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Tectonic background of the study region. The yellow star denotes the epicenter of the 1920 Haiyuan earthquake (M 8.5). The yellow and white lines denote active faults in Holocene and boundaries of tectonic blocks, respectively (Q. Deng et al., 2003; P. Zhang et al., 2003): The Longmenshan fault (LMSF), the Kunlun fault (KLF), the north Qinling fault (QLNF), the Zhuanlanghe fault (ZLHF), the Maxianshan fault (MXSF), the Liupanshan fault (LPSF), the Haiyuan fault (HYF), and the Qingtongxia‐Guyuan fault (QTX‐GYF); the Alxa block (ALXB), the Ordos basin (ODB), the eastern boundary region of the Tibetan Plateau (AEB), the Qilian block (QLB), the eastern Kunlun‐Qaidam block (EKQB), the Bayan Har block (BYHB), the Qinling fold zone (QLFZ), and the Sichuan basin (SCB). Sepia arrows denote GPS vectors with a reference frame of the stable Eurasia (Gan et al., 2007). Red and black arrows denote uplift and subsidence vectors, respectively, with respect to the ITRF2014 reference frame (Su et al., 2019). The blue line shows location of the vertical cross sections in Figure 5. In the inset map, the black arrows show the distribution of the proposed crustal channel flow (Clark & Royden, 2000), and the blue box shows the present study region. A gray scale of the surface topography (Amante & Eakins, 2009) is shown at the bottom.
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We present high‐resolution tomographic images of isotropic P wave velocity and azimuthal anisotropy in the crust and uppermost mantle beneath NE Tibet by jointly inverting 62,339 arrival times of the first P and later PmP waves from 6,602 local earthquakes and 9 seismic explosions. Widespread low‐velocity zones in the middle crust contribute most o...
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Citations
... Therefore, Zhang et al. (1996) and Song et al. (2018) adopted the concept of a lithospheric interchange structure, which uses the analogy of a highway interchange where traffic is allowed to pass between two highways running in different directions, to describe the various extents and movements of lithospheric layers in central China. The validity of various models and the extent of crust-mantle coupling/decoupling has remained controversial (Chang et al., 2017;Clark and Royden, 2000;Deng et al., 2018;Gao et al., 2019;Hao et al., 2021b;Phillips et al., 2007;Song et al., 2018;Sun and Zhao, 2020;Ye et al., 2017;Zhao et al., 2021), largely due to the lack of combining surface deformation with lithospheric structure at depth. ...
... The crustal motions and its geodynamic mechanisms around the WQS are debated (Chang et al., 2017;Clark and Royden, 2000;Deng et al., 2018;Gao et al., 2019;Hao et al., 2021b;Phillips et al., 2007;Song et al., 2018;Sun and Zhao, 2020;Ye et al., 2017;Zhao et al., 2021). The controversy is mainly about whether upper crustal or deeper lithospheric forces predominate. ...
... Moreover, abundant evidence suggests that the plastic low-velocity lithospheric mantle beneath the WQS drives crustal deformation. Various recent seismic tomography studies, including shear-wave tomography (Bao et al., 2015;Hao et al., 2021b;Zhang et al., 2018), teleseismic P-wave tomography (Lei and Zhao, 2016), Pn tomography (Phillips et al., 2007), anisotropic tomography (Sun and Zhao, 2020), receiver functions and surface wave dispersions (Deng et al., 2018), consistently reveal a widespread low-velocity zone in the lithospheric mantle that terminates under the CLSB and defines the lithospheric boundary between the Tibet and the rigid Sichuan Basin/Ordos Block. The low velocity of the deep crust (Fig. 6) and lithospheric mantle (Bao et al., 2015) beneath the WQS could reflect high temperatures and partial melting, which could then be easily extruded by plateau growth. ...
