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On 21 May 2021 a magnitude Mw 6.1 earthquake occurred in Yangbi region, Yunan, China, which was widely felt and caused heavy casualties. Imaging of the source region was conducted using our improved double-difference tomography method on the huge data set recorded by 107 temporary stations of ChinArray-I and 62 permanent stations. Pronounced structural heterogeneities across the rupture source region are discovered and locations of the hypocenters of the Yangbi earthquake sequence are significantly improved as the output of the inversion. The relocated Yangbi earthquake sequence is distributed at an unmapped fault that is almost parallel and adjacent (∼15 km distance) to the Tongdian–Weishan fault (TWF) at the northern end of the Red River fault zone. Our high-resolution 3D velocity models show significant high-velocity and low-VP/VS ratios in the upper crust of the rupture zone, suggesting the existence of an asperity for the event. More importantly, low-VS and high-VP/VS anomalies below 10 km depth are imaged underlying the source region, indicating the existence of fluids and potential melts at those depths. Upward migration of the fluids and potential melts into the rupture zone could have weakened the locked asperity and triggered the occurrence of the Yangbi earthquake. The triggering effect by upflow fluids could explain why the Yangbi earthquake did not occur at the adjacent TWF where a high-stress accumulation was expected. We speculate that the fluids and potential melts in the mid-to-lower crust might have originated either from crustal channel flow from the southeast Tibet or from local upwelling related to subduction of the Indian slab to the west.

How the crust in the core of the Eastern Himalayan Syntaxis (EHS) deforms responding to the India-Asia collision remains ambiguous. Here we present the first high-resolution receiver functions image of crustal structure along a new NW-SE trending dense nodal array crossing the core of the EHS. Two sets of low velocity zones (LVZs) are clearly observed: one with a flat style beneath the western Lhasa terrane and Higher Himalaya at 18–20 km depth and the other with two west-dipping shapes below the western Yarlung-Zangbo suture within 10–30 km depth. These LVZs caused by partial melting and aqueous fluids are disconnected, impeding the formation of crustal flow. A discontinuous east-dipping intra-crustal discontinuity and a sharp Moho offset of 7 km under the Aniqiao-Motuo shear zone are identified, suggesting that the underthrusting of the Indian lower crust and pure shear mechanisms jointly dominate crustal deformation in the core of the EHS.

Various studies report on temporal changes of seismic velocities in the crust and attempt to relate the observations to changes of stress and material properties around faults. Although there are growing numbers of observations on coseismic velocity reductions, generally there is a lack of detailed observations of the healing phases. Here we report on a pronounced coseismic reduction of velocities around two locked sections (asperities) of the Longmenshan fault with a large slip during the 2008 Mw 7.9 Wenchuan earthquake and subsequent healing of the velocities. The healing phase accelerated significantly at the southern asperity right after the nearby 2013 Mw 6.6 Lushan earthquake. The results were obtained by joint inversions of travel time data at four different periods across the Wenchuan and Lushan earthquakes. The rapid acceleration of healing in response to the Lushan earthquake provides unique evidence for the high sensitivity of seismic velocities to stress changes. We suggest that stress redistribution plays an important role in rebuilding fault strength. After a fault ruptures, the recovery of its strength may be accelerated by earthquakes that redistribute stress, according to an analysis of temporal variation in crustal seismic velocity on the Longmenshan fault in China.

We present detailed lithospheric images of the NE Tibetan Plateau by applying the depthmigration technique toSreceiver functions derived from 113 broadband stations. Our migrated imagesindicate that the lithosphere‐asthenosphere boundary (LAB) lies at depths of 105–120 km beneath the Qilianterrane and reaches depths of 126–140 km below the Alxa and Ordos blocks. The most prominent variationin the LAB depth is the presence of LAB steps of no less than 20 km in the transition zone between theactive NE Tibetan Plateau and the surrounding cratonic Alxa and Ordos blocks, which conflicts with themodel of southward subduction of the Alxa and Ordos blocks. Furthermore, the marked LAB steps occur at130 ± 10 km away from the southern surficial boundary faults between the NE Tibetan Plateau and thesurrounding tectonic provinces, corresponding to the North Qilian fault and the Liupanshan fault,respectively. Therefore, we propose that these scenarios of LAB can be attributed to the delamination offragmented mantle lithosphere in the transition zone between the NE Tibetan Plateau and the surroundingAlxa and Ordos blocks, triggered by lateral asthenosphericflow. In addition, our observations of a thinlithosphere with thickness of 107–115 km beneath the Songpan‐Ganzi terrane and the west Qinlin orogengreatly facilitate the process of underlying lateral asthenosphericflow. The isostatic uplift of the plateaucaused by the delamination of fragmented mantle lithosphere, together with increased horizontalcompressive stress, may have led to the outward growth of the NE Tibetan Plateau.

Detailed crustal structure of large earthquake source regions is of great significance for understanding the earthquake generation mechanism. Numerous large earthquakes have occurred in the NE Tibetan Plateau, including the 1920 Haiyuan M8.5 and 1927 Gulang M8 earthquakes. In this paper, we obtained a high-resolution three-dimensional crustal velocity model around the source regions of these two large earthquakes using an improved double-difference seismic tomography method. High-velocity anomalies encompassing the seismogenic faults are observed to extend to depths of 15 km, suggesting the asperity (high-velocity area) plays an important role in the preparation process of large earthquakes. Asperities are strong in mechanical strength and could accumulate tectonic stress more easily in long frictional locking periods, large earthquakes are therefore prone to generate in these areas. If the close relationship between the aperity and high-velocity bodies is valid for most of the large earthquakes, it can be used to predict potential large earthquakes and estimate the seismogenic capability of faults in light of structure studies.

