Crustal Shear Wave Velocity Structure Research Articles

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

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

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

Imaging the Upper 10 km Crustal Shear-Wave Velocity Structure of Central Myanmar via a Joint Inversion of P-Wave Polarizations and Receiver Functions

Abstract Myanmar occupies a complex region in the active Indo-Burma subduction system. To illuminate the upper 10 km crustal structure of central Myanmar and obtain new insight into the subduction system, we jointly use P-wave polarizations and receiver functions (RFs) to construct a high-resolution VS profile based on a Bayesian Markov chain Monte Carlo approach. This obtained profile clearly delineates six tectonic units and their boundaries, including the Indo-Burman ranges (IBR), the IBR-fore-arc basin boundary, the fore-arc basin, the volcanic arc, the back-arc basin, and the Sunda plate. The Sunda plate has relatively higher upper crustal VS (>3.0 km/s) and thinner sedimentary cover (∼1 km) compared with the Central Myanmar basin in the Burma plate. The fore-arc basin, containing thick sediments (>10 km), and the back-arc basin, with thinner sediments (∼1–6 km), are separated by a region with higher VS (∼3.0 km/s), which represents crystallized magma beneath the volcanic arc. A narrow zone of relatively high-VS (∼2.6–2.7 km/s) ophiolites is situated between the fore-arc basin and the IBR. We also find a narrow zone of high-VS (∼2.9 km/s) metamorphic rocks contained within the low-VS (≲2.3 km/s) IBR. This study suggests that the proposing joint inversion of two types of single-station measurements, that is, P-wave polarizations and RFs, can robustly and computationally efficiently image the shallow VS structure and provide a reliable uncertainty estimation.

High-Resolution Crustal Shear-Wave Velocity Structure in the Pearl River Delta, South China

Abstract We present a high-resolution shear-wave velocity model of the upper-middle crust beneath the Pearl River Delta. The sedimentary basins are characterized by low-velocity anomalies at depths of 6–8 km and reflect the lateral migration of the deposition centers. High-velocity structures are located predominantly in Hong Kong, and outline the supervolcano conduit and magma reservoir in the crust. We observe contrasting velocity structures across the Lianhuashan fault zone (LFZ) where low-velocity anomaly zones (LVZs) are developed in different strikes. Microseismic swarms at depths of 8–16 km mainly occurred at the intersection of the LVZs. Metamorphic fluids and migration in the crust are proposed to interpret the seismic activity. The intersecting LVZs would increase the lithologic permeability to allow deep-rooted fluids to flow up, whereas the overlying high-velocity structures representing the overflowing magma act as a permeable barrier that confines the seismicity in the middle-lower crust. We anticipate that the detailed subsurface structures will help correlate magmatic remnants with the distribution of intraplate earthquakes and assess the earthquake risks in the Greater Bay Area of China.

Growth of Northern Tibet: Insights From the Crustal Shear Wave Velocity Structure of the Qilian Shan Orogenic Belt

AbstractWe collected continuous ambient noise data recorded between 2014 and 2015 at 288 broadband stations. Phase velocity maps of Rayleigh waves are constructed from 2 to 35 s periods. Combining phase dispersions at long periods (40, 45, 50, 55, and 60 s) obtained from earthquake data, we construct a three‐dimensional crustal S‐wave velocity (Vs) model with a lateral resolution of ∼20 km beneath the Qilian orogenic belt (QLOB) and its surrounding areas. The velocity structure above 20 km is constrained by faults and agrees well with subblocks. The middle‐lower crustal shear wave velocity in the QLOB is relatively low and has an unevenly distributed low‐velocity zone (LVZ, Vs < 3.4 km/s) at depths of 20–40 km, while that in the Alxa block is normal except for a narrow region protruding through the Longshou Shan. Deformation of the upper and lower crust in the QLOB is decoupled, and the growth of northern Tibet occurs through ductile lower crustal thickening and upper crustal overthrusting. The emergence of the mid‐crust LVZ in the QLOB could be due to high heat flow, more felsic composition of the mid‐lower crust, and especially shear heating. The weak lower crust from the QLOB to the Alxa block through the Longshou Shan is conducive to the expansion of the plateau, and the active front of northern Tibet is north of the Longshou Shan.