Continental rejuvenation results from the tectonic reactivation of crustal structures and lithospheric reworking by mantle flow. Geochemical observations and field mapping have traditionally provided the primary evidence for the secular evolution of crustal composition and tectonic processes during continental rejuvenation. Nonetheless, the impact of continental rejuvenation on the observed present-day strain rate and orogenic-scale lithospheric structure has not been well constrained. The pre-existing E-W−trending Central China Orogenic Belt has been overprinted by the N-S−trending Central Longitudinal Seismic Belt and constitutes the intracontinental West Qinling Syntaxis in central China, where the tectonic setting changes eastward from contraction to extension. Combining updated global positioning system data and high-resolution crustal seismic tomography, we reveal a modern continental rejuvenation process within the West Qinling Syntaxis in central China. The northward extrusion of the Tibetan Plateau’s weak lithospheric layer (middle-lower crust and lithospheric mantle) of southwestern China relative to the rigid Sichuan Basin/Ordos Block of the eastern West Qinling Syntaxis results in regional dextral shearing that shapes the Central Longitudinal Seismic Belt and defines the eastern Tibetan Plateau margin. The pre-existing E-W−trending Central China Orogenic Belt has been preserved above the brittle-ductile transition zone, and the northward movement of the deep lithospheric layer drives the deformation of the upper crust in the West Qinling Syntaxis. Our results, along with previous studies, suggest the presence of an intracontinental lithospheric interchange structure in central China. The continental rejuvenation of the West Qinling Syntaxis results from a combination of fault reactivation in the upper crust (Stage I, Eocene−Oligocene) and reworking of the deep lithosphere (Stage II, middle−late Miocene) related to the plateau-wide shift in stress accommodation ultimately driven by the redistribution of mass outward from the central Tibetan Plateau. At present, the transition zone between the high- and low-velocity anomalies along the Central Longitudinal Seismic Belt not only shapes the landscape boundary but controls the size and recurrence interval of earthquakes within the West Qinling Syntaxis in central China.
... Recent studies have shown that the trade-off problem can be resolved when earthquakes and seismic stations used in a tomographic inversion are densel y and uniforml y distributed in the study region (e.g. Huang et al. 2015 ;Zhao et al. 2023 ), then both the 3-D Vp heterogeneity and anisotropy can be determined reliably Sun & Zhao 2020 ;Wang et al. 2022 ). We performed synthetic tests to further examine the trade-of f ef fect between the Vp isotropic and anisotropic components (Figs S3 -S16 and S23 -S26 ). ...
We present high-resolution 3-D images of isotropic P-wave velocity (Vp), azimuthal anisotropy (AAN), and radial anisotropy (RAN) down to 700 km depth beneath the North China Craton (NCC) and adjacent areas, which are obtained by inverting a great number of high-quality arrival time data recorded at 1,374 portable seismic stations and 635 permanent stations in the study region. Our results reveal new and detailed features of the upper mantle structure beneath the NCC. Varying structural heterogeneities are revealed beneath different tectonic blocks, and differences also exist between northern and southern parts of each block. The fast velocity directions (FVDs) of azimuthal anisotropy are mainly NW-SE under the Alaxa block, and NE-SW beneath the Tibetan Plateau. The FVDs present an arc transition along the boundary faults separating the Tibetan Plateau, the Alaxa block, the western NCC and the Sichuan basin. Low-Vp anomalies with positive RANs (i.e. horizontal Vp > vertical Vp) are revealed at 100-200 km depths under the Tibetan Plateau, reflecting frozen-in anisotropy in the thick lithosphere. Significant low-Vp anomalies with a circular AAN pattern exist at 0-700 km depths beneath the Datong volcano. In addition, negative RAN occurs right below the volcano, whereas positive RANs appear around it, suggesting that the Datong volcano is fed by hot upwelling flow from the lower mantle associated with collapsing of subducted slab materials down to the lower mantle. The eastern NCC shows complex Vp AANs and RANs. Seismic anisotropy exhibits east-west variations in the upper mantle across the Tanlu fault zone. The west of the Tanlu fault shows negative RANs (vertical Vp > horizontal Vp), whereas its east shows positive RANs at 300-500 km depths. The low-Vp anomaly under the Datong volcano is connected with a large low-Vp anomaly beneath the eastern NCC above ~250 km depth, suggesting that the hot upwelling flow under Datong may migrate laterally to the asthenosphere under the eastern NCC and contribute to the lithospheric delamination and destruction there.