To explore the west‐east variations in the underthrusting Indian lithosphere, we investigate the lithospheric structure using P and S receiver functions obtained from a west‐east linear array of 19 broadband stations spanning the Tethyan Himalaya at 85.5–88.5°E. Our observations of the Main Himalayan Thrust (MHT) and Moho serving as the top and bottom boundaries at depths of 42–54 km and 60–72 km, respectively, suggest that a thinning Indian lower crust with an eastward inclined geometry is underthrusting beneath the Tethyan Himalaya. An intra‐crustal low velocity zone (LVZ) immediately above the MTH at depths of 32–42 km is imaged, likely resulting from the presence of crustal dehydration partial melting. We propose that this LVZ induces the southward extrusion of melt‐weakened crustal material with itself as a channel and facilitates the development of duplexing structures, by which Indian lower crustal materials could be accreted into the Himalayan wedge along the north‐south mid‐crustal ramp in the MHT. The eastward dipping lithosphere‐asthenosphere boundary is clearly observed ranging from 90 to 126 km, which generally coincides with the changing trends of the MHT and Moho. These observations can be attributed to the eastward steepening subduction angle of the Indian lithosphere.

In recent years, many small- to moderate-size earthquakes occurred in central northern Oklahoma, likely associated with wastewater injection. The most recent one is the M5.8 Pawnee earthquake occurred on 3 September 2016. It is still not clear what controls the locations of these injection-induced earthquakes. Here we conduct 2-D Pg wave tomography with anisotropy to image seismogenic structures in this region, using more than 10 years of Pg arrivals recorded by 81 seismic stations. Our high-resolution Pg wave tomography shows two high-velocity zones with northwest fast direction alternated by low-velocity zones with northeast fast direction. The two dominant anisotropy directions are consistent with conjugated microfractures from surface faults and two nodal planes from focal mechanisms of moderate-size earthquakes. Most moderate-size (M > 4) earthquakes occurred either close to the boundaries between high- and low-seismic velocity zones or within the high-velocity zones, suggesting that they are associated with geological boundaries of different basement rock properties or with strong material properties in the upper crust. We suggest that although these moderate-size earthquakes were induced by fluid injection, their spatial locations were likely controlled by local geologic structures.

We analyze the teleseismic waveform data recorded by 42 temporary stations from the Y2 and ANTILOPE-1 arrays using the P and S receiver function techniques to investigate the lithospheric structure beneath western Tibet. The Moho is reliably identified as a prominent feature at depths of 55-82 km in the stacked traces and in depth migrated images. It has a concave shape and reaches the deepest location at about 80 km north of the Indus-Yarlung suture (IYS). An intra-crustal discontinuity is observed at ~55 km depth below the southern Lhasa terrane, which could represent the upper border of the eclogitized underthrusting Indian lower crust. Underthrusting of the Indian crust has been widely observed beneath the Lhasa terrane and correlates well with the Bouguer gravity low, suggesting that the gravity anomalies in the Lhasa terrane are induced by topography of the Moho. At ~ 20 km depth, a mid-crustal low velocity zone (LVZ) is observed beneath the Tethyan Himalaya and southern Lhasa terrane, suggesting a layer of partial melts that decouples the thrust/fold deformation of the upper crust from the shortening and underthrusting in the lower crust. The Sp conversions at the lithosphere-asthenosphere boundary (LAB) can be recognized at depths of 130-200 km, showing that the Indian lithospheric mantle is underthrusting with a ramp-flat shape beneath southern Tibet and probably is detached from the lower crust immediately under the IYS. Our observations reconstruct the configuration of the underthrusting Indian lithosphere and indicate significant along strike variations.

High-resolution 3-D P and S wave velocity (Vp and Vs) models of the crust and uppermost mantle beneath the northeastern (NE) Tibetan Plateau and the surrounding areas were obtained by applying an improved double-difference seismic tomography method on abundant body wave travel-time data of local and regional earthquakes collected from 1985 to 2018. The improved method not only increases constraints from observed Moho depth information but also incorporates the later-arriving Pg and Sg phases to improve the ray coverage in the middle-lower crust and uppermost mantle. The results show significant low-velocity anomalies in the crust beneath the NE Tibetan Plateau and high-velocity features beneath the surrounding stable Alxa and Ordos blocks. The areas near the boundaries of the cratonic blocks show high velocity in the upper crust and uppermost mantle beneath the Hexi Corridor, and high velocity in the upper crust beneath the Liupan Shan belt. Both of those regions have significant low-velocity anomalies in the middle-lower crust similar to the Tibetan Plateau, which suggests they probably were parts of the Alxa and Ordos blocks respectively, and were cut off from the two cratonic blocks during the lateral expansion of the Tibetan Plateau because of middle-lower crustal ductile deformation and asthenospheric flow. We propose a new tectonic model controlled by middle-lower crustal ductile deformation and asthenospheric flow to illuminate the tectonic evolution and growth mechanism of the NE Tibetan Plateau.

The Mw7.8 Gorkha Earthquake struck Nepal and ruptured the boundary between the Indian and Eurasian plates. We conducted 2D Pg-wave tomography to clarify the seismogenic structure and try to understand causal mechanisms for this large earthquake, using the aftershock data recorded by 15 broadband seismic stations located near the China-Nepal border. Our high-resolution results show that coseismic slip area of the main shock is consistent with the high P-wave velocity anomaly, and the region of maximum slip has a larger area with higher velocity than the region of initial slip, possibly resulting in the dominant low-frequency radiation of energy observed after the dominant high-frequency radiation of energy in the source rupture process. The boundary between these regions of contrasting high and low seismic velocity anomalies suggests a potential crustal tearing at the southern end of the Tangra Yum Co Rift, possibly resulting from different thrust speeds in the Greater Himalaya.

