Neotectonics around the Ordos Block,North China

The Eurasian continent was subject to multiphase intensive intracontinental deformation in the Cenozoic(Fig.1A).However,its Cenozoic intra-continental deformation process and the driving force has long been disputed,which is only associated with the Indo-Asian collision(Molnar and Tapponnier,1975;Jolivet et al.,1990;Tapponnier et al.,2001;Yin,2010;Xu et al.,2013;Zhao et al.,2016),is caused by the Pacific-Asian collision(Cui,1997;Schellart and Lister,2005;Fan et al.,2019),or is connected with a combined effect of the Indo-Asian collision and the Pacific-Eurasia convergence(Ren et al.,2002;Li et al.,2013;Shi et al.,2015;Liu et al.,2019).

Crustal structure of the northeastern Tibetan plateau,the Ordos block and the Sichuan basin from ambient noise tomography

We apply ambient noise tomography to significant seismic data resources in a region including the northeastern Tibetan plateau, the Ordos block and the Sichuan basin. The seismic data come from about 160 stations of the provincial broadband digital seismograph networks of China. Ambient noise cross-correlations are performed on the data recorded between 2007 and 2009 and high quality inter-station Rayleigh phase velocity dispersion curves are obtained between periods of 6 s to 35 s. Resulting Rayleigh wave phase velocity maps possess a lateral resolution between 100 km and 200 kin. The phase velocities at short periods （〈20 s） are lower in the Sichuan basin, the northwest segment of the Ordos block and the Weihe graben, and outline sedimentary deposits. At intermediate and long periods （〉25 s）, strong high velocity anomalies are observed within the Ordos block and the Sichuan basin and low phase velocities are imaged in the northeastern Tibetan plateau, reflecting the variation of crustal thickness from the Tibetan plateau to the neighboring regions in the east. Crustal and uppermost mantle shear wave velocities vary strongly between the Tibetan plateau, the Sichuan basin and the Ordos block. The Ordos block and the Sichuan basin are dominated by high shear wave velocities in the crust and uppermost mantle. There is a triangle-shaped low velocity zone located in the northeastern Tibetan plateau, whose width narrows towards the eastern margin of the plateau. No low velocity zone is apparent beneath the Qinling orogen, suggesting that mass may not be able to flow eastward through the boundary between the Ordos block and the Sichuan basin in the crust and uppermost mantle.

Segmentations of active normal dip-slip faults around Ordos block according to their surface ruptures in historical strong earthquakes

From the results of researches of active faults in resent years, a correlation analysis between segments of the faults according to surface ruptures in nine historical strong earthquakes occurring in downfaulted system and active structures around Ordos block is conducted in paper. The result shows that there is a good correlation between them, except few individual data that have more uncertain parameters. It shows that intensity and segments of surface ruptures in these strong earthquakes are intrinsically related with the active structures. These strong earthquakes produced stable and unstable rupture boundaries of characteristic-earthquake type and successive occurrence of strong earthquakes on the different boundary faults in the same tectonic unit.

Tectonic and climatic controls for fluvial terraces of the Yellow River over the past 2.6 Ma at Northeast Tibetan Plateau and Ordos Block