A joint inversion of receiver function and Rayleigh wave phase velocity dispersion data to estimate crustal structure in West Antarctica

SUMMARY We determine crustal shear wave velocity structure and crustal thickness at recently deployed seismic stations across West Antarctica, using a joint inversion of receiver functions and fundamental mode Rayleigh wave phase velocity dispersion. The stations are from both the UK Antarctic Network (UKANET) and Polar Earth Observing Network/Antarctic Network (POLENET/ANET). The former include, for the first time, four stations along the spine of the Antarctic Peninsula, three in the Ellsworth Land and five stations in the vicinity of the Pine Island Rift. Within the West Antarctic Rift System (WARS) we model a crustal thickness range of 18–28 km, and show that the thinnest crust (∼18 km) is in the vicinity of the Byrd Subglacial Basin and Bentley Subglacial Trench. In these regions we also find the highest ratio of fast (Vs = 4.0–4.3 km s–1, likely mafic) lower crust to felsic/intermediate upper crust. The thickest mafic lower crust we model is in Ellsworth Land, a critical area for constraining the eastern limits of the WARS. Although we find thinner crust in this region (∼30 km) than in the neighbouring Antarctic Peninsula and Haag-Ellsworth Whitmore block (HEW), the Ellsworth Land crust has not undergone as much extension as the central WARS. This suggests that the WARS does not link with the Weddell Sea Rift System through Ellsworth Land, and instead has progressed during its formation towards the Bellingshausen and Amundsen Sea Embayments. We also find that the thin WARS crust extends towards the Pine Island Rift, suggesting that the boundary between the WARS and the Thurston Island block lies in this region, ∼200 km north of its previously accepted position. The thickest crust (38–40 km) we model in this study is in the Ellsworth Mountain section of the HEW block. We find thinner crust (30–33 km) in the Whitmore Mountains and Haag Nunatak sectors of the HEW, consistent with the composite nature of the block. In the Antarctic Peninsula we find a crustal thickness range of 30–38 km and a likely dominantly felsic/intermediate crustal composition. By forward modelling high frequency receiver functions we also assess if any thick, low velocity subglacial sediment accumulations are present, and find a 0.1–0.8-km-thick layer at 10 stations within the WARS, Thurston Island and Ellsworth Land. We suggest that these units of subglacial sediment could provide a source region for the soft basal till layers found beneath numerous outlet glaciers, and may act to accelerate ice flow.

Crustal shear-wave velocity structure in Western Java, Indonesia from analysis of teleseismic receiver functions

We analysed receiver functions from teleseismic events recorded at 11 broadband seismometers in the western part of Java Island, Indonesia. The stations are mostly located at three main geological environment including Northwest Java Basin, Bogor Zone, and Southern Mountains Arc. A total of about 341 receiver functions were computed using iterative time domain deconvolution. We derived shear-wave velocity structure and crustal Vp/Vs ratio by inverting stacked radial receiver functions using non-linear neighbourhood algorithm. Inversion results show sediment thickness varies between 1 and 2 km thick in Western Java. Our inversion shows that crustal thickness in this region varies between 25 and 32 km. Average crustal Vp/Vs ratio is estimated to be about 1.69–1.78. We hope the study may provide useful information for velocity model and crustal thickness for Indonesia region.