In this study, we analyze available weak‐ and strong‐motion seismograms to discern the main source characteristics of the 12 August 2015 Tianjin explosions. The disaster involved two large explosions, which can be clearly identified in the near‐field seismic recordings. The first explosion (event 1) took place at 15:34:04.68 (UTC) and the second at 15:34:36.94 (UTC). The average time interval between the two events was about 32.3 s, which is consistent with the result determined by cross correlation of the whispering gallery wave ( WG phase). The two events did not occur at the same site, and the second explosion was located 65±10 m northwest (N352°W) of the first one. The local magnitudes M L of the two explosions were further measured as 2.4 and 2.9, respectively. The Fourier spectra from the two closest stations, DAG and BET, show that the predominant frequency of the waveforms ranged from 0.7 to 0.9 Hz after the long‐period wavetrains had arrived. According to the distribution of the three‐component peak ground acceleration, the area with a shaking intensity of modified Mercalli intensity VI spanned up to 131 km2. However, the peak intensity of this explosion did not reach an intensity of VII, based on the ground‐motion attenuation relation.

We have conducted a systematic shear wave splitting analysis using 1000 selected aftershocks with M > 2 from the 2013 Ms 7.0 Lushan earthquake along the Longmenshan fault system in southwest China. Polarization directions of fast shear waves show a bimodal distribution with one dominant direction approximately parallel to the fault strike and the other close to the regional maximum horizontal compressive stress direction. This indicates that in this area mechanisms causing crustal seismic anisotropy are both stress induced and fault zone structure controlled. Delay times between fast and slow shear waves do not show a clear trend of increase for deeper events, suggesting the anisotropic zone is mostly above the aftershocks, which are generally located below 8 km. We further applied a shear wave splitting tomography method to measured delay times to characterize the spatial distribution of seismic anisotropy. The three-dimensional anisotropic percentage model shows strong anisotropy above 8 km but low anisotropy below it. The mainshock slip zone and its aftershocks are associated with very low or negligible anisotropy and high velocity, indicating that the zones with high anisotropy and low velocity above 8 km are mechanically weak and it is difficult for stress to accumulate there. The main and back reverse fault zones are associated with high anisotropic anomalies above ∼8 km, likely caused by shear fabric or microfractures aligned parallel to the fault zone.

Seismic anisotropy in the upper mantle has been inferred from shear wave splitting measurements at the temporary seismic array (ANTILOPE-2) in the central-southern Tibet from September 2005 to October 2006. Teleseismic shear waves (mainly SKS and SKKS) are used to determine the splitting parameters (fast polarization direction and delay time). Weak splitting with average delay time of 0.3 s is observed at stations in the southern Lhasa terrane and northernmost Tethyan Himalaya. Significant seismic polarization anisotropy with delay times from 0.8 to 1.5 s and NE-oriented fast direction is obtained for stations located farther north in the northern Lhasa terrane. The transition in shear wave splitting characteristics occurs at 30.5°N and suggests distinct upper mantle deformation from south to north. The negligible anisotropy south of this transition indicates that there is no appreciable large-scale mantle seismic polarization anisotropy. The substantial anisotropy in the north may be caused by the eastward flow in a lithospheric crush zone which is squeezed and sheared between the advancing Indian plate to the south and Eurasian plate to the north.

We examine the Pn and Sn travel time data with long and short epicentral distance ranges separately and obtain 2.5-dimensional tomographic models for the uppermost mantle velocity and the Vp/Vs ratio beneath the Sichuan–Yunnan and adjacent regions. The tomographic models show that the average Vp and Vs of the upper layer are approximately 8.10 km/s and 4.60 km/s and those of the deeper layer are 8.19 km/s and 4.65 km/s, respectively; the average velocity ratio is 1.76 under the study area. A comparison of the Pn and Sn velocity distribution reveals that the high- and low-velocity anomalies are consistent. High velocity and low Vp/Vs ratios exist in the Sichuan Basin and low velocity and high ratios distribute in the Myanmar–Yunnan, Songpan–Ganzi, and Hainan regions, respectively. Our models provide more evidence for the subduction of the Indian plate beneath the Myanmar–Yunnan region and the existence of the hot material upwelling beneath the Hainan region. In our velocity model, there is a significant difference between the southern and the northern parts of the Yunnan region (i.e. north and south of 26° N); we infer that these regions are controlled by different dynamic processes. The velocity distributions at different depths are mainly similar in most of the regions of the study area, and the differences between the upper layer and the deeper layer beneath the Yunnan region indicate that the heat flow from the upper mantle has complex characteristics.

