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SUMMARYConvolutional neural network (CNN) is presented to implement quick quality classification and inversion for teleseismic P-wave receiver functions (RF). For the first case, a CNN is trained using field measured RFs from NE margin of the Tibetan Plateau to efficiently predict the quality of each input waveform. Signal-to-noise ratio and correlation are introduced to quantitatively determine the quality label of RF, avoiding the subjectivity of manual labelling. The trained network reduces the time needed for data processing and has higher accuracy and efficiency than conventional methods. Its good performance is confirmed by comparing it with manually selected data from NE of the Tibetan Plateau. The second case is an example of joint inverting teleseismic P-wave RF and surface wave dispersions for the estimation of earth S-wave structure and associated uncertainties. We train a UNet based on synthetic global Crust 5.1 models and standard earth models, as well as associated perturbed models to ensure enough generalization capacity. We find that the UNet inversion is robust and has a better performance to reconstruct subsurface ${V}_s$ distributions than the damping least-squares method, but at the expense of slightly higher data misfits. The pre-trained network can predict subsurface ${V}_s$ models and associated uncertainties beneath NE of the Tibetan Plateau, which is consistent with the published models.

SUMMARY The seismic quality factor (Q) of the Earth’s mantle is of great importance for the understanding of the physical and chemical properties that control mantle anelasticity. The radial structure of the Earth’s Q is less well resolved compared to its wave speed structure, and large discrepancies exist among global 1-D Q models. In this study, we build a global data set of amplitude measurements of S, SS, SSS and SSSS waves using earthquakes that occurred between 2009 and 2017 with moment magnitudes ranging from 6.5 to 8.0. Synthetic seismograms for those events are computed in a 1-D reference model PREM, and amplitude ratios between observed and synthetic seismograms are calculated in the frequency domain by spectra division, with measurement windows determined based on visual inspection of seismograms. We simulate wave propagation in a global velocity model S40RTS based on SPECFEM3D and show that the average amplitude ratio as a function of epicentral distance is not sensitive to 3-D focusing and defocusing for the source–receiver configuration of the data set. This data set includes about 5500 S and SS measurements that are not affected by mantle transition zone triplications (multiple ray paths), and those measurements are applied in linear inversions to obtain a preliminary 1-D Q model QMSI. This model reveals a high Q region in the uppermost lower mantle. While model QMSI improves the overall datafit of the entire data set, it does not fully explain SS amplitudes at short epicentral distances or the amplitudes of the SSS and SSSS waves. Using forward modelling, we modify the 1-D model QMSI iteratively to reduce the overall amplitude misfit of the entire data set. The final Q model QMSF requires a stronger and thicker high Q region at depths between 600 and 900 km. This anelastic structure indicates possible viscosity layering in the mid mantle.

The origin and mantle dynamics of the Quaternary intraplate sodic and potassic volcanism in northeast China have long been intensely debated. We present a high-resolution, three-dimensional (3-D) crust and upper-mantle S-wave velocity (Vs) model of northeast China by combining ambient noise and earthquake two-plane wave tomography based on unprecedented regional dense seismic arrays. Our seismic images highlight a strong correlation between the basalt geochemistry and upper-mantle seismic velocity structure: Sodic volcanoes are all characterized by prominent low seismic velocities in the uppermost mantle, while potassic volcanoes still possess a normal but thin upper-mantle “lid” depicted by high seismic velocities. Combined with previous petrological and geochemical research findings, we propose that the rarely erupted Quaternary potassic volcanism in northeast China results from the interaction between asthenospheric low-degree melts and the overlying subcontinental lithospheric mantle. In contrast, the more widespread Quaternary sodic volcanism in this region is predominantly sourced from the upwelling asthenosphere without significant overprinting from the subcontinental lithospheric mantle.