The formation of the Dabashan orocline, central China: Insights from high-resolution 3D crustal shear-wave velocity structure

The Dabashan orocline is a large thrust belt in central China. Determination of its high-resolution three-dimensional (3D) crustal shear-wave (S-wave) velocity structure can enhance our understanding of the intracontinental evolution of the South China plate and North China plate. In this study, we estimated the Rayleigh wave phase and group velocities at periods of 5–50 s, P-wave receiver functions, and Ps and SsPmp travel times from 63 permanent stations and 29 portable stations; we also constructed a 3D S-wave velocity model and crustal thickness and Vp/Vs ratio maps of the Dabashan orocline and adjacent areas. The crustal thickness and Vp/Vs ratio maps show prominent thick crust (50–55 km) and high Vp/Vs ratios (1.8–1.85), suggesting a mafic lower crust in the Dabashan orocline. The 3D S-wave velocity structure shows northeast-dipping low-velocity anomalies in the upper crust of the Dabashan orocline, and high velocities extended from near the surface to the lower crust in the Hannan-Micang and Shennong-Huangling domes, suggesting deep roots of the two domes. We propose that the two domes acted as rigid indenters and rotated clockwise together with the South China plate, penetrating into the Qinling-Dabie orogenic belt. Weak sediments in the upper crust of the Dabashan orocline were compressed and blocked by the two domes and the rigid Qinling-Dabie orogenic belt, resulting in vertical accretion of the sediments and crust and subsequent formation of the northeast-dipping low-velocity anomalies and the thick crust beneath the Dabashan orocline.

Crustal structure beneath the Malawi and Luangwa Rift Zones and adjacent areas from ambient noise tomography

Crustal shear wave velocity structure beneath the Malawi and Luangwa Rift Zones (MRZ and LRZ, respectively) and adjacent regions in southern Africa is imaged using fundamental mode Rayleigh waves recorded by 31 SAFARI (Seismic Arrays for African Rift Initiation) stations. Dispersion measurements estimated from empirical Green's functions are used to construct 2-D phase velocity maps for periods between 5 and 28 s. The resulting Rayleigh wave phase velocities demonstrate significant lateral variations and are in general agreement with known geological features and tectonic units within the study area. Subsequently, we invert Rayleigh wave phase velocity dispersion curves to construct a 3-D shear wave velocity model. Beneath the MRZ and LRZ, low velocity anomalies are found in the upper-most crust, probably reflecting the sedimentary cover. The mid-crust of the MRZ is characterized by an ~3.7% low velocity anomaly, which cannot be adequately explained by higher than normal temperatures alone. Instead, other factors such as magmatic intrusion, partial melting, and fluid-filled deep crustal faults might also play a role. Thinning of the crust of a few kilometers beneath the rifts is revealed by the inversion. A compilation of crustal thicknesses and velocities beneath the world's major continental rifts suggests that both the MRZ and LRZ are in the category of rifts beneath which the crust has not been sufficiently thinned to produce widespread syn-rifting volcanisms.

Crustal shear wave velocity structure in the northeastern Tibet based on the Neighbourhood algorithm inversion of receiver functions

Crustal shear wave velocity structure in the northeastern Tibet based on the Neighbourhood algorithm inversion of receiver functions

Stepwise joint inversion of surface wave dispersion, Rayleigh wave ZH ratio, and receiver function data for 1D crustal shear wave velocity structure

Accurate determination of seismic velocity of the crust is important for understanding regional tectonics and crustal evolution of the Earth. We propose a stepwise joint linearized inversion method using surface wave dispersion, Rayleigh wave ZH ratio (i.e., ellipticity), and receiver function data to better resolve 1D crustal shear wave velocity (vS) structure. Surface wave dispersion and Rayleigh wave ZH ratio data are more sensitive to absolute variations of shear wave speed at depths, but their sensitivity kernels to shear wave speeds are different and complimentary. However, receiver function data are more sensitive to sharp velocity contrast (e.g., due to the existence of crustal interfaces) and vP/vS ratios. The stepwise inversion method takes advantages of the complementary sensitivities of each dataset to better constrain the vS model in the crust. We firstly invert surface wave dispersion and ZH ratio data to obtain a 1D smooth absolute vS model and then incorporate receiver function data in the joint inversion to obtain a finer vS model with better constraints on interface structures. Through synthetic tests, Monte Carlo error analyses, and application to real data, we demonstrate that the proposed joint inversion method can resolve robust crustal vS structures and with little initial model dependency.