The Longmen-Shan fault zone, at the eastern margin of the Tibetan Plateau, is one of the most extensively studied areas in the world, yet the deformation model and earthquake-generating mechanism remain subjects of vigorous debate. This paper presents a new three-dimensional (3-D) velocity model determined using a large volume of seismic data and two-dimensional (2-D) magnetotelluric (MT) profiles from previous studies, to investigate the mechanisms of crustal deformation and earthquake generation along the reverse-thrust and strike-slip fault zone. It has been observed that low-velocity, and low-resistivity anomalies related to the Sichuan foreland basin, is in sharp contrast to high-velocity and high-resistivity anomalies in the Songpan-Ganze block in the upper crust. The tomographic model presented here reveals two crustal bodies with low-velocity and high-conductivity anomalies underneath the Longmen-Shan fault zone, separated into three contrasting segments by the two crustal bodies. The two low-velocity and low-resistivity bodies have been interpreted as being associated with extrusion of either fluids or products of partial melting from the lower crust and/or the upper mantle. This suggests strong variations in the rheological strength of the rock along the fault zone. This finding implies that coupling between these presumably fluid-bearing bodies and earthquake generation could be extremely complex and that there is dramatic variation from the southwestern area to the northeastern segment along the fault belt It is suggested here that this complex and variable deformation system along the fault zone played a principal role in controlling seismic generation and rupturing during the 2008 Wenchuan earthquake (Ms 8.0) and that it will do so again during possible future earthquakes in the region. (C) 2014 Elsevier B.V. All rights reserved.

We present a tomographic Sn velocity model of the uppermost mantle beneath the Himalayan collision zone and surrounding regions. A total of 43,905 Sn phases are used in the investigation. The average Sn velocity in the study area is approximately 4.6 km/s, and the velocity perturbations reach 0.2 km/s. The Sn velocity distribution is consistent with Pn tomography results obtained previously. High velocities are found under the Indian plate, the Tarim Basin, and the Sichuan Basin, whereas low Sn velocities are found beneath the Myanmar region, the Hindu-Kush region, and the Lhasa block and western Qiangtang block. These results support the idea that the lithosphere of the Indian plate is subducted into the mantle and causes the upwelling of hot material. The east-west variability of the Sn velocity beneath the Indian plate and southern Tibet indicates that the underthrusting of the Indian continental lithosphere may be in a piecewise manner. The differences between the thermal structure of the crust and upper mantle in southern Tibet suggest that this region may be represented by a tectonic model of hot crust and cold mantle, supporting the idea that crustal material flow may occur in this region.

[1] We employ the P and S receiver function technique to data from the 44 seismic stations deployed in the eastern Himalayan syntaxis to investigate the crustal thickness, the average Poisson's ratio, and the depth of the lithosphere-asthenosphere boundary (LAB). The observed crustal thickness exhibits an overall NE-deepening trend, varying from 55 to 75 km. Two anomalous areas lie in the west and east of the Namche Barwa syntaxis characterized by thinner and thicker crust, respectively. The average Poisson's ratios within the study area are low in the north and moderate elsewhere with some high values in the south, consistent with felsic and intermediate rocks forming the crust. Our migrated images reveal that (1) the LAB of the Tibetan plate exists at relatively shallow depths (~110 km) and exhibits a gap beneath the Namche Barwa syntaxis, which may have formed by the delamination of mantle lithosphere due to local mantle upwelling, and (2) the LAB of the Asian plate is observed at a depth of ~180 km, which implies that the Asian plate has advanced southward to about 30°N under the Lhasa terrane. Our results provide new insights into the understanding of continental subduction and lithospheric deformation of the eastern Himalayan syntaxis.

We investigate the crustal thickness, Poisson’s ratio, and the thickness variation of the mantle transition zone using the P receiver function technique on data recorded by the 68 seismic station of the ASCENT experiment beneath the northeast Tibetan plateau. Our results reveal a distinct crustal and upper mantle structure contrast between the western and eastern part of the northeast Tibetan plateau along two migrated profiles. In the eastern region the Moho is relatively smooth and continuous and varies in depth between ∼80 and 51 km, whereas the depth of the Moho beneath the western region varies between ∼77 and 52 km and is offset ∼20 km beneath the boundary between the Kunlun fault and Qaidam basin. The delay times for the Ps phases from both the 410-km and 660-km discontinuities in the western region are ∼1.2 s later than that in the eastern region, indicating that the average velocities of the upper mantle in the western region are lower than that in the eastern region. Crustal rocks with low- to moderate Poisson’s ratios are interpreted to be dominated by felsic to intermediate compositions, implying that large scale middle-lower crustal flow does not occur easily. The nearly constant thickness of the mantle transition zone, with an average value of ∼255 km (24.4 s) from south to north, implies that no lithospheric fragments correlating to the India–Asia collision have entered into the mantle transition zone.

To investigate upper crust anisotropy, a tomography algorithm was reformulated to include lateral variations in both velocity and azimuthal anisotropy, and it was applied to Pg travel time data around the epicenter of the 14 April 2010 Ms = 7.1 Yushu earthquake. A high-resolution two-dimensional (horizontal) seismic velocity and azimuthal anisotropy model was obtained using this simple tomography method including both station and event depth corrections. The results show clear anisotropy in the upper crust and it is as important as the velocity variation in explaining the travel time residuals. The most striking result is a high velocity anomaly with large anisotropy at the epicenter on the Yushu–Garze fault. The main rupture originated within this high velocity anomaly and propagated southeastward into a low velocity anomaly with small anisotropy at Yushu. The azimuthal anisotropy shows fast velocity direction along Yushu–Garze fault and larger anisotropy exists in the epicenter than surrounding regions. These results demonstrate a clear example that lateral variation and anisotropy in seismic structures of the upper crust controlled the origination (stress accumulation) and rupture propagation of the 2010 Yushu earthquake and distribution of aftershocks as well. The simple 2D Pg-wave travel time tomography method presented here introduces a new approach utilizing abundant aftershocks data for investigating the rupturing process of a major earthquake and possible fracture distribution around seismogenic zone.