... Previous P-wave tomographic models (e.g., Sun and Zhao, 2020;Zuo and Chen, 2018;Yin et al., 2022) indicated that a high-velocity anomaly exists in the crust of the 2022 Menyuan mainshock source area (Fig. 12). We can infer that the edge of the high-velocity anomaly zone at the intersection of anomalies is more likely to generate an unstable crustal stress, leading to the frequent seismic activities of the Menyuan region in the northeastward extension of the Tibetan plateau (Lei and Zhao, 2009;Lei and Zhao, 2016;Lei et al., 2019;Song et al., 2023). ...
... Besides, previous tomographic models showed that there are obvious (Fig. 12) (e.g., Zuo and Chen, 2018;Sun and Zhao, 2020;Yin et al., 2022). These results suggest that the Menyuan earthquakes could also be related to the partial melting resulting from a hybrid impact of fluids, mineral components, and thermal structures (e. g., Huang et al., 2011;Liu and Zhao, 2016;Wang et al., 2022;Zhao, 2021). ...
... In the sidewall ripout model, there is adhesion or increased friction on the main strike-slip fault (Swanson, 1989), which leads to a lateral jump of stress to the adjacent curving fault and activation of the adjacent restraining and releasing bends (Mann, 2007). The P-wave tomographic model showed that there is a high-velocity anomaly at depths of 5-15 km around the 2016 Menyuan earthquake (Sun and Zhao, 2020;Zuo and Chen, 2018;Yin et al., 2022), suggesting that this high-velocity anomaly may be related to the increase of the adhesion on the Lenglongling fault, leading to the locked main fault (Figs. 12 and 15). Moreover, the blocked shear stress on the Lenglongling fault jumped to the northern Lenglongling fault and sidewall ripout occurred. ...
On 8 January 2022, an earthquake with Ms 6.9 struck Menyuan County, Qinghai Province, China, which provides an opportunity to investigate the seismotectonics of the Qilian-Haiyuan fault in northeast Tibet. We collect seismic observation data from China Seismic Network Center from 8 January to 6 February 2022 around the 2022 Menyuan mainshock. The double-difference relocation algorithm is applied to relocate 904 earthquakes, and 820 high-precision hypocenter locations are obtained. The mainshock epicenter was located at 10.2 km depth in the Tuolaishan fault. The aftershock sequence extended towards nearly E-W along the Tuolaishan fault and NW-SE along the Lenglongling fault, parts of the Qilian-Haiyuan fault, but there were fewer aftershocks at the beginning of the rupture near the intersection between the Lenglongling and Tuolaishan faults, suggesting a conjugated type of rupturing with a seismic gap. Integrating with focal mechanism solutions of mainshock and one large aftershock, we suggest that the rupture is a left-lateral strike-slip event with a high dip. The stress change around the faults intersection. The 1986 Ms 6.4 earthquake may trigger the 2016 Ms 6.4 earthquake, and then the extrusion stress could transfer to around the 2022 Ms 6.9 earthquake source area due to the Indo-Eurasian collision. Previous tomographic models revealed low-velocity anomalies under the Menyuan earthquakes, reflecting that fluids could decrease friction coefficient and normal stress on the fault planes and further promote the generation of these earthquakes. These fluids could be contained in the lower crustal flow and mantle upwelling flow under northeast Tibet.
... The inserted map displays blocks around the study area ( Enkelmann et al. (2006) surmised that this exhumation was partly in response to the crustal flow from the TP by analyzing the cooling history of the Qinling belt and by kinematic and dynamic characterization of active faults in and around the Qinling belt. A crustal flow model was also employed to interpret the absence of steep topography in the Qinling belt (Clark & Royden 2000), and a few seismic tomography studies have suggested the existence of crustal flow beneath this area (Yang et al. 2012;Guo & Chen 2016;Song et al. 2018;Sun & Zhao 2020;Li et al. 2022b). In addition, based on GPS data indicating the eastward extrusion of the NETP (Zhang et al. 2004), Enkelmann et al. (2006) proposed that the extruding material of the NETP could be compensated by shortening and uplift in part of the Qinling belt. ...