The detailed images of the mantle transition zone (MTZ) give insights into the nature of mantle upwelling from the deep Earth. We use receiver function to investigate the MTZ structure beneath the Mongolia Plateau using earthquakes recorded by 223 stations deployed in the Central Asian Orogenic Belt. The topographies of the 410 km (D410) and 660 km (D660) discontinuities are both characterized by slight depressions in most parts of the Mongolia Plateau. An obvious low-velocity layer (LVL) atop the MTZ with an average thickness of ∼40 km is observed beneath this study area, indicating the hydrous partial molten layer atop the D410 and a hydrous MTZ. The D410 depth of ∼410 km and the LVL up to ∼60 km thick beneath the Dariganga volcano imply that there is a chemically distinct hydrous upwelling related to the subducted Pacific slab rather than a classic thermal-dominated upwelling. The ∼10 km thinned MTZs are found beneath the western Hangay Dome and the Middle Gobi volcano. A narrow NS-trending MTZ thickening zone extending from the Hovsgol rift to the central Hangay Dome is mainly caused by a ∼10 km depression of D660. We speculate that the mantle upwelling beneath the Hangay Dome may be related to the stagnant slab descending through the MTZ, rather than being rooted from the deep lower mantle. Hydrous melting-driven upwellings from the MTZ beneath the Mongolia Plateau play a key role in intraplate volcanism. Published version This work is supported by National Natural Science Foundation of China (42030310, 42074102) and research grants from National Institute of Natural Hazards, Ministry of Emergency Manage-ment of China (Grant ZDJ2020-11).

SUMMARY A uniformly spaced linear transect through the northeastern Tibetan Plateau was constructed using 54 stations from ChinaArray Phase II. We used a set of colocated earthquakes to form receiver function beams that were then used to construct a 2-D image of main converting boundaries in our region and to investigate lateral changes in main impedance contrasts along the transect. The image revealed obvious mid-crustal low-velocity zones beneath the Qilian Orogen and the Alxa Block. We developed a new procedure that uses harmonically decomposed receiver functions to characterize seismic anisotropy, and that can determine both the orientations of symmetry axes and their type (fast or slow). We tested our technique on a number of synthetic models, and subsequently applied it to the data from the transect. We found that: (1) within the upper crust the orientations of slow symmetry axes are nearly orthogonal to the strike directions of faults, and thus anisotropy is likely caused by the shape preferred orientation of fluid-saturated cracks or fractures and (2) together with the low-velocity zones revealed from receiver functions stacks, anisotropic layers in the middle-to-lower crust could be explained by the crustal channel flow that was proposed for this region by previous studies. The shear within the boundary layers of crustal flow forms anisotropy with symmetry axes parallel to the flow direction.

Over the past decades, there have been hot debates in the geodynamic community regarding to the deep evolution mechanisms of Cenozoic volcanism in the Central Asian Orogenic Belt (CAOB). Of all the constraints available, high resolution structure of upper mantle discontinuities, especially the discontinuities at depths of 410 km (D410) and 660 km (D660), is of the most important, which may provide reliable clues on the magma channel as well as its evolution. In this work, we adopt the common conversion point stacking technique with teleseismic radial P-wave receiver functions to examine the D410 and D660 discontinuities. The primary results exhibit that the major characteristics of the mantle transition zone (MTZ) obtained by different velocity models are largely consistent. Obviously elevated D410 and slightly depressed D660 are observed beneath the Hannuoba Volcano, suggesting possible delamination of the local lithosphere deposited at the D410. This process may induce upwelling of the asthenospheric materials filling the space left by the delaminated lithosphere, and subsequently trigger volcanic eruptions. Strong depressions are observed at both D410 and D660 depths beneath west of the Dariganga Volcano, and the depression of D660 is more pronounced. It leads to the apparently thickened MTZ, indicating the presence of cold material at the D660. This cold material is speculated as a stagnant slab from the subducted Pacific Plate or the remains of a detached island arc system from the collision and formation of the CAOB. Slightly thinner MTZ is found beneath the Hentey Mountains and the Middle Gobi Volcano. Apparently, this thinner MTZ is not significant enough to support the existence of high thermal anomalies, which may rule out the possibility of large-scale hot material upwelling from either the local MTZ or even the lower mantle.