The Yellow River is usually assumed to record tectonic activities and climatic changes;however,a systematic study was lack in the sedimentology,stratigraphy,geomorphology and geochronology for the entire Yellow River though various geologic scholars have conducted numerous works in individual basins.This review focused on well-preserved fluvial terrace sequences that formed along this river on northeastern(NE)Tibetan Plateau and Ordos Block over the past 2.6 Ma.After comparing numerous initial incision ages at different segments along the Yellow River,we found out that the youngest initial incision may occur at ca.150 ka at the Longyang Gorge.The Yellow River may transit from multiple separated endorheic drainages to an entire external drainage after 150 ka,which may cause differentiations in the apparent incision rates before and after 150 ka;thus apparent net incision rates were calculated respectively for the Yellow River before 150 ka and the drainage network post 150 ka.Apparent net incision rates prior to 0.15 Ma were calculated as 0.15,0.29,0.10,0.12 and 0.03 mm/a respectively in Tongde-Xunhua,Lanzhou-Linxia basins,Heishan,Jinshan and Fenwei-Sanmen Gorges in this review,which mainly reflected Kunhuang-Gonghe Tectonic Event,generated by the Indo-Asian collision and diminishing as the NE Tibetan Plateau eastward extruding at ca.1.8-0.15 Ma.Apparent net incision rates post 0.15 Ma were calculated respectively for NE Tibetan Plateau and Ordos Block,considering their different base level.On NE Tibetan Plateau,four fluvial degradational phases were identified between ca.105~70,53~40,25~16 and 12~6 ka associated with terrace levels respectively,at average elevations of 96,40,20 and 10.5 meters above the current river level(m arl)within a range of 5~96 m arl;and four broad periods in the last 150 ka on Ordos Block:possibly marine oxygen isotope stage(MIS)5,ca.118 to 72 ka,most of MIS 3,ca.44~28 ka,transition from LGM to last deglacial ca.20 to 16 ka,and 4~3 ka at average elevations of 67.5,26,19 and 11.5 m arl.These degradational phases post 0.15 Ma were associated with multiple processes including enhanced fluvial discharge with an increase in monsoonal precipitation and/or melt water in deglaciation.

Study on Shear Wave Velocity Structure and Velocity Ratio Beneath Ordos Block and Its Eastern and Southern Margins

Using pure S wave fitting method, we studied the shear wave velocity structures under the Ordos block and its eastern and southern marginal areas. The results show that the velocity structure beneath Yulin station in the interior of Ordos block is relatively stable, where no apparent change between high and low velocity layers exists and the shear wave velocity increases steadily with the depth. There is a 12km thick layer at the depth of 25km under this station, with an S wave velocity (V S=3.90km/s) lower than that at the same depth in its eastern and southern areas (V S≥4.00km/s). The crust under the eastern margin of Ordos block is thicker than that of the Yulin station, and the velocity structures alternate between the high and low velocity layers, with more low velocity layers. It has the same characteristic as having a 10km-thick low velocity layer (V S=3.80km/s) in the lower crust but buried at a depth of about 35km. Moreover, we studied the V P/V S ratio under each station in combination with the result of P wave velocity inversion. The results show that, the average velocity ratio of the Yulin station at the interior of Ordos block is only 1.68, with a very low ratio (about 1.60) in the upper crust and a stable ratio of about 1.73 in the mid and lower crust, which indicates the media under this station is homogenous and stable, being in a state of rigidity. But at the stations in the eastern and southern margins of the Ordos block, several layers of high velocity ratio (about 1.80) have been found, in which the average velocity ratio under Kelan and Lishi stations at the eastern margin is systemically higher than that of the general elastical body waves (1.732). This reflects that the crust under the marginal areas is more active relatively, and other materials may exist in these layers. Finally, we discussed the relationship among earthquakes, velocity structures beneath stations and faults.

Crustal structure of northeastern Tibetan plateau and Ordos block:Waveform interpretation of the Maqen-Jingbian seismic refraction profile

The Maqen-Jingbian wide-angle seismic reflection and refraction experiment was carried out in 1998, which aims at determining detailed structure in the crust and top of the upper mantle and understanding structural relation between the northeastern Tibetan plateau and the Ordos block. The 1-D crustal models inferred by waveform inversion show strong variations in crustal structure, which can be classified into four different types： ① an Ordos platform with the Proterozoic crust and two high-velocity layers in the northeast section, ② a transitional crust between the northeastern Tibetan plateau and the Ordos block across the Haiyuan earthquake zone, ③ the Qilian orogenic zone in the central part, and ④ the Qinling orogenic zone in the southwestern section. The Moho depth increases from -42 km to -62 km from the NE part to the SW part of the profile. The crystalline crust consists of the upper crust and lower crust in northeastern Tibetan plateau. There is an obviously low P-wave velocity layer dipping northeastward, which is 12-13 km thick, at the bottom of the upper crust in Qinling orogenic zone and Haiyuan earthquake zone. The lower crust is characterized by alternating high and low P-wave velocity layers. Beneath Ordos block, i.e., the NE part of the profile, the crust shows quite a smooth increase in P-wave ve- locity down to the Moho at a depth of about 42 km.