... The deformation of the WQB is prominently linked to the evolution of the NETP (He et al. 2017). Based on seismic tomography studies, many researchers have observed significant low-velocity anomalies in the middle-lower crust beneath the WQB (Hu & Wang 2018;Sun & Zhao 2020;Li et al. 2022b). They considered that the significant low-velocity anomalies represent the Downloaded from https://academic.oup.com/gji/article/234/1/263/7040568 by guest on 16 February 2024 middle-lower crustal flow from the TP. ...
The Qinling belt is a transitional zone lying among three units: the North China block (NCB), the South China block (SCB) and the northeastern Tibetan Plateau (NETP). Owing to the interaction of these units, complex deformation has occurred in the Qinling belt. Although many studies have been conducted to understand the deformation mechanism in the Qinling belt, some key issues are still under debate, such as whether middle-lower crustal flow exists beneath the western Qinling belt (WQB). High-resolution images of subsurface structures are essential to shed light on the deformation mechanism. In this article, high-resolution images of the velocity structure and azimuthal anisotropy beneath the Qinling belt are obtained by using an eikonal equation-based traveltime tomography method. Our seismic tomography inverts 38 719 high-quality P-wave first arrivals from 1 697 regional earthquakes recorded by 387 broadband seismic stations. In the WQB, our tomography results show low-velocity anomalies but relatively weak anisotropy in the middle-lower crust. These features suggest that middle-lower crustal flow may not exist in this area. In the central Qinling belt (CQB), we find low-velocity anomalies in the middle-lower crust; however, the fast velocity directions (FVDs) no longer trend E–W but vary from NNE–SSW to N–S. These characteristics can be ascribed to the convergence and collision between the NCB and the SCB. In addition, we find strong low-velocity anomalies in the uppermost mantle beneath the CQB, which may indicate delamination of the lower crust. In the southern Qinling belt (SQB), we observe significant high-velocity anomalies in the upper crust beneath the Hannan–Micang (HNMC) and Shennong–Huangling (SNHL) domes. These high-velocity anomalies indicate a mechanically strong upper crust, which is responsible for the arc-shaped deformation process of the Dabashan (DBS) fold. Based on the P-wave velocity and azimuthal anisotropic structures revealed by the inversion of high-quality seismic data, the deformation of the Qinling belt is affected mainly by the convergence between the NCB and the SCB rather than by the middle-lower crustal flow from the Tibetan Plateau.
... Numerous geophysical investigations have been implemented to study the tectonic pattern and evolution of the lower crustal ductile flow beneath the tectonic zone (e. g., Cheng et al., 2016;Klemperer, 2006;Royden et al., 2008;Zhao et al., 2021). Previous regional seismic anisotropy studies had provided important information of this ductile deformation associated with tectonic stress in the crust (e.g., Huang et al., 2017;Li et al., 2011;Sun and Zhao, 2020). Furthermore, regional fluid geochemistry investigation has been performed in the tectonic zone Sun et al., 2016;Wang and Zhang, 1991;Wang, 2008;Xu, 2014;Zhang, 2016). ...
... Previous studies have shown that the crack density in the middleupper crust is small (ε ≤ 0.2; Sun et al., 2021) and that seismic anisotropy is strong in the deep crust (Sun and Zhao, 2020) under NE Tibet. Hence, following the instructions of O'Connell and Budiansky (1974), we estimated three-dimensional (3-D) distributions of crack density (ε) and saturation rate (ξ) for the crust of the study area. ...
... 2021Zhao et al., 2005). Based on P-Wave azimuthal anisotropic results (Sun and Zhao, 2020), obvious longitudinal boundary between lowvelocity zone and high-velocity zone under NE Tibet was delineated, which has been confined by the Haiyuan fault in Zone A and the Liupanshan fault in Zone B (Fig. 2). ...