We studied a magnetotelluric transect from Tongliao, Inner Mongolia, to Baishan, Jilin, that crosses the northern segment of the Tan-Lu fault zone. Based on the profiling data, we calculated and analyzed the two-dimensional (2D) deviation degree and the tectonic strike, inverted the data using the 2D nonlinear conjugate gradient inversion method, and obtained an electrical structure model of the crust and upper mantle along the transect. Our analysis reveals that the western portion of the transect is within and at the margin of a typical sedimentary basin, and a high-conductivity layer is present in the lower crust. In comparison, the eastern portion of the transect is in the Jihei fold zone and has a relatively stable electrical structure, but its resistivity is one to two orders of magnitude higher than that of the western portion. A distinct electrical anomaly exists between the western and eastern sides. The Yilan-Yitong Fault of the northern segment of the Tan-Lu fault zone dips to the northwest, has strike-slip characteristics, extends to a depth of 40–50 km, and induces upwelling of upper mantle materials. The Dunhua-Mishan Fault of the northern segment of the Tan-Lu fault zone has a clear electrical gradient, and similar to the Yilan-Yitong Fault, it also dips to the northwest. However, the Dunhua-Mishan Fault is mostly fractured, extends to a depth of approximately 20 km and is slightly smaller than the Yilan-Yitong Fault. The inferred distribution of deep resistivity values indicates that the lithosphere is generally thinner at the center of the transect and thicker at the edges: the lithosphere is approximately 150 km thick on the western end, thins eastward to approximately 60 km thick near the Tan-Lu fault zone and then gradually thickens eastward to 100 km thick.

The eastern region of Tibetan Plateau (ERTP) and surrounding regions have gentle to moderate slope in topography, contrasting to the steep margins in the north and south. Many geodynamic models have been proposed to explain plateau growth in the east part of Tibetan Plateau and no consensus have yet been reached. In part, this is because of lack of detailed tomography images of the deep structure, which is essential for understanding the deep dynamics that govern the evolution of ERTP. Two stages of ChinArray have been conducted from 2011 to 2015. Nearly 1000 temporary seismic stations, with an average spacing ~40 km, were installed in ERTP and its adjacent regions. In addition, there was 297 temporary stations deployed in eastern margin of Tibetan Plateau during September 2006 to July 2009. Additional data from 271 permanent stations in 2007−2015 were also included. The overall station distribution densely covers the northeastern, eastern and southeastern margins of Tibetan Plateau. We imaged high-resolution 3D P - wave velocity structures of the crust and upper mantle beneath ERTP and its surrounding regions through teleseismic traveltime tomography. The horizontal and vertical resolutions are 0.8° and 80 km, respectively. The imaged velocity anomalies correlate well with geological blocks. Slow velocity anomalies, extending down to ~200 km, are found beneath the Tibetan Plateau. Fast anomalies associated with cratonic blocks around the plateau are imaged within 200−300 km depth. Below 400 km there are less velocity anomalies, indicating a normal mantle close to the global average. Neither fast subducted slabs nor slow mantle upwellings are obeserved beneath the study region. The plateau growth is mainly caused by the deformation of the weak lithosphere at the shallow depth. Surrounded by the Indian plate and surrounding rigid blocks, the weak lithosphere beneath Tibetan Plateau is horizontally shortened and vertically stretched. The horizontal shortening helps accommodate the ongoing collision between the Indian and Asian plates and the plateau growth is resulted from the vertical stretching.

We estimate over 2,000 shear wave splitting parameters from 84 teleseismic earthquakes recorded at 465 temporary broadband stations distributed over the western and central North China Craton to constrain lithospheric deformation and mantle flow. In the interior of the Ordos Block, the W-E trending fast polarization directions with small delay times are indicative of stable terrane, where a thick Archean cratonic keel has been imaged. The dominant W-E fast orientations with relatively large delay times observed in southern Trans-North China Orogen show a systematic directional deviation of ~30° from the absolute plate motion (APM) direction and are explained by a combined effect of APM-induced mantle fabric and a mantle flow around the southeast edges of the Ordos Block. The prominent rift-orthogonal fast orientations beneath rifts to the northern Ordos Block imply the existence of active mantle flow. Also, it is inferred that a mantle plume is responsible for generating the parabolic fast direction pattern beneath the Datong Volcano.

Two years of high-quality broadband seismic data from 69 temporary stations deployed in south-central Mongolia provide an opportunity to study the anisotropy-forming mechanisms in this area. The majority of shear wave splitting observations determined from the analysis of teleseismic SKS phase are characterized by NW–SE trending fast directions with large splitting delay times (greater than 2.0 s at six stations), which is inferred to be generated by active asthenospheric flow. The variation of the fast direction may be associated with deflection of asthenosphere around the deep Siberian cratonic keel at the base of the lithosphere. Several of the NE–SW trending fast directions with relatively small delay times observed in the Gobi Desert are parallel to the strike of the main faults and sutures, which may represent lithospheric deformation. In addition, it is inferred that small-scale hot mantle upwelling is responsible for generating a cluster of null measurements observed on the south of the Hentiy Mountain.