Seismic evidence of crustal low velocity beneath Eastern Ghat Mobile Belt, India

The Eastern Ghat Mobile Belt (EGMB), a tectonically active area extends along the eastern margin of Peninsular India, is divided into three provinces, namely, Eastern Ghat Province, the Jeypore Province, and the Krishna Province. The Ongole domain of Krishna Province is a seismically active region that has experienced four moderate earthquakes of magnitude ⩾5.0, of which largest one is of magnitude 5.4 occurred on 27th March 1967. The crustal shear wave velocity structure in the Eastern Ghat Mobile Belt has been investigated using joint inversion of receiver functions and Rayleigh wave group velocity at 5 locations in the study region. The results show crustal thickness variation from 37 to 42km and average shear velocity variation from 3.67 to 3.78km/s in the study region. A low velocity layer of variable thickness and velocities 3.54–3.7km/s) is also observed in the region. The low velocity layer in most of the stations is observed at a depth of ∼20km. This low velocity layer may be due to the presence of fluid in the crust, which also be one of the causes of the intraplate earthquakes in the study region.

Crustal shear velocity structure in the Southern Lau Basin constrained by seafloor compliance

AbstractSeafloor morphology and crustal structure vary significantly in the Lau back‐arc basin, which contains regions of island arc formation, rifting, and seafloor spreading. We analyze seafloor compliance: deformation under long period ocean wave forcing, at 30 ocean bottom seismometers to constrain crustal shear wave velocity structure along and across the Eastern Lau Spreading Center (ELSC). Velocity models obtained through Monte Carlo inversion of compliance data show systematic variation of crustal structure in the basin. Sediment thicknesses range from zero thickness at the ridge axis to 1400 m near the volcanic arc. Sediment thickness increases faster to the east than to the west of the ELSC, suggesting a more abundant source of sediment near the active arc volcanoes. Along the ELSC, upper crustal velocities increase from the south to the north where the ridge has migrated farther away from the volcanic arc front. Along the axial ELSC, compliance analysis did not detect a crustal low‐velocity body, indicating less melt in the ELSC crustal accretion zone compared to the fast spreading East Pacific Rise. Average upper crust shear velocities for the older ELSC crust produced when the ridge was near the volcanic arc are 0.5–0.8 km/s slower than crust produced at the present‐day northern ELSC, consistent with a more porous extrusive layer. Crust in the western Lau Basin, which although thought to have been produced through extension and rifting of old arc crust, is found to have upper crustal velocities similar to older oceanic crust produced at the ELSC.

Crustal shear-wave velocity structure beneath Sumatra from receiver function modeling

We estimated the shear-wave velocity structure and Vp/Vs ratio of the crust beneath the Sumatra region by inverting stacked receiver functions from five three-component broadband seismic stations, located in diverse geologic setting, using a well known non-linear direct search approach, Neighborhood Algorithm (NA). Inversion results show significant variation of sediment layer thicknesses from 1km beneath the backarc basin (station BKNI and PMBI) to 3–7km beneath the coastal part of Sumatra region (station LHMI and MNAI) and Nias island (station GSI). Average sediment layer shear velocity (Vss) beneath all the stations is observed to be less (∼1.35km/s) and their corresponding Vp/Vs ratio is very high (∼2.2–3.0). Crustal thickness beneath Sumatra region varies between 27 and 35km, with exception of 19km beneath Nias island, with average crustal Vs ∼3.1–3.4km/s (Vp/Vs ∼1.8). It is well known that thick sediments with low Vs (and high Vp/Vs) amplify seismic waves even from a small-magnitude earthquake, which can cause huge damage in the zone. This study can provide the useful information of the crust for the Sumatra region. Since, Sumatra is an earthquake prone zone, which suffered the strong shaking of Great Andaman–Sumatra earthquake; this study can also be helpful for seismic hazard assessment.