The Tibetan Plateau is growing by both vertical uplift and horizontal extension. It is a continuing debate how the Tibetan Plateau interacts with its surrounding plates and blocks. Due to intense tectonic activity, which produced catastrophic earthquakes, the tectonic zone between the northeast margin of the horizontal extending Tibetan Plateau and the stable Ordos Block has garnered considerable interest. This study investigated the spatial distribution of gas geochemical anomalies (e.g., high flux of CO2 in correspondence of the main faults) at regional scale together with the seismic tomography in correspondence of this tectonic zone with the aim to figure out the domain of convergent boundary between the Ordos block and Tibetan plateau, and trace the tectonic discontinuities which are able to transfer fluids through the crustal layers between the two main geological units. From northwest to southeast, obvious difference of spatial distributions of geochemical and geophysical features in the tectonic zone between the northeast margin of the Tibetan Plateau and the Ordos Block is inferred. The northeast area (Zone A) is dominated by thrust and strike-slip faults with clear velocity boundary underneath, where low crack density (ε), saturation rate (ξ) and Poisson’ ratio (σ) in the middle-lower crust coincided with the low values of heat flow and CO2 emissions, tectonic compression and regional locked-fault can be inducements. The southeast area (Zone C) is dominated by extensional tectonics with roughly E-W fast-velocity direction (FVD) of P-wave azimuthal anisotropy, where high permeability and porosity can be deduced from crustal high ε, ξ and relatively high σ anomalies, resulting in high heat flow, CO2 concentrations and fluxes at the surface, and predominantly crustal-derived gases. The intermediate area (Zone B) also dominated by thrust and strike-slip faults is an extraordinary zone, where intensely locked-fault were clearly revealed, while the predominant anisotropic FVDs in the middle crust changed obviously, more contribution of shallow gas component was detected, and CO2 flux, heat flow, and regional ε, ξ, and σ in the upper crust were higher, compared with those in Zone A, which indicated the regional crushing fragmentation underneath Zone B. The adopted multidisciplinary approach demonstrated that Zone B is the convergent boundary between the Tibetan Plateau and the Ordos Block.
... In the Helan-Liupan area in west-central China, ductile flow in the middle-lower crust was also revealed, and fluids in the flow affected the generation of large crustal earthquakes there (Cheng et al. 2016). After Sun and Zhao (2020) Significant depth-dependent anisotropy is revealed beneath Southern California (Fig. 7). FVDs in the lithospheric mantle closely follow the strike of the San Andreas fault, whereas FVDs in the asthenosphere exhibit a circular pattern centered in the high-V Isabella anomaly under the Great Valley (Yu and Zhao 2018). ...
Seismic anisotropy tomography is the updated geophysical imaging technology that can reveal 3-D variations of both structural heterogeneity and seismic anisotropy, providing unique constraints on geodynamic processes in the Earth’s crust and mantle. Here we introduce recent advances in the theory and application of seismic anisotropy tomography, thanks to abundant and high-quality data sets recorded by dense seismic networks deployed in many regions in the past decades. Applications of the novel techniques led to new discoveries in the 3-D structure and dynamics of subduction zones and continental regions. The most significant findings are constraints on seismic anisotropy in the subducting slabs. Fast-velocity directions (FVDs) of azimuthal anisotropy in the slabs are generally trench-parallel, reflecting fossil lattice-preferred orientation of aligned anisotropic minerals and/or shape-preferred orientation due to transform faults produced at the mid-ocean ridge and intraslab hydrated faults formed at the outer-rise area near the oceanic trench. The slab deformation may play an important role in both mantle flow and intraslab fabric. Trench-parallel anisotropy in the forearc has been widely observed by shear-wave splitting measurements, which may result, at least partly, from the intraslab deformation due to outer-rise yielding of the incoming oceanic plate. In the mantle wedge beneath the volcanic front and back-arc areas, FVDs are trench-normal, reflecting subduction-driven corner flows. Trench-normal FVDs are also revealed in the subslab mantle, which may reflect asthenospheric shear deformation caused by the overlying slab subduction. Toroidal mantle flow is observed in and around a slab edge or slab window. Significant azimuthal and radial anisotropies occur in the big mantle wedge beneath East Asia, reflecting hot and wet upwelling flows as well as horizontal flows associated with deep subduction of the western Pacific plate and its stagnation in the mantle transition zone. The geodynamic processes in the big mantle wedge have caused craton destruction, backarc spreading, and intraplate seismic and volcanic activities. Ductile flow in the middle-lower crust is clearly revealed as prominent seismic anisotropy beneath the Tibetan Plateau, which affects the generation of large crustal earthquakes and mountain buildings.