We investigated the crustal structure at 34 stations using the H-κ stacking method and jointly inverting receiver functions with Rayleigh-wave phase and group velocities. These seismic stations are distributed along a profile extending across the Songpan-Ganzi Terrane, Qinling-Qilian terranes and southwestern Ordos Basin. Our results reveal the variation in crustal thickness across this profile. We found thick crust beneath the Songpan-Ganzi Terrane (47–59 km) that decreases to ~ 45–47 km in the west Qinling and Qilian terranes, and reaches its local minimum beneath the southwestern Ordos Block (43–51 km) at an average crustal thickness of 46.7 ± 2.5 km. A low-velocity zone in the upper crust was found beneath most of the stations in NE Tibet, which may be indicative of partial melt or a weak detachment layer. Our observations of low to moderate Vp/Vs (1.67–1.79) represent a felsic to intermediate crustal composition. The shear velocity models estimated from joint inversions also reveal substantial lateral variations in velocity beneath the profile, which is mainly reflected in the lower crustal velocities. For the Ordos Block, the average shear wave velocities below 20 km are ~ 3.8 km/s, indicating an intermediate-to-felsic lower crust. The thick NE Tibet crust is characterized by slow shear wave velocities (3.3–3.6 km/s) below 20 km and lacks high-velocity material (Vs ≥ 4.0 km/s) in the lower crust, which may be attributed to mafic lower crustal delamination or/and the thickening of the upper and middle crust.

From the 1960s to 1970s, North China has been hit by a series of large earthquakes. During the past half century, geophysicists have carried out numerous surveys of the crustal and upper mantle structure, and associated studies in North China. They have made significant progress on several key issues in the geosciences, such as the crustal and upper mantle structure and the seismogenic environment of strong earthquakes. Deep seismic profiling results indicate a complex tectonic setting in the strong earthquake areas of North China, where a listric normal fault and a low-angle detachment in the upper crust coexist with a high-angle deep fault passing through the lower crust to the Moho beneath the hypocenter. Seismic tomography images reveal that most of the large earthquakes occurred in the transition between the high- and low-velocity zones, and the Tangshan earthquake area is characterized by a low-velocity anomaly in the middle-lower crust. Comprehensive analysis of geophysical data identified that the deep seismogenic environment in the North China extensional tectonic region is generally characterized by a low-velocity anomalous belt beneath the hypocenter, inconsistency of the deep and shallow structures in the crust, a steep crustalal-scale fault, relative lower velocities in the uppermost mantle, and local Moho uplift, etc. This indicates that the lithospheric structure of North China has strong heterogeneities. Geologically, the North China region had been a stable craton named the North China Craton or in brief the NCC, containing crustal rocks as old as ~3.8 Ga. The present-day strong seismic activity and the lower velocity of the lower crust in the NCC are much different from typical stable cratons around the world. These findings provide significant evidence for the destruction of the NCC. Although deep seismic profiling and seismic tomography have greatly enhanced knowledge about the deep-seated structure and seismogenic environment, some fundamental issues still remain and require further work.

We present new constraints on the upper mantle transition zone structure beneath eastern and southeastern Tibet based on P wave receiver functions for a large broadband data set from two very dense seismic arrays. A clear depression of both the 410 km and 660 km discontinuities is detected west of the Red River fault relative to the east. The correlated topographic variations across the Red River fault are indicative of temperature changes in the upper mantle above the transition zone, which suggests that the fault is a deep-rooted structure that penetrates into the upper mantle and separates Indochina from South China. West of the Red River fault, the transition zone thickness under the Tengchong volcano is found to be normal compared to the global average. This strongly suggests that the intraplate volcano may originate from slab tearing of the eastward subducting Indian plate at shallow depths in the upper mantle rather than from dehydration of a flattened plate within the transition zone. Our results further show that the 660 km discontinuity is significantly depressed under the western Yangtze Craton and that the transition zone therefore thickens by up to 20 km. This thickening is suggestive of lowered temperatures associated with a remnant of detached lithosphere in response to overlying asthenospheric escape flow in and around the western Yangtze Craton. In addition, we find that the transition zone thickness beneath much of the Sichuan Basin is similar to the global average.