Westward Growth of Laurentia by Pre–Late Jurassic Terrane Accretion, Eastern Oregon and Western Idaho, United States: A Reply

Previous article No AccessWestward Growth of Laurentia by Pre–Late Jurassic Terrane Accretion, Eastern Oregon and Western Idaho, United States: A ReplyTodd A. LaMaskin and Rebecca J. DorseyTodd A. LaMaskin1. Department of Geography and Geology, University of North Carolina at Wilmington, 601 South College Road, Wilmington, North Carolina 28403-5944, USA*Author for correspondence; e-mail: [email protected]. Search for more articles by this author and Rebecca J. Dorsey2. Department of Geological Sciences, University of Oregon, 1272 University of Oregon, Eugene, Oregon 97403-1272, USA Search for more articles by this author PDFPDF PLUSFull Text Add to favoritesDownload CitationTrack CitationsPermissionsReprints Share onFacebookTwitterLinkedInRedditEmail SectionsMoreDetailsFiguresReferencesCited by The Journal of Geology Volume 124, Number 1January 2016 Article DOIhttps://doi.org/10.1086/684120 Views: 206Total views on this site Citations: 5Citations are reported from Crossref HistoryReceived September 04, 2015Accepted September 09, 2015Published online December 16, 2015 © 2015 by The University of Chicago. All rights reserved.PDF download Crossref reports the following articles citing this article:Todd A. LaMaskin, Jonathan A. Rivas, David L. Barbeau, Joshua J. Schwartz, John A. Russell, Alan D. Chapman A crucial geologic test of Late Jurassic exotic collision versus endemic re-accretion in the Klamath Mountains Province, western United States, with implications for the assembly of western North America, GSA Bulletin 134, no.3-43-4 (Jul 2021): 965–988.https://doi.org/10.1130/B35981.1Paul M. Bremner, Mark P. Panning, R. M. Russo, Victor Mocanu, A. Christian Stanciu, Megan Torpey, Sutatcha Hongsresawat, John C. VanDecar, Todd A. LaMaskin, D. A. Foster Crustal Shear Wave Velocity Structure of Central Idaho and Eastern Oregon From Ambient Seismic Noise: Results From the IDOR Project, Journal of Geophysical Research: Solid Earth 124, no.22 (Feb 2019): 1601–1625.https://doi.org/10.1029/2018JB016350B. Tikoff, J. Vervoort, J.A. Hole, R. Russo, R. Gaschnig, A. Fayon Introduction: EarthScope IDOR project (deformation and magmatic modification of a steep continental margin, western Idaho–eastern Oregon) themed issue, Lithosphere (Feb 2017): L628.1.https://doi.org/10.1130/L628.1N. Braudy, R.M. Gaschnig, D. Wilford, J.D. Vervoort, C.L. Nelson, C. Davidson, M.J. Kahn, B. Tikoff Timing and deformation conditions of the western Idaho shear zone, West Mountain, west-central Idaho, Lithosphere (Dec 2016): L519.1.https://doi.org/10.1130/L519.1Tracy L. Vallier, Keegan L. Schmidt, Todd A. LaMaskin Geology of the Wallowa terrane, Blue Mountains province, in the northern part of Hells Canyon, Idaho, Washington, and Oregon, (): 211–249.https://doi.org/10.1130/2016.0041(07)

Crustal shear-wave velocity structure of northeastern Tibet revealed by ambient seismic noise and receiver functions