... The Chuandian and Indochina blocks, which are extruded by massive low-Vs materials, have been identified to be the southeastern branch of the crustal flow (Bai et al., 2010;Bao et al., 2015b;Liu et al., 2014). The eastern rigid blocks, such as the Ordos and Sichuan blocks, restrict the crustal flow, and the West Qinling block, the passage between the Ordos and Sichuan blocks, cannot serve as the eastern branch of the crustal flow because the LVZs do not extrude into the passage and terminate in the Tibet-Qinling boundary (Ye et al., 2017;Sun and Zhao, 2020;Tian et al., 2021). In addition, Sun et al. (2021) claimed that the low-Vs extrusion has passed the Liupanshan fault (LPSF) arriving eastwards at the Xiaoguanshan fault in the Ordos block, and extended northwards into the Alxa block up to the Longshoushan fault by imaging 3D Vp and Vs models. ...
The uplift mechanism and geodynamic model of the northeastern (NE) Tibetan Plateau remain controversial. Two competing models have been proposed for the uplift of the NE Tibetan Plateau, including crustal shortening through folding or thrust faulting and the middle-lower crustal flow model. Here, we applied the joint inversion of surface wave dispersions and receiver functions with P-wave velocity constraints on a dense linear broadband seismic array to obtain the Vs and Vp/Vs profiles of the crust and uppermost mantle across northeastern Tibet. The inversion yields robust structural images that show stark Vs and Vp/Vs jumps across the West Qinling fault from the Songpan-Ganzi and Kunlun-West Qinling terranes (south) to the Qilian and North China terranes (north). The results reveal that regional low-Vs/high-Vp/Vs anomalies present in the middle-to-lower crust, which may be the signature of viscous flow, are distinctly limited south of the West Qinling fault. In contrast, the imaged physical properties of the crust and mantle lithosphere north of this fault manifest a rigid pattern, which is inferred to be originated from the North China craton underthrust on the northern margin of the Tibetan plateau. Our seismic imaging indicates that the NE corner of the Tibetan Plateau does not currently serve as the flow channel of the ductile material from the central and northern Tibet.
... In the crust beneath our study region, the Vp anisotropic tomographic images (Fig. 6) provide another piece of evidence for the structural heterogeneity. Several mechanisms can cause the crustal anisotropy (e.g., Huang et al., 2011;Sun and Zhao, 2020), such as preferential closure of random fractures (Boness and Zoback, 2004), stress-aligned fluid-saturated microcracks (Crampin, 1987), fossil fabrics relevant to paleo-stress (Blenkinsop, 1990), and preferential mineral orientation (Brocher and Christensen, 1990). ...
We apply local seismic tomography to 159,754 P-wave and 101,751 S-wave arrival times of 5287 local earthquakes to determine 3-D images of P- and S-wave velocity (Vp, Vs), Poisson's ratio (ν) and Vp azimuthal anisotropy in the source zone of the 2019 Ridgecrest earthquake in Southern California. Its big foreshock (M6.4) and mainshock (M7.1) took place in a high-velocity (high-V) and low-velocity (low-V) transition belt. The source zone exhibits significant low-Vs and high-ν anomalies in the lower crust and uppermost mantle, probably reflecting crustal fluids associated with upwelling mantle flow. A high consistency between the seismogenic fault of the 2019 Ridgecrest earthquake and a low-Vs and high-ν anomaly suggests that the fluids in the fault zone affected the rupture process. Fast-velocity directions (FVDs) of Vp azimuthal anisotropy in the upper crust reflect effects of faults and cracks, whereas mineral alignments affect the FVDs in the low-V parts of the lower crust. Dominant NW–SE and E–W FVDs are revealed in the study area, indicating that the Pacific-North American plate relative motion causes the crustal deformation of the study area.
... Thus, it is essential to propose a suitable model for understanding the mechanism of the growth and deformation in the northeastern margin of the Tibetan plateau. The crustal channel flow [5,57] and vertical coherent deformation of the entire lithosphere [58,59] have been debated recently to predict the very different seismic anisotropy and velocity structures in the northeastern margin of the Tibetan plateau [24,40,60,61]. ...