We present the results of a seismic wide-angle reflection/refraction profile across the central Qaidam basin, the largest basin within the Qinghai-Tibetan plateau. The 350-km‐long profile extends from the northern margin of the East-Kunlun Shan to the southern margin of the Qilian Shan. The P- and S-wave velocity structure and Poisson's ratio data provide constraints on composition. The crust here consists of a near-surface sedimentary layer and a four-layered crystalline crust having several significant features. (1) The sedimentary fill of the Qaidam basin reaches a maximum thickness of 8 km, and the basin shape mirrors the uplifted Moho. (2) Within the four layers of the crystalline crust, P- (S-) wave velocities increase with depth and fall within the following velocity ranges: 5.9–6.3 km/s (3.45–3.65 km/s), 6.45–6.55 km/s (3.7 km/s), 6.65 km/s (3.8 km/s), and 6.7–6.9 km/s (3.8–3.9 km/s), respectively; (3) low-velocity zones with a 3–5% reduction in seismic velocity are detected in the lower half of the crust beneath the Qaidam basin and its transition to the Qilian Shan. (4) The crystalline crust is thickest beneath the northern margin of the basin towards the Qilian Shan (58–62 km) and thinnest beneath the center of the basin (52 km). Variations in crustal thickness are caused most pronouncedly by thickness variations in the lowermost layer of the crust. (5) Poisson's ratio and P-wave velocity values suggest that the Qaidam crust has an essentially felsic composition with an intermediate layer at its base. Based on the crustal structure reported here, we suggest that late Cenozoic convergence is accommodated by thick-skinned tectonic deformation with thickening involving the entire crust across the Kunlun–Qaidam–Qilian system.

As the western end point of continental collision between the Indian and Eurasian plates, Pamir is an ideal place to research uplifting mechanisms in the Tibetan plateau. In this study, 141 644 Pn arrivals were used to obtain seismic wave velocities and anisotropy in the uppermost mantle beneath Pamir and its adjacent regions by performing tomographic inversion of Pn travel times. The data were selected from multiple databases, including ISC/EHB, the Annual Bulletin of Chinese Earthquakes, and regional bulletins of Xinjiang. The tomography results reveal significant features with high resolution and correlate well with geological structures. The main results are as follows: (1) The Pn wave velocities are particularly high in the old stable blocks such as Tarim basin, Indian plate and Tajik basin, while the low Pn velocities always lie in tectonically active regions like the western Tibetan plateau, Pamir, Tianshan and Hindu Kush. (2) Strong Pn anisotropy is found beneath the Indian-Eurasian collision zone; its direction is parallel to the collision arc and nearly perpendicular to both the direction of maximum compression stress and relative crustal movement. The result is probably caused by the pure shear deformation in the uppermost mantle of the collision zone. (3) A geodynamic continent-continent collision model is proposed to show anisotropy and collision mechanisms between the Indian plate and the Tarim and Tajik basins.

A high‐resolution 2D (horizontal) seismic velocity model is presented for the upper crust around the epicenter of the 14 April 2010 M s 7.1 Yushu earthquake from aftershock tomography. The velocity model is obtained using a simple Pn ‐like tomography method including both station and event depth corrections. The most striking result is a fast‐velocity anomaly at the epicenter on the Yushu–Garze fault (YGF). The main rupture originated within this fast‐velocity anomaly and propagated southeastward into a slow‐velocity anomaly at Yushu. The second largest aftershock on 29 May 2010 on a nearby fault also occurred within this fast‐velocity anomaly. These results demonstrate a clear example that lateral variation in seismic velocity structures of the upper crust controlled the origination (stress accumulation) and rupture propagation of the 2010 Yushu earthquake and distribution of aftershocks as well. The simple 2D Pg wave travel‐time tomography method presented here introduces a new approach utilizing abundant aftershocks data for investigating the rupturing process of a major earthquake.

We present tomographic velocity and anisotropy models of the uppermost mantle beneath the Iran region. A total of 74,375 Pn phase readings from 86 stations of the Iranian network and 133 stations of the International Seismological Centre are used in the investigation. The study uses the Pn travel‐time tomography method proposed by Hearn (1996). The tomography results show some interesting anomalies. The average Pn velocity under the Iran region is approximately 8.0 km/s, and the maximum velocity perturbations are approximately 3%–4%. High Pn velocities under the Zagros Mountains and the Caspian Sea may be due to the presence of oceanic crust/lithosphere material. Low Pn velocities were found under the Alborz and Caucasus regions and may be due to higher temperatures or partial melting resulting from volcanoes and mid‐Cenozoic volcanic/plutonic rocks in these regions. The inversion velocities support the idea that the subduction of the Arabian plate into the mantle beneath the Iranian plateau may have resulted in the upwelling of hot material. The well‐resolved Pn anisotropy model is jointly obtained with a velocity model for the areas with good ray‐path coverage. In the plate collision regions (Zagros and Alborz), the fast Pn anisotropy direction is oriented parallel to the collision arc and to large reverse faults due to pure shear deformation from cross‐fault compression and along‐fault extension. Under the Caucasus regions, the Pn anisotropy results indicate that the preferred alignment of olivine crystals is parallel to the plate movement direction; however, the surface fault strike is at an angle of nearly 45° with the crustal movement direction and anisotropy. These differing deformations suggest potential decoupling between the crust and upper mantle. The possible decoupling and differing deformation between the crust and upper mantle are easily enhanced under high temperatures due to volcanoes and supported by low velocities beneath the Caucasus. We validate the existence of Pn anisotropy under these regions by azimuthal averaging of the apparent Pn velocity.