The Tibetan plateau is formed by the persistent convergence between the Indian and Eurasian plates. The northeastern Tibetan plateau is undergoing young deformation that has been noticed for a long time. We conduct a passive-source seismic profile with 22 stations in NE Tibet in order to investigate the crustal shear-wave velocity structure and its relationship with tectonic processes. In this paper we obtain the Rayleigh-wave phase velocity dispersion data among all station pairs within the period bandwidth of 5–20s from the method of ambient noise cross-correlations. Phase velocity variations correlate well with surface geological boundaries and tectonic features, for instance, low phase velocity beneath the Songpan–Ganzi block and the Guide basin. We also compute P-wave receiver functions based on the selected teleseismic events with similar ray parameters, and perform the joint inversion of surface wave dispersion data and receiver functions to obtain the 2-D crustal shear-wave velocity structure along the profile. The inversion results show that low shear-wave velocities beneath the Songpan–Ganzi block are widespread in the middle-to-lower crust. In together with high crustal Vp/Vs ratios and high temperature suggested by the P-wave velocities obtained from the active-source seismic study, we suggest that the low velocity zone beneath the Songpan–Ganzi block is probably attributed to partial melting. Across the North Kunlun fault, there is no crustal LVZ found beneath the Kunlun block. This structural difference may have already existed before the collision of the two blocks, or due to limit of the northward extension for the crustal LVZ across the North Kunlun fault.

Crustal shear-wave velocity structure beneath northeast India from teleseismic receiver function analysis

We investigated the seismic shear-wave velocity structure of the crust beneath nine broadband seismological stations of the Shillong–Mikir plateau and its adjoining region using teleseismic P-wave receiver function analysis. The inverted shear wave velocity models show ∼34–38km thick crust beneath the Shillong Plateau which increases to ∼37–38km beneath the Brahmaputra valley and ∼46–48km beneath the Himalayan foredeep region. The gradual increase of crustal thickness from the Shillong Plateau to Himalayan foredeep region is consistent with the underthrusting of Indian Plate beyond the surface collision boundary. A strong azimuthal variation is observed beneath SHL station. The modeling of receiver functions of teleseismic earthquakes arriving the SHL station from NE backazimuth (BAZ) shows a high velocity zone within depth range 2–8km along with a low velocity zone within ∼8–13km. In contrast, inversion of receiver functions from SE BAZ shows high velocity zone in the upper crust within depth range ∼10–18km and low velocity zone within ∼18–36km. The critical examination of ray piercing points at the depth of Moho shows that the rays from SE BAZ pierce mostly the southeast part of the plateau near Dauki fault zone. This observation suggests the effect of underthrusting Bengal sediments and the underlying oceanic crust in the south of the plateau facilitated by the EW-NE striking Dauki fault dipping 300 toward northwest.

Distinct variations of crustal shear wave velocity structure and radial anisotropy beneath the North China Craton and tectonic implications

This study presents the crustal shear wave velocity structure and radial anisotropy along two linear seismic arrays across the North China Craton (NCC) from ambient noise tomography. About a half to one year long ambient noise data from 87 stations were used for obtaining the inter-station surface wave empirical Green's functions (EGFs) from cross-correlation. Rayleigh and Love dispersion curves within the period band 5–30s were measured from the EGFs of the vertical and transverse components, respectively. These dispersion data were then used to determine the crustal shear wave velocity structure (VSV and VSH) and radial anisotropy (2(VSH−VSV)/(VSH+VSV)) from point-wise linear inversion with constraints from receiver function analysis. Our results reveal substantial structural variations among different parts of the NCC. The Bohai Bay Basin in the eastern NCC is underlain by a thin crust (~30km) with relatively low velocities (particularly VSV) and large positive radial anisotropy in the middle to lower crust. Such a crustal structure is no longer of a cratonic type and may have resulted from the widespread tectonic extension and intensive magmatism in this region since late Mesozoic. Beneath the Ordos Basin in the western NCC, the crust is relatively thicker (≥40km) and well stratified, and presents a large-scale low velocity zone in the middle to lower crust and overall weak radial anisotropy except for a localized lower crust anomaly. The overall structural features of this region resemble those of typical Precambrian shields, in agreement with the long-term stability of the region. The crustal structure under the Trans North China Orogen (TNCO, central NCC) is more complicated and characterized by smaller scale velocity variations, strong positive radial anisotropy in the middle crust and rapid change to weak-to-negative anisotropy in the lower crust. These features may reflect complex deformations and crust–mantle interactions, probably associated with tectonic extension and magmatic underplating during the Mesozoic to Cenozoic evolution of the region. Our structural images in combination with previous seismic, geological and geochemical observations suggest that the Phanerozoic lithospheric reactivation and destruction processes may have affected the crust (especially the middle and lower crust) of the eastern NCC, and the effect probably extended to the TNCO, but may have minor influence on the crust of the western part of the craton.