Central Mongolia is marked by wide spread recent volcanism as well as significant topographic relief even though it is far from any plate tectonic boundaries. The cause of the recent magmatism and topography remains uncertain partially because little is known of the underlying mantle seismic structure due to the lack of seismic instrumentation in the region. From August 2011 through August 2013, 69 broadband seismic stations were deployed in central Mongolia. Teleseismic traveltime residuals were measured using waveform correlation and were inverted to image upper mantle P and S velocity variations. Significant lateral variations in seismic velocity are imaged in the deep upper mantle (100 to 800 km depth). Most significant are two continuous slow anomalies from the deep upper mantle to near the surface. One slow feature has been imaged previously and may be a zone of deep upwelling bringing warm mantle to beneath the Hangay Dome resulting in uplift and magmatism including the active Khanuy Gol and Middle Gobi volcanoes. The second, deep low velocity anomaly is seen in the east from 800 to 150 km depth. The anomaly ends beneath the Gobi Desert that is found to have fast shallow mantle indicating a relatively thick lithosphere. We interpret the second deep slow anomaly as a mantle upwelling that is deflected by the thick Gobi Desert lithosphere to surrounding regions such as the Hentay Mountains to the north. The upwellings are a means of feeding warmer than normal asthenospheric mantle over a widely distributed region beneath Mongolia resulting in distributed volcanic activity and uplift. There is no indication that the upwellings are rooted in the deep lower mantle i.e. classic plumes. We speculate the upwellings may be related to deep subduction of the Pacific and Indian plates and are thus plumes anchored in the upper mantle.

We present a Pn wave velocity and anisotropy model of the central segment of the North-South Seismic Belt in China, where there are numerous stable basins and active faults, making this segment attractive for extensive studies. The model was obtained by a tomographic analysis of 49973 Pn wave phase readings collected by the China Earthquake Networks Center and temporary stations in Yunnan and Sichuan. The tomographic velocity model shows that the average Pn wave velocity is 8.06 km/s; prominent high-velocity (high-V) anomalies are visible under the Sichuan Basin, the Zoige Basin and the Ordos block, which clearly outline their tectonic margins. A pronounced low-velocity (low-V) zone is observed from the Songpan-Ganzi block to the Chuan-Dian and Daliangshan blocks, suggesting the presence of hot material upwelling. The station delay data show a gradual variation from negative to positive values, possibly reflecting a crustal thickness variation from the southwest to the northeast of the study area. A correlation between the Pn wave anisotropy and the distribution of velocity anomalies is observed: anisotropy is relatively weaker in the high-V anomaly zones beneath stable basins, while it is stronger in the low-V anomaly zones and the high-to-low-V anomaly transition zones. The high-resolution velocity and anisotropy tomographic model that we obtained could also provide a better understanding of the study area seismicity, since the occurrence of strong earthquakes seems to be related to the presence and strength of lateral heterogeneities at the uppermost mantle level., The 28th IUGG General Assembly (IUGG2023) (Berlin 2023)

Seismic data from northeast (NE) China and the Sino-Korean craton were combined to image the upper mantle discontinuities at 410 and 660 km. Fine-scale topographic variations on these two discontinuities provide important clues for both delineating geometry of the subducting Pacific slab particularly at arc–arc junction and interpreting regional Cenozoic intraplate volcanism. We used over 90,000 receiver functions from 1916 teleseismic earthquakes recorded by 584 broadband seismic stations, primary those of temporary seismic arrays. We found the average depths of the two discontinuities to be 410 km and 672 km, respectively, beneath the study area. Results show that the 660-km discontinuity is strongly depressed by about 20–30 km in a narrow region beneath and around the Changbaishan volcano, consistent with the results of previous receiver function studies. In contrast, much of the Sino-Korean craton exhibits typical transition zone thickness (~ 260 km) and thus offers no evidence of a stagnated Pacific slab. Our results also reveal an elevated 660-km discontinuity and a thinner transition zone both to the west of the observed depression region and beneath the Kuril–Japan arc junction. This feature is most likely due to a tearing of the descending Pacific plate at both its leading and junction edges. An additional elevated 660-km discontinuity together with a thinner transition zone appears in the vicinity of the Dariganga lava field, supporting interpretations of a deep-rooted mantle plume. Our observations of an elevated 410-km discontinuity and a thicker transition zone correlate spatially with the diffuse distribution of volcanism around Hannuoba, Aershan and Wudalianchi. This correlation may suggest lithospheric removal as a mechanism for these magmatic activities.