To investigate the generation mechanism of the 2011 great East Japan (GEJ) earthquake (M 9.0), we determined a three-dimensional (3-D) seismic model under the NE Japan forearc region using a large number of P and S wave arrival times from local earthquakes. Our results show that the GEJ main-shock occurred in a high-velocity (Vp, Vs) zone with higher Poisson's ratio (σ). The aftershocks relocated with a master-event location (MEL) procedure indicate that most of them are located at the corner of the mantle wedge along the upper boundary of the subducting Pacific slab, and their focal depths are slightly shallower than the background seismicity. The tomographic images and spatial distribution of the aftershocks imply strong interplate coupling (asperity) in the main-shock area and weak coupling in the surrounding areas of the megathrust zone. We think that fluids in the crust and uppermost mantle in the subduction zone could have affected the earthquake generation through the physical role of fluid pressure and a variety of chemical effects. We conclude that the 2011 GEJ earthquake initiation and the rupture processes of the aftershock sequence were influenced by fluid extrusion into the rupture zone due to the dehydration of the subducting Pacific slab. Fluid-bearing structural heterogeneities with thermal–petrologic variations in the megathrust zone played a key role in the initiation of the 2011 GEJ earthquake and its aftershocks.

SUMMARY Detailed high-resolution images of the crust–mantle and lithosphere–asthenosphere boundaries (the Moho and the LAB, respectively) have been well observed by applying an S-receiver function technique to data collected by the Hi-CLIMB (Himalayan-Tibetan Continental Lithosphere During Mountain Building) experiment. The Moho depth variation in the range of ∼50–70 km is in good agreement with that from previous P-receiver function results. The significant variation in the LAB depth indicates that the subducting Indian lithosphere drops northwards from a depth of ∼80 km beneath the Himalayas to ∼130 km just north of the Bangong–Nujiang suture at ∼33.0 ◦ N, and undergoes a transition from low angle to flat subduction beneath the Yarlung–Zangbo suture. Our findings provide new seismic constraints on the 3-D subducting configuration of the Indian lithosphere beneath Tibet.

We have obtained a Poisson's ratio image of the uppermost mantle beneath China by performing tomographic inversion of travel-time differences between Sn and Pn. The arrival pairs were selected from the Annual Bulletin of Chinese Earthquakes from 1985 to 2007 and from the International Seismological Center (ISC) data set between 1985 and 2005. The data include 58,663 arrival pairs from 10,486 earthquakes recorded at 204 stations. The average Poisson's ratio is 0.266. The preliminary tomographic results show that (1) the pseudowave velocity is high and the velocity ratio (V(P)/V(S)) and Poisson's ratio are low in the stable cratons around the Tibetan Plateau such as the Tarim and Junggar basins, the Ordos craton, and the southern region of the Sichuan basin; (2) a low pseudowave velocity and high velocity ratio and Poisson's ratio exist in the central and northern Tibetan Plateau, the North-South Seismic Zone, and north China; and (3) the high velocity ratio and Poisson's ratio region in the Tibetan Plateau extends to the surrounding cratons, suggesting that the uplift of the Tibetan Plateau results from a high Poisson's ratio or partially melted rocks beneath the plateau and that the softer rocks have intruded into the upper mantle of surrounding cratons.

Seismic imaging together with global positioning system (GPS) and crustal stress data analyses show that the Mw7.9 2008 Wenchuan earthquake occurred within a distinct area of high crustal stress (∼ 17.5 MPa) and high Poisson's ratio (7–10%) anomalies centered on the Longmen-Shan (Shan means Mountain in Chinese) tectonic fault belt. Low P-wave and S-wave velocities in the southwest (SW) segment contrast with high-velocity anomalies in the central portion (CP) and northeast (NE) segment within the uppermost ∼ 15 km depths along the tectonic fault belt, though a presumably ductile zone with low-velocity anomalies separates the CP and NE segment. The rupture initiated near the southwestern end of the CP at a zone of high Poisson's ratio (σ) which extends down into the lower crust. These low-velocity and high-σ anomalies immediately below the source hypocenter, together with the high crustal stress, indicate the presence of high-pressure fluids from the lower crust, which might have reduced the mechanical strength of the fractured rock matrix and triggered the earthquake. Our study suggests that the structural heterogeneity and high crustal stress played an important role in the nucleation of the Wenchuan earthquake and its rupture process.

Seismic tomography reveals an “R-shape” regional flow constrained between the depths of 50 to 80 km in the Southeastern Tibetan Plateau (STP) which demonstrates some of the differences revealed by the magnetotelluric (MT) soundings in some areas. The “R-shape” flow could be present in both the lower crust and uppermost mantle, but not in the lower crust above the Moho discontinuity. Lateral flow has been imaged under the Qiangtang and Songpan-Ganzi blocks while two channel flows have been revealed beneath the south part of the STP with the eastward lateral flow from the Qiangtang block separating into two channel flows. One branch turns southwards at the south Qiangtang block, along the Bangong-Nujiang fault reaching to the Indochina block, and another is across the Songpan-Ganzi block (fold system) which then separates into northward and southward parts. The northward branch is along the edge of the north Sichuan basin reaching to the Qingling fault and the southward channel turns south along the Anninghe fault, then turns eastward along the margins of the south Sichuan basin. Our study suggests that the crustal deformation along the deep, large sutures (such as the Longmen Shan fault zone) is maintained by dynamic pressure from the regional flow intermingled with the hot upwelling asthenosphere. The material in the lower crust and uppermost mantle flowing outward from the center of the plateau is buttressed by the old, strong lithosphere that underlies the Sichuan basin, pushing up on the crust above and maintaining steep topography through dynamic pressure. We therefore consider that the “R-shape” regional flow played a key role in the crustal deformation along the deep suture zones of the Bangong-Nujiang, the Longmen-Shan faults, and other local heavily faulted zones.