... According to the continuous low-velocity layer in the mid-lower crust associated with the observed seismic anisotropy, the crustal channel-flow model may explain the lateral crustal deformation along the southeastern margin of the Tibetan plateau [62][63][64]. In the Songpan-Ganzi terrane, the crustal thickening and deformation were also considered to be driven by the crustal channel flow based on the existence of regional low-velocity zones [60,61]. However, the low-velocity anomalies of Rayleigh wave phase velocity at periods of 30-40 s (Figure 7c,d), sampling the mid-lower crust (Figures 8 and 9), are discontinuous, coinciding with the distribution of S-wave velocity revealed from ambient noise tomography [65]. ...
The North China Craton (NCC) has experienced strong tectonic deformation and lithospheric thinning since the Cenozoic. To better constrain the geodynamic processes and mechanisms of the lithospheric deformation, we used a linear damped least squares method to invert simultaneously Rayleigh wave phase velocity and azimuthal anisotropy at periods of 10–80 s with teleseismic data recorded by 388 permanent stations in the NCC and its adjacent areas. The results reveal that the anomalies of Rayleigh wave phase velocity and azimuthal anisotropy are in good agreement with the tectonic domains in the study area. Low-phase velocities appear in the rift grabens and sedimentary basins at short periods. A rotation pattern of the fast axis direction of the Rayleigh wave together with a distinct low-velocity anomaly occurs around the Datong volcano. A NW–SE trending azimuthal anisotropy and a low-velocity anomaly at periods of 60–80 s are observed subparallel to the Zhangbo fault zone. The whole lithosphere domain of the Ordos block shows a high-phase velocity and counterclockwise rotated fast axis. The northeastern margin of the Tibetan plateau is dominated by a low-velocity and coherent NW–SE fast axis direction. We infer that the subduction of the Paleo-Pacific plate and eastward material escape of the Tibetan plateau mainly contribute to the deformation of the crust and upper mantle in the NCC.
... In the past two decades, the lower crustal ductile flow model has acquired much attention (Clark and Royden, 2000;Royden et al., 2008). Many seismological studies have revealed widespread low-velocity (low-V) zones in the lower crust beneath NE Tibet (e.g., Lei and Zhao, 2016;Sun and Zhao, 2020;Wei et al., 2013), which provide direct geophysical evidence for the lower crustal flow. To clarify the nature of the low-V zones and their role in the current deformation and seismotectonics of NE Tibet, we need to investigate the detailed 3-D structure and geodynamics of the region. ...
... Strong anisotropy has been widely revealed in the crust and upper mantle beneath NE Tibet by shear-wave splitting (SWS) measurements, which supports the existence of lower crustal flow and crust-mantle decoupling beneath the region (e. g., Huang et al., 2017;Sun and Zhao, 2020). SWS studies also suggested coherent vertical deformation throughout the lithosphere (e.g., Chang et al., 2017). ...
... However, the SWS measurements have poor depth resolution. In contrast, 3-D P-wave anisotropic tomography has good depth resolution and could provide new insight into crustal deformation and seismotectonics (e.g., Cheng et al., 2016;Sun and Zhao, 2020;Zhao and Hua, 2021). ...
We present detailed 3-D tomographic images of P and S wave velocity (Vp, Vs), Poisson's ratio (ν) and Vp azimuthal anisotropy of the crust and uppermost mantle beneath the source area of the 21 May 2021 Maduo earthquake (M 7.4) in the NE Tibetan Plateau. The images are obtained by inverting a large number of P and S wave arrival-time data of 11,235 local earthquakes and Maduo aftershocks recorded at 67 seismic stations. Our results show that the 2021 Maduo mainshock occurred in a low-Vs and high-ν anomaly, probably reflecting crustal fluids that affected the rupture nucleation. Our Vp anisotropy results show that at 40 km depth under the southern part of the study region, the fast-velocity direction (FVD) is NW-SE, which is mainly controlled by the India-Eurasia collision. At 60 km depth under the study region and at 40 km depth under the northern part of the region, the FVDs are NE-SW to N-S, reflecting lower crustal flow. The FVDs are roughly E-W at 60 km depth beneath the Qilian mountain range, which reflect the lower crustal flow that is blocked by a rigid terrane in the north. The lower crustal flow may lead to intra-crustal and crust-mantle decoupling and affect seismotectonics in NE Tibet.