A shear-wave velocity model of the crust and uppermost mantle beneath the SE Tibetan plateau was derived by inverting Rayleigh-wave group-velocity measurements of periods between 10 and 70 s. Rayleigh-wave group-velocity dispersions along more than 3,000 interstation paths were measured based on analysis of teleseismic waveform data recorded by temporary seismic stations. These observations were then utilized to construct 2D group-velocity maps in the period range of 10–70 s. The new group-velocity maps have an enhanced resolution compared with previous global and regional group-velocity models in this region because of the denser and more uniform data coverage. The lateral resolution across the region is about 0.5° for the periods used in this study. Local dispersion curves were then inverted for a 3D shear-wave velocity model of the region by applying a linear inversion scheme. Our 3D shear-wave model confirms the presence of low-velocity zones (LVZs) in the crust beneath the northern part of this region. Our imaging shows that the upper-middle crustal LVZ beneath the Tengchong region is isolated from these LVZs beneath the eastern and northern part of this region. The upper–middle crustal LVZ may be regarded as evidence of a magma chamber in the crust beneath the Tengchong Volcanoes. Our model also reveals a slow lithospheric structure beneath Tengchong and a fast shield-like mantle beneath the stable Yangtze block.

We present a new crustal thickness map of the Chinese mainland and adjacent areas based on a compilation of previously published data. More than 1900 estimates of crustal thickness from receiver function studies were collected and the weighted averages were used to develop a contour map of crustal thickness beneath the Chinese mainland. The combined dataset provided good coverage, especially over eastern and Central China, and our analysis revealed crustal thickness variations of the Chinese mainland with considerably more detail than in previous models. Our crustal thickness map shows a large variation, from a thickness of less than 30 km in the Mesozoic basin of eastern China to over 80 km in the Tibetan plateau. The thickest crust was found in central Tibet, while the thinnest continental crust was observed beneath eastern China. Normal to slightly thick crust (38–51 km) was found for the stable Precambrian basins and orogenic belts. Comparison between our model and other models reveals that the models share similar large-scale and smaller-scale features over large portions of eastern China where the data coverage is good; however, large differences (up to ± 25 km) in crustal thickness occur in areas with complex crustal structure or poor data coverage. We also evaluated the quality of previous models by comparing them with our newly compiled point observations of crustal thickness.

We present a new shear velocity model of the upper mantle beneath the East Asia region derived by inverting Rayleigh wave group velocity measurements between 10 and 145 s combined with previously published Rayleigh wave phase velocity measurements between 150 and 250 s. Rayleigh wave group velocity dispersion curves along more than 9500 paths were measured and combined to produce 2D dispersion maps for 10–145 s periods. The group velocity maps benefit from the inclusion of new data recorded by the China National Seismic Network and surrounding global stations. The increase in available data has resulted in enhanced resolution compared with previously published group velocity maps; the horizontal resolution across the region is about 3° for the periods used in this study. The new shear-wave velocity models indicate varying velocity structure beneath eastern China, which yields estimates of a lithosphere–asthenosphere boundary depth from around 160 km beneath the Yangtze block to approximately 140 km beneath the western part of the North China Craton (NCC), up to depths of 70–100 km beneath the eastern NCC, Northeast China, and the Cathaysia block. The models reveal the subduction of two opposite-facing continental plates under the southern and northern margin of Tibet. An obvious low-velocity anomaly appears in the top 200 km of the upper mantle beneath northern Tibet, which is inconsistent with the presence of subducted Asian or Indian mantle lithosphere beneath northern Tibet. The Cenozoic volcanism fields in the Mongolian plateau are characterized by an obvious upper mantle negative anomaly, but no signature of deep-seated plume was observed.