Phase velocity maps of Rayleigh waves in the Ordos block and adjacent regions

We analyze continuous waveform data from 257 broadband stations of the portable seismic array deployed under the "China Seismic Array-northern part of NS seismic belt" project as well as data from a permanent seismic network from January 2014 to December 2015. The phase velocity dispersion curve of 7,185 Rayleigh waves is obtained with a method based on the image analysis of phase velocity extraction, and the inversion is obtained. The period of Rayleigh wave phase velocity distribution has a range of 5–40 s, and minimum resolution close to 20 km. The results show that the phase velocity structure image well reflects the geological structural characteristics of the crust and uppermost mantle, and that the phase velocity distribution has obvious lateral heterogeneity. The phase velocity of the 5–15 s period is closely linked to the surface layer and sedimentary layer, the low-velocity anomalies correspond to loose sedimentary cover, and the high-velocity anomalies correspond to orogenic belts and uplifts and the boundary between high and low velocity anomalies is consistent with the block boundary. The phase velocity of the 5–15 s period is strongly affected by the crust layer thickness, the northeastern Tibetan plateau has low-velocity anomalies in the middle to lower crust, the west side of the Ordos block is consistent with the northeastern Tibetan plateau, which may imply the material exchange and fusion in this area. The velocity variation is inversely related to the Moho depth in the 40 s period of Swave, and the lateral velocity heterogeneity represents the lateral variation of the Moho depth. The Ordos block and the northern margin of Sichuan basin are located in the uppermost mantle at this depth, and the depth in the transition zone is still located in the lower crust.

Electrical Structure of the Crust Beneath the Ordos Block

The Ordos block is a stable tectonic unit since the Cenozoic. Whether low-resistivity layers exist in the middle and lower crust of this kind block is an open question. This work attempts to reveal the entire crustal structure of the block based on interpretation of magnetotelluric data collected along the profile across this region. The result shows that a layered structure characterizes the crust of the Ordos block, with a low-resistivity layer at depth of about 20km, presumably associated with fluids there. In contrast, in the areas of active tectonics on the east and west of the block, there are no such layered electric structures in the crust, and the low-resistivity zones may be related to the decollement zones (or ductile shear zones) in the crust. The difference in electric structure of crust between the Ordos Block and neighboring areas is of significance to analyze the movement and deformation of varied blocks in the continent.

Precursors Revealed by the Multifractal Dimension Around Ordos Block,North China

Seismicity around the Ordos Block in the northern part of China has been studied by the multifractal dimensions with the help of complete data set of earthquakes with M≥2.5 since 1977. Seven regions around this block have been examined where the eight large earthquakes with M≥6.0 for the complete data occurred. In order to display the temporal distribution of the correlation dimension D 2, a moving window of fixed number of events is used in every of the seven regions. Except for two regions with less seismic activity, all other five regions have obviously been found to display sudden changes of the correlation dimension for the events with M≥4.5.

