Group Velocity Dispersion Data Research Articles

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

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.

Crust–Uppermost Mantle Structure beneath the Caribbean Region from Seismic Ambient Noise Tomography

ABSTRACT We present a new 3D shear velocity model for the crust–uppermost mantle structure beneath the Caribbean region from the surface down to 150 km depth. Our velocity model was derived from joint inversion of group and phase velocity dispersion data obtained from ambient noise and earthquake data. The group and phase dispersion curves estimated from ambient noise were calculated from cross correlation using up to 4 yr of continuous data. Perturbations in group and phase surfaces-wave velocities within a resolution of 1° × 1° show the relevant geotectonic units in the Caribbean plate. Plate boundaries, ocean basins, rises, rifts, and microplates are well defined by shear-wave velocity impedances. The 3D shear-wave velocity inversion along profiles shows the thickening of the crust from the ocean to continental margins. We present a new Moho interface map with depths undulating between 11 and 17 km beneath most parts of the sea and 25–45 km below the continental areas. Low-velocity zones were found in the uppermost mantle indicating a highly laterally heterogeneous area.

Evidence of structural segmentation of the Uttarakhand Himalaya and its implications for earthquake hazard

The earthquake hazard associated with the Main Himalayan Thrust (MHT) is a critical issue for India and its neighbouring countries in the north. We used data from a dense seismic network in Uttarakhand, India, to model the lateral variations in the depths of MHT (2–6% drop in Vs at 12–21 km depths), Moho (a sharp increase in Vs (by ~ 0.5–0.7 km/s) at 39–50 km depths) and lithosphere (a marked decrease in Vs(~ 1–3%) at 136–178 km depths), across the Himalayan collisional front. Our joint inversion of radial PRFs and group velocity dispersion data of Rayleigh waves detects three NNE trending transverse lithospheric blocks segmenting the lithosphere in Uttarakhand Himalaya, which spatially correlate well with the northward extension of the Delhi -Haridwar Indian basement ridge, an inferred tectonic boundary and great boundary fault, respectively. Our radial receiver function imaging detects highly deformed and segmented crustal and lithospheric structures associated with three mapped transverse lithospheric blocks, suggesting a reduction in rupture lengths of future earthquakes, thereby, reducing earthquake hazards in Uttarakhand.

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.

Variations in crustal and lithospheric structure across the Eastern Indian Shield from passive seismic source imaging: Implications to changes in the tectonic regimes and crustal accretion through the Precambrian

Changes in the dominant tectonic regimes and the modes of formation and maturation of the continental crust during the Precambrian are a subject of active debate. Here, we discuss these issues in terms of new information on the crustal and lithospheric architecture of the different terranes in the Eastern part of the Indian Shield (EIS). We present well-constrained estimates of the crustal and lithospheric thickness characterizing different terranes of the EIS through the joint inversion of P-receiver functions (PRF) and fundamental mode group velocity dispersion data of Rayleigh waves (10–100 s). The lithospheric thicknesses are also computed independently using the S-receiver function (SRF) modeling. The study involved 2167 PRFs and 100 SRFs from 15 broadband stations. Our modeling detects a marked crustal (~4–8 km) and lithospheric (~15–20 km) thinning beneath the Singhbhum-Odisha-Craton (SOC), spatially correlating with the central part of the craton comprising the gneissic terrain contrasted by a crustal thickening (~2–6 km) beneath the horseshoe-shaped Iron Ore Group. We propose that such a crustal configuration beneath the Paleoarchean craton may suggest the dominance of vertical tectonic processes such as thickening of an oceanic mafic plateau and recurrent melting producing the TTGs and the Singbhum Granite suite. Infra-crustal reworking involving Rayleigh-Taylor instabilities and gravitational processes could explain the crustal architecture of the SOC much similar to a few other Paleoarchean cratons such as Pilbara (Australia), Barberton and Kappvaal (South Africa). Our modeling has also revealed a relatively smaller degree of crustal (2–4 km) and lithospheric thinning (4–10 km) beneath the Eastern Ghat Mobile Belt, south of the SOC. We observe a relatively thicker crust (~42 km) characterized by a flat Moho over a large area (~40,000 sq. km) encompassing the Chota Nagpur Granite Gneiss Terrane (CGGT). Such thickening is consistent with the convergence and collision tectonics involving the SOC and CGGT. However, we note a 15–20 km lithospheric thinning associated with the CGGT. Such thinning may mainly relate to Phanerozoic rifting along the Damodar graben and Rajmahal magmatism. Broadly, our model of the crustal and lithospheric architecture of the EIS reflects the imprints of northward subduction of SOC beneath the CGGT.