SUMMARY We imaged the crust and uppermost mantle structure beneath northeastern (NE) China with fundamental mode Rayleigh waves recorded by 125 broad-band stations deployed in the region. Rayleigh wave phase and group velocities along more than 700 interstation paths were estimated using the wavelet transformation method and then these data were utilized to construct 2-D phase and group velocity maps in the period range of 15–60 s. Owing to the dense ray coverage and short ray path, our results provide better lateral resolution in the NE China region compared with previous phase and group velocity studies. The regularized Rayleigh wave phase and group velocity dispersions at each cell were jointly inverted to determine 1-D shear wave velocity structure using the linear inversion method and then assembled into a 3-D model. Our results show that obvious low velocities exist in the uppermost mantle beneath the Changbaishan volcanic region, which may be due to asthenospheric upwelling. The thin lithosphere with fast S-wave velocity in the lower crust of the Songliao Basin implies that the lithospheric mantle beneath NE China is partly removed.

Triplicate waveform modeling is used to resolve SH (Vs) and P (Vp) wave velocity structures in the upper mantle transition zone (TZ) beneath northwestern (NW) Tibet. Focal depth move out stacking is proposed to enhance the identification of triplicate phases, and can be used to test consistency of our data. Our results show that the Vs and Vp structures are decorrelated, and that a large Vs jump occurred across the 660-km discontinuity, with a small Vs gradient above it. Conversely, the Vp model is characterized by a relatively small contrast across the discontinuity, accompanied by a high Vp gradient in the TZ. There seem no significant depth anomalies of the 660-km discontinuity in both models. The seismic structures in TZ beneath NW Tibet are similar to recent studies beneath the central Qiangtang and western Lhasa terrains. Taking the lower TZ structures under India as references, Vs is normal but Vp appears slightly high, and thus a high ratio of Vp/Vs was indicated beneath NW Tibet. Combined results with experiment information from mineral studies, we suggest that the differential anomalies of Vp and Vs can be attributed to a chemical heterogeneity, such as increased Al content in the lower TZ. Considering the tectonic evolution of Tibet, the chemical heterogeneity may be associated with subduction or detachment of the Tethys oceanic slab.

Finite-frequency travel time tomography is a newly developing method. The main procedure in this new method is to compute the traveltime sensitive kernel. The travel time of the same scatterer needs to be used for computing the traveltime sensitive kernel many times. It is a time-consuming task. It is easy and fast to get the travel time from analytic equations in a simple model such as a homogenous or linear velocity media. However, most of the earth models are layered. It is cumbersome to get the travel time from analytic equations. In order to enhance the computation efficiency, we used the table look-up method to compute the finite-frequency travel time sensitive kernel for P-waves in a layered structure model. We chose the AK135 earth model for the velocity model. The table look-up method saved about 50% of the computation time. We enhanced the computation speed by using the table lookup method in the same velocity model, which was very useful for enhancing the computation efficiency for the finite-frequency travel time tomography.

We present shear wave splitting results obtained from the analysis of teleseismic SKS, SKKS and PKS phases recorded by 70 permanent seismographic stations located in the Northeastern Tibetan Plateau. We identify a contrast in the splitting pattern complexity beneath different parts of NE Tibet. In the western and northern part, anisotropy observations are well explained by a single layer of anisotropy with a fast anisotropic direction trending NWW–SEE or NW–SE. In Xining and its adjacent area, the anisotropy shows strong azimuthal dependence of splitting parameters that can be modeled by two anisotropic layers. The fast direction for the upper layer lies in the N75–95°E range, which is consistent with the surface movement direction determined from GPS, and could be associated with middle to lower crustal flow. The fast direction in the lower layer is in the N105–125°E range and similar to the direction observed in the western and northern part where only a single layer is required. These NWW–SEE or NW–SE fast feature could be related to the current orogenesis induced from the India–Eurasia collision, or flow in the asthenosphere related to the absolute motion of Eurasia. Comparison between the anisotropy patterns expected from proposed models with our shear wave splitting observation suggests that no unique geodynamic model can reconcile all splitting measurements for such a complex region.

In this paper, we present wavelet transformation method to measure interstation phase velocity. We use Morlet wavelet function as mother wavelet to filter two seismograms at various period of interest, and correlate the wavelet filtered seismograms to form cross-correlogram. If both wavelet filtered signals are in phase at that period, the phase of the cross-correlogram is a minimum. Using 3-spline interpolation to transform cross-correlation matrix to a phase velocity verse period image, it is convenient for us to measure interstation phase velocity.