S-wave velocity of the crust around Tianshan Mountains inverted from seismic ambient noise tomography

We process ambient noise data from seismic stations deployed in central Asia to determine the crustal shear wave velocity structure beneath the Tianshan Mountians and surrounding area. About 748 inter-station Rayleigh wave empirical Green’s functions have been recovered to estimate the phase velocity dispersions over periods from 6 to 50 s using the image transformation technique. Results show that for short periods (6–20 s), the distribution of Rayleigh wave phase velocities is generally consistent with surface geology, with high velocities corresponding to mountain ranges and low velocities to sedimentary basins. Along two profiles, which trend from NE-SW and NW-SE, the shear wave velocity shows a pair of high velocity anomalies dipping in opposite directions beneath the Tianshan Mountains. At shallow depths, those high velocity anomalies roughly correlate with areas where the mountain front and the surrounding basin are connected. The profiles also show a narrow zone beneath the Tianshan Mountains, which may represent a route for the upwelling from upper mantle. Those observations suggest that the underthrusting of the Tarim Basin and Kazakh Shield combine with the weakness of the crust, which is heated by the upwelling from upper mantle, may play an important role on the reactivation of the Tianshan Mountains associated with the India-Eurasia collision.

Crustal shear wave velocity structure of the western United States inferred from ambient seismic noise and earthquake data

Surface wave dispersion measurements from ambient seismic noise and array‐based measurements from teleseismic earthquakes observed with the EarthScope/USArray Transportable Array (TA) are inverted using a Monte Carlo method for a 3‐D VS model of the crust and uppermost mantle beneath the western United States. The combination of data from these methods produces exceptionally broadband dispersion information from 6 to 100 s period, which constrains shear wave velocity structures in the crust and uppermost mantle to a depth of more than 100 km. The high lateral resolution produced by the TA network and the broadbandedness of the dispersion information motivate the question of the appropriate parameterization for a 3‐D model, particularly for the crustal part of the model. We show that a relatively simple model in which VS increases monotonically with depth in the crust can fit the data well across more than 90% of the study region, except in eight discrete areas where greater crustal complexity apparently exists. The regions of greater crustal complexity are the Olympic Peninsula, the MendocinoTriple Junction, the Yakima Fold Belt, the southern Cascadia back arc, the Great Central Valley of California, the Salton Trough, the Snake River Plain, and the Wasatch Mountains. We also show that a strong Rayleigh‐Love discrepancy exists across much of the western United States, which can be resolved by introducing radial anisotropy in both the mantle and notably the crust. We focus our analysis on demonstrating the existence of crustal radial anisotropy and primarily discuss the crustal part of the isotropic model that results from the radially anisotropic model by Voigt averaging. Model uncertainties from the Monte Carlo inversion are used to identify robust isotropic features in the model. The uppermost mantle beneath the western United States is principally composed of four large‐scale shear wave velocity features, but lower crustal velocity structure exhibits far greater heterogeneity. We argue that these lower crustal structures are predominantly caused by interactions with the uppermost mantle, including the intrusion and underplating of mafic mantle materials and the thermal depression of wave speeds caused by conductive heating from the mantle. Upper and middle crustal wave speeds are generally correlated, and notable anomalies are inferred to result from terrane accretion at the continental margin and volcanic intrusions.