Upper crustal shear wave velocity and radial anisotropy beneath Jeju Island volcanoes from ambient noise tomography

SUMMARYJeju Island offshore of the southern Korean Peninsula is an isolated intraplate volcano formed by multiple basaltic eruptions from the Pleistocene (∼1.8 Ma) to the Holocene (∼3.7 ka). Due to the lack of available seismic data, magma structures at upper crustal depths of the island have not been clearly revealed. In this study, we imaged upper crustal isotropic and radial anisotropic structures beneath the island using ambient noise data from a temporary seismic network. A series of transdimensional hierarchical Bayesian inversions were performed to construct upper crustal (1–10 km) isotropic and anisotropic structures. Surface wave (Rayleigh and Love wave) group and phase velocity dispersion data were jointly inverted for 2–15 s. The results show that layers of negative anisotropy (VSH < VSV) are predominant at shallower (<2 km) and deeper (>5 km) depths, which was interpreted as reflecting dyke swarms responsible for the more than 400 cinder cones at the surface and the vertical plumbing systems supplying magma from deeper sources, respectively. Additionally, a layer with significantly positive radial anisotropy (VSH > VSV, up to 5 per cent) was found at middle depths (2–5 km), and was interpreted as horizontally aligned magma plumbing systems (e.g. sills) through comparisons with several other volcanoes worldwide. In comparison with the isotropic structure, the positive anisotropic layer was separated into upper and lower layers with locally neutral to slightly fast and slower shear wave velocities, respectively, beneath the largest central crater (Mt Halla). Such a structure indicates that the cooled upper part of the magma plumbing systems formed within the horizontally developed sill complex, and is underlain by still-warm sill structures, potentially with a small fraction of melting. With dykes predominant above and below, the island-wide sill layer and locally high-temperature body at the centre explain the evolution of the Jeju Island volcanoes by island-forming surface lava flows and central volcanic eruptions before and after the eruptions of cinder cones.

A Seismological Study of the Sos Enattos Area—the Sardinia Candidate Site for the Einstein Telescope

AbstractThe recent discovery of gravitational waves (GWs) and their potential for cosmic observations prompted the design of the future third-generation GW interferometers, able to extend the observation distance for sources up to the frontier of the Universe. In particular, the European detector Einstein Telescope (ET) has been proposed to reach peak strain sensitivities of about 3×10−25 Hz−1/2 in the 100 Hz frequency region and to extend the detection band down to 1 Hz. In the bandwidth [1,10] Hz, the seismic ambient noise is expected to represent the major perturbation to interferometric measurements, and the site that will host the future detectors must fulfill stringent requirements on seismic disturbances. In this article, we conduct a seismological study at the Italian ET candidate site, the dismissed mine of Sos Enattos in Sardinia. In the range between few mHz to hundreds of mHz, out of the detection bandwidth for ET, the seismic noise is compatible with the new low-noise model (Peterson, 1993); in the [0.1,1] Hz bandwidth, we found that seismic noise is correlated with sea wave height in the northwestern Mediterranean Sea. In the [1,10] Hz frequency band, noise is mainly due to anthropic activities; within the mine tunnels (≃100 m underground), its spectrum is compliant with the requirements of the ET design. Noise amplitude decay with depth is consistent with a dominance of Rayleigh waves, as suggested by synthetic seismograms calculated for a realistic velocity structure obtained from the inversion of phase- and group-velocity dispersion data from array recording of a mine blasting. Further investigations are planned for a quantitative assessment of the principal noise sources and their spatiotemporal variations.

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.

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.

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.

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.

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.

