⁠>15° NE⁠) beneath the Pir Panjal range from a depth of ∼40 km south of the Main Central thrust to ∼58 km on the southwestern flank of the valley and undergoes an ∼4 km upwarp beneath the valley, thinning the overlying crust. Further northeast (NE) of the valley, the Moho dips gently (⁠∼3°⁠) beneath the Zanskar range. A persistent low‐velocity zone at a depth of ∼12–16 km is interpreted as a sheared zone associated with the decollement separating Himalayan rocks from the Indian plate. Relocated seismicity from the International Seismological Centre (ISC, 2013) catalog (1964–2013), together with a small number of local earthquakes recorded by our network, is apparently confined south of the NE edge of the valley, suggesting that the transition of the Indian plate from locked decollement to aseismic creep lies near here. This result is consistent with the geodetic findings that a broad zone of partial seismic coupling exists beneath the Kashmir valley.