This is the first of two papers that describes a regional tomography investigation, which combines P-wave arrival times of both regional and teleseismic earthquakes to obtain 3D mantle structures of East Asia up to 1 000 km depth. The most important findings of this tomography study are reported in this paper as follows. (1) No fast P-wave velocity anomalies can be related to subducted oceanic slabs beneath the 660 km discontinuity; instead the subducted oceanic slabs become flattened and stagnant within the transition zone. (2) The high velocity anomalies in the transition zone extend up to 1 500 km to the westward of the active trenches, which is a unique feature in the worldwide subduction systems. (3) Slow P-wave velocity anomalies are visible up to ∼250 km underneath most of the East Asia on the east of 115°E, similar to the area of the stagnant slabs. These observations have important implications for the geodynamic process at depths beneath the East Asia, which might in turn control the widespread Cenozoic volcanism and associated extensional tectonics seen at the Earth’s surface.

We have updated the lateral variations of the quality factor Q0 (Q at 1 Hz) beneath the crust of North China using ML amplitude tomography with near three times data. The data were selected from the Annual Bulletin of Chinese Earthquakes (ABCE) in 1985–2009, including 26 283 ML amplitude readings from 4 204 events recorded by 38 stations. The result is similar with previous research but has higher resolution. Estimated Q0 values are consistent with tectonic and topographic structure in North China. Q0 is low in the active tectonic regions having many faults, such as Bohai bay, North China basin, the Shanxi and Yinchuan grabens, while it is high in the stable Ordos craton. Q0 values are low in several topographically low-lying areas, such as the North China, Taikang-Hefei, and Subei-Huanghai Sea basins, whereas it is high in mountainous and uplift regions exhibiting surface expressions of crystalline basement rocks: the Yinshan, Yanshan, Taihang, Qinling and Dabie mountains, Luxi and Jiaoliao uplifts. Quality factor estimates are also consistent with Pn and Sn velocity patterns. High velocity values in general correspond with high Q0 and vice versa. This coincides with a common temperature influence in the crust and uppermost mantle.

P-wave arrival times of both regional and teleseismic earthquakes were inverted to obtain mantle structures of East Asia. No fast (slab) velocity anomalies was not find beneath the 660-km discontinuity through tomography besides a stagnant slab within the transition zone. Slow P-wave velocity anomalies are present at depths of 100–250 km below the active volcanic arc and East Asia. The western end of the flat stagnant slab is about 1 500 km west to active trench and may also be correlated with prominent surface topographic break in eastern China. We suggested that active mantle convection might be operating within this horizontally expanded “mantle wedge” above both the active subducting slabs and the stagnant flat slabs beneath much of the North China plain. Both the widespread Cenozoic volcanism and associated extensional basins in East Asia could be the manifestation of this vigorous upper mantle convection. Cold or thermal anomalies associated with the stagnant slabs above the 660-km discontinuity have not only caused a broad depression of the boundary due to its negative Clapeyron slope but also effectively shielded the asthenosphere and continental lithosphere above from any possible influence of mantle plumes in the lower mantle.

We present three-dimensional (3D) seismic compressional wave velocity ( V p ), shear wave velocity ( V s ) and V p / V s models around the Longmen Shan fault, Sichuan, China region, using aftershocks associated with the 2008 Wenchuan Ms 8.0 earthquake. The velocity and ratio models are obtained using a new version of the double-difference seismic tomography code tomoDDPS (Zhang, 2003) to simultaneously solve for V p , V s , V p / V s and event locations. The data used in inversion include 73,013 P arrival times, 62,287 S arrival times and 61,823 S–P travel times recorded by 63 stations from both permanent and temporary networks in a region 400 km northeast-southwest by 200 km northwest-southeast. The velocity model shows structure heterogeneity both along and across the fault zone. Generally, the velocity model is consistent with the local geology, with older rocks having high velocity and younger rocks having low velocity. The Longmen Shan fault zone is a clear boundary in V p , V s and V p / V s ratio. Down to the depth around 15 km, higher V p and V s and lower V p / V s ratio exist to the west of the fault, corresponding to the Songpan–Ganze Fold System, while lower V p and V s and higher V p / V s ratio exist to the east, corresponding to the Sichuan Basin. Along the fault zone, the velocity structure is generally consistent with various rupture slip models, with two high velocity bodies corresponding well to the two large slip patches. This shows the structure control on the slip distribution along the fault plane.

The Eastern Himalayan Syntaxis (EHS) is a singular point of continental collision between the Indian and Eurasian plates, and has been a focus of geoscientific researches. We added the ISC data of 1964~2006 to a previous Chinese data set used by Pei et al., and 349475 Pn arrivals were selected in all. We have obtained seismic velocities and anisotropy in the uppermost mantle around EHS and surrounding regions by performing tomographic inversion of Pn travel times. Comparing Pn velocity variation with geological results, we find that they have distinct correlation: particularly high velocities always exist in old stable regions such as Sichuan Basin and Indian Plate. Low Pn velocities lie in active tectonic regions, volcanic areas and magmatic rock regions, for example, eastern Tibetan Plateau and North-South Seismic Zone, southern Yunnan province and northern Indochina. The Pn anisotropy shows that the fast directions of Pn turn around the EHS from NE in southeastern Tibet Plateau, then SE to NS in North-South Seismic Zone. But the direction changes sharply to EW in the northern Indochina. The rotation of Pn fast direction may be a result of mass extrusion from Tibet under simple shear, which is caused by mass flow in southeastern Tibetan Plateau and North-South Seismic Zone relative to stable EHS and Sichuan Basin. The EW Pn direction probably results from pure shear, which is caused by compression in NS and extension in EW in Burma back-arc extensional area. The polarization directions of SKS have similar rotation to Pn, but in northern Indochina the area of EW-oriented fast direction of Pn has a southward offset of about 3° than SKS, which may result from the faster movement of upper lithosphere in this area.