Joint inversion of seismic and gravity data for imaging seismic velocity structure of the crust and upper mantle beneath Utah, United States

We present a method for using any combination of body wave arrival time measurements, surface wave dispersion observations, and gravity data to simultaneously invert for three-dimensional P- and S-wave velocity models. The simultaneous use of disparate data types takes advantage of the differing sensitivities of each data type, resulting in a comprehensive and higher resolution three-dimensional geophysical model. In a case study for Utah, we combine body wave first arrivals mainly from the USArray Transportable Array, Rayleigh wave group and phase velocity dispersion data, and Bouguer gravity anomalies to invert for crustal and upper mantle structure of the region. Results show clear delineations, visible in both P- and S-wave velocities, between the three main tectonic provinces in the region. Without the inclusion of the surface wave and gravity constraints, these delineations are less clear, particularly for S-wave velocities. Indeed, checkerboard tests confirm that the inclusion of the additional datasets dramatically improves S-wave velocity recovery, with more subtle improvements to P-wave velocity recovery, demonstrating the strength of the method in successfully recovering seismic velocity structure from multiple types of constraints.

Single-station and single-event marsquake location and inversion for structure using synthetic Martian waveforms

In anticipation of the upcoming InSight mission, which is expected to deploy a single seismic station on the Martian surface in November 2018, we describe a methodology that enables locating marsquakes and obtaining information on the interior structure of Mars. The method works sequentially and is illustrated using single representative 3-component seismograms from two separate events: a relatively large teleseismic event (Mw5.1) and a small-to-moderate-sized regional event (Mw3.8). Location and origin time of the event is determined probabilistically from observations of Rayleigh waves and body-wave arrivals. From the recording of surface waves, averaged fundamental-mode group velocity dispersion data can be extracted and, in combination with body-wave arrival picks, inverted for crust and mantle structure. In the absence of Martian seismic data, we performed full waveform computations using a spectral element method (AxiSEM) to compute seismograms down to a period of 1s. The model (radial profiles of density, P- and S-wave-speed, and attenuation) used for this purpose is constructed on the basis of an average Martian mantle composition and model areotherm using thermodynamic principles, mineral physics data, and viscoelastic modeling. Noise was added to the synthetic seismic data using an up-to-date noise model that considers a whole series of possible noise sources generated in instrument and lander, including wind-, thermal-, and pressure-induced effects and electromagnetic noise. The examples studied here, which are based on the assumption of spherical symmetry, show that we are able to determine epicentral distance and origin time to accuracies of ∼0.5–1° and ±3–6s, respectively. For the events and the particular noise level chosen, information on Rayleigh-wave group velocity dispersion in the period range ∼14–48s (Mw5.1) and ∼14–34s (Mw3.8) could be determined. Stochastic inversion of dispersion data in combination with body-wave travel time information for interior structure, allows us to constrain mantle velocity structure to an uncertainty of ∼5%. Employing the travel times obtained with the initially inverted models, we are able to locate additional body-wave arrivals including depth phases, surface and Moho (multiple) reflections that may otherwise elude visual identification. This expanded data set is reinverted to refine interior structure models and source parameters (epicentral distance and origin time).

Joint inversion of receiver functions and surface waves with enhanced preconditioning on densely distributed CNDSN stations: Crustal and upper mantle structure beneath China

AbstractWe present shear wave velocity structure beneath China by joint modeling of teleseismic receiver function and Rayleigh wave group velocity dispersion data observed at +1000 permanent broadband seismic stations in the Chinese National Digital Seismic Network (CNDSN). A ray‐parameter‐based stacking method is employed to minimize artifacts in stacking receiver functions from different sources. The Rayleigh wave dispersion curve is extracted from group velocity tomographic models at all applicable periods. Enhanced preconditions are applied on the linearized iterative inversion to regularize and balance multiple types of data. The velocity profile inversion at each station starts from an initial model derived from sediments, crustal thickness, Vp/Vs ratio and Pn/Sn models. This multistep approach not only reduces uncertainty and nonuniqueness of the velocity inversion but also efficiently fills information gap in each data set. We then generate a 3‐D S velocity model by combining and smoothing all the 1‐D models. The obtained 3‐D model reveals crustal and upper mantle velocity structures that are well correlated with tectonic features of China, for example, our model shows a clear east‐west bimodal distribution at 35 km deep, low velocity in the crust beneath central and eastern Tibetan plateau, and sedimentary structure in major cratons and basins. Our model is consistent with existing tomographic models in large scale but provides more structural details in regional and local scales.