SUMMARY Joint inversion of interstation phase and group velocity dispersion data of Rayleigh waves provides the regional shear-wave velocity structure of the lithosphere across the central and eastern block of the North China Craton (NCC). Both the velocity dispersion curves and V s structure show strong lateral variation across the NCC, which correlates well with the regional tectonic evolution. At the lower crustal level, the characteristics of V s structure reveal differences in composition of the two blocks. In the upper mantle, the lithospheric removal in the eastern NCC is more severe than in the central NCC. The V s differences between the central NCC and the eastern NCC imply that multistage reactivation of the NCC occurred in the Mesozoic and Cenozoic time.

Sliding-window cross-correlation method is firstly adopted to identify sPn phase, and to constrain focal depth from regional seismograms, by measuring the time separation between sPn and Pn phases. We present the focal depths of the 17 moderate-sized aftershocks (MS⩾5.0) of the Wenchuan MS8.0 earthquake, using the data recorded by the regional seismic broadband networks of Shaanxi, Qinghai, Gansu, Yunnan and Sichuan. Our results show focal depths of aftershocks range from 8 to 20 km, and tend to cluster at two average depths, separate at 32.5°N, i.e., 11 km to the south and 17 km to the north, indicating that these aftershocks are origin of upper-to-middle crust. Combined with other results, we suggest that the Longmenshan fault is not a through-going crustal fault and the Pingwu-Qingchuan fault may be not the northward extension of the Longmenshan thrust fault.

We constrain SH wave velocity structure for the upper mantle beneath western Qiangtang Terrane by comparing regional distance seismic triplicated waveforms with synthetic seismograms, based on an intermediate event (∼220 km) recorded by the INDEPTH-III seismic array. The ATIP model reveals a low-velocity anomaly with up to −4% variation at the depth of 190–270 km and a relatively small velocity gradient above the depth of 410 km in the upper mantle, which is in agreement with previous results. In combination with other geological studies, we suggest that the depth of top asthenosphere is 190 km and no large-scale lithosphere thinning occurs in western Qiangtang Terrane, besides, Qiangtang Terrane has the same kind of upper mantle structure as the stable Eurasia.

National Science Foundation of China[40574040, 40704011]; Science Foundation for Institute of Geophysics; Ministry of Science and Technology of China[DQJB06B07]

Receiver functions can be estimated by minimizing the square errors of Wiener filter in time-domain or spectrum division in frequency domain. To avoid the direct calculation of auto-correlation and cross-correlation coefficients in Toeplitz equation or of auto-spectrum and cross-spectrum in spectrum division equation as well as empirically choosing a damping parameter, autoregressive deconvolution is presented to isolate receiver function from three-component teleseismic P waveforms. The vertical component of teleseismic P waveform is modeled by an autoregressive model, which can be forward and backward, predicted respectively. The optimum length of the autoregressive model is determined by the Akaike criterion. By minimizing the square errors of forward and backward predicting filters, autoregressive filter coefficients can be recursively solved, and receiver function is also estimated in the similar procedure. Both synthetic and real data tests show that autoregressive deconvolution is an effective method to isolate receiver function from teleseismic P waveforms in time-domain.

SUMMARY We present a wavelet modelling approach to invert for S-wave velocities from broad-band receiver functions. Taking spline function as the basic wavelet, the broad-band receiver function is decomposed into five resolution scales by Mallat's pyramid algorithm. The linearized least-squares inversion procedure is applied to every resolution scale. The fifth-scale approximation of receiver function is first inverted to recover the slowly varying background velocity variations with respect to a reference model. This solution is then taken as the initial model for fitting the fourth-scale wavelet coefficients of receiver function to further tune the solution to resolve sharper variations. This procedure is iteratively carried out up to the first-scale wavelet coefficients of receiver function. In this manner, the model neighbourhood containing the global minimum is first searched from the coarsest-scale receiver function, and the search gradually focuses on the global minimum by introducing finer-scale information of receiver function. Noise-free synthetic receiver function tests show that wavelet modelling of receiver functions can guide a certain range of initial models to converge to the true velocity distribution. Tests on actual data indicate that wavelet modelling can provide results very similar to those inferred by joint inversion of receiver function and surface wave dispersion.