Knowledge of tectonic crustal stress in highland region can provide important insights into relative plate motion, mountain building and genesis of larger crustal earthquakes underneath tectonically active regions. Here we focus on the highland regions in Southwest (SW) China, including southeast Tibet and Sichuan and Yunnan provinces. To better understand the correlation between crustal stress and structural heterogeneities, three-dimensional (3-D) velocity (Vp, Vs) structures in the crust are imaged using a large number of P- and Pn-wave, and S- and Sn-wave arrival times simultaneously. A slow-velocity layer at depths of 16–26 km was clearly imaged in most regions of SW China, suggesting the presence of a ductile layer that may be associated with partial melting and/or aqueous fluids in the crust. The azimuths of the maximum compressional stresses estimated using 147 high-quality stress data revealed a characteristic regional stress field along the fault belts in the study region. The trajectories of the stress axes along the southeastern Tibetan plateau vary from NE–SW in southwest Tibet to E–W in southeast Tibet. Our study indicates that, apart from the influence from the collision between the Indian and Eurasian plates, the present-day crustal stress field along the tectonic zones in SW China is significantly affected by ductile deformation in the crust, suggesting that crustal structural heterogeneity plays an important role in the tectonic stress.

By analyzing teleseismic waveforms recorded by 53 stations of Hi-Climb profile passing through the central Bangong-Nujiang suture (BNS), a total of 4764 high- quality receiver functions are obtained. The average crustal thickness and Poisson’s ratio beneath each station are estimated using the travel time of Ps and PpPs of the Moho. The discontinuities such as the Moho, the 410- and 660-km interfaces are also studied using the common converted points (CCP) time to depth migration of receiver functions. The main results are as follows: (1) The Moho of Lhasa terrane and that of Qiangtang terrane nearby BNS are overridden and offset by ∼10 km. The structural geometry shows a northward uplifting and the southward deepening for the Moho of Lhasa terrane and Qiangtang terrane, respectively, which is related to the reactivated structure beneath BNS since Cenozoic era. (2) The variation range of Poisson’s ratios along the profile is between 0.237 and 0.280, indicating that the crust is mainly composed of felsic and intermediate rocks. The anti-correlation between the crustal thickness and Poisson’s ratio suggests that thicker crust beneath the southern Qiangtang terrane may be related to the successive thrust of felsic and intermediate rock of Lhasa terrane. (3) The thickness of the mantle transition zone along the profile remains about 255 km, implying that the tectonic activities caused by the India-Asia collision are confined to the depths above 410 km.

We use teleseismic body waveforms to explore S-wave layered velocity structures beneath 30 portable digital seismic stations deployed around western Yunnan Province. Results show that the Moho depth in this region is 40 km and decreases in general from north to south, consistent with previous geophysical studies. Associated with this lateral variation of the Moho depth, the lower crust above the Moho discontinuity has a 15– 25 km thick zone with an S-wave velocity lower than that of the upper crust. This lower velocity zone might be interpreted as a lower crust weak channel, which may mechanically partially decouple the upper-crust deformation from the underlying mantle. Thus, the inverted S-wave velocity structure could provide new evidence for the lateral flow of lower crust in the build-up of the south-eastern Tibetan plateau.

Analyses of global seismic waveform data for the 2008 devastating Wenchuan Earthquake have indicated where the rupture started and how it expanded. Here we reveal that the rupture process of this earthquake was strongly controlled by structural heterogeneities of the upper crust. We made a tomographic inversion of 200,000 local travel time data, according to which the rupture was confined in a presumably brittle layer of high P- and S-wave velocity anomalies within the upper ~ 15 km depths. The rupture propagated northeastward through the southwest (SW) segment of the fault and then jumped to the northeast (NE) segment with little slip at the intervening, presumably ductile zone characterized by low P and S velocities with high Poisson's ratio. The rupture initiated near the southwestern end of the SW segment within a body of high Poisson's ratio, which extends down into the lower crust, suggesting upward migration of deep-seated aqueous fluid to trigger the earthquake by reducing the mechanical strength of the brittle layer.

Seismic attenuation ( Q -value) can be estimated by extracting the amplitude-frequency information contained in seismic waveforms. We apply the attenuation tomography method of Pei et al. (2006) using M L amplitude data to estimate attenuation within the upper crust in Japan. More than 60,000 Sg -wave maximum amplitude readings from 5559 events, recorded by 971 stations, were selected from the dense High-Sensitivity Seismography Network (Hi-net) under the condition that epicentral distance is less than 2o and event depth is less than 10 km. The lateral S -wave Q variations of the upper crust at 1 Hz in Japan were obtained. The results indicate that low Q -values exist in the central Japanese islands, with almost the same distribution as volcanoes, while high Q -values exist mainly between the front of volcanoes and the Japanese east coast. In addition, a low Q was found between the eastern coast and the subducted trench. Most large crustal earthquakes occur in or around zones of low Q or the boundaries between areas of low and high Q , which will be very helpful in estimating the risk of large earthquakes.