Study on the Characteristics of Seismic Activity in the Interior of the Ordos Block

The temporal and spatial distribution characteristics of earthquakes in the Ordos block are studied by using historical earthquake data,instrument data of the regional seismic network around the Ordos block and the historical felt earthquake data,and the relationship between seismicity in the Ordos block and seismicity around the Ordos block is discussed. The result shows that the Ordos block is a typical moderate-strong earthquake active region where many M_S≥5.0 destructive earthquakes have occurred. The temporal and spatial distribution of earthquakes in the Ordos block is asymmetrical. The temporal distribution of earthquakes shows a periodic characteristic and the activity of earthquakes in the southeastern Ordos block is higher than in the northwest Ordos block. The M_S≥5.0 moderate size earthquakes in the Ordos block are controlled by the M_S≥6.0 earthquake around the Ordos block.

Earthquake detection probabilities and completeness magnitude in the northern margin of the Ordos Block

The assessment of the completeness of earthquake catalogs is a prerequisite for studying the patterns of seismic activity.In traditional approaches,the minimum magnitude of completeness(MC)is employed to evaluate catalog completeness,with events below MC being discarded,leading to the underutilization of the data.Detection probability is a more detailed measure of the catalog's completeness than MC;its use results in better model compatibility with data in seismic activity modeling and allows for more comprehensive utilization of seismic observation data across temporal,spatial,and magnitude dimensions.Using the magnitude-rank method and Maximum Curvature(MAXC)methods,we analyzed temporal variations in earthquake catalog completeness,fi nding that MC stabilized after 2010,which closely coincides with improvements in monitoring capabilities and the densifi cation of seismic networks.Employing the probability-based magnitude of completeness(PMC)and entire magnitude range(EMR)methods,grounded in distinct foundational assumptions and computational principles,we analyzed the 2010-2023 earthquake catalog for the northern margin of the Ordos Block,aiming to assess the detection probability of earthquakes and the completeness of the earthquake catalog.The PMC method yielded the detection probability distribution for 76 stations in the distance-magnitude space.A scoring metric was designed based on station detection capabilities for small earthquakes in the near fi eld.From the detection probabilities of stations,we inferred detection probabilities of the network for diff erent magnitude ranges and mapped the spatial distribution of the probability-based completeness magnitude.In the EMR method,we employed a segmented model fi tted to the observed data to determine the detection probability and completeness magnitude for every grid point in the study region.We discussed the sample dependency and low-magnitude failure phenomena of the PMC method,noting the potential overestimation of detection probabilities for lower magnitudes and the underestimation of MC in areas with weaker monitoring capabilities.The results obtained via the two methods support these hypotheses.The assessment results indicate better monitoring capabilities on the eastern side of the study area but worse on the northwest side.The spatial distribution of network monitoring capabilities is uneven,correlating with the distribution of stations and showing signifi cant diff erences in detection capabilities among diff erent stations.The truncation eff ects of data and station selection aff ected the evaluation results at the edges of the study area.Overall,both methods yielded detailed descriptions of the earthquake catalog,but careful selection of calculation parameters or adjustments based on the strengths of diff erent methods is necessary to correct potential biases.

Upper mantle anisotropy in the Ordos Block and its margins

Based on the polarization analysis of teleseismic data,SKS (SKKS) fast-wave directions and delay times between fast and slow shear waves were determined for each of the 111 seismic stations from both permanent and temporary broadband seismograph networks deployed in the Ordos Block and its margins.Both the Silver and Chan and stacking analysis methods were used.In this way,an image of upper mantle anisotropy in the Ordos Block and its margins was acquired.In the western and northern margins of the Ordos Block,the fast-wave directions are consistently NW-SE.The fast-wave directions are mainly NWW-SEE and EW in the southern margin of the Ordos Block.In the eastern margin of the Ordos Block,the fast-wave directions are generally EW,although some run NEE-SWW or NWW-SEE.In the Ordos Block,the fast-wave directions trend near N-S in the north,but switch to near EW in the south.The delay time between fast and slow waves falls into the interval 0.48-1.50 s,and the average delay time at the stations in the Ordos Block is less than that in its margins.We suggest that the anisotropy of the stable Ordos Block is mainly caused by "fossil" anisotropy frozen in the ancient North China Craton.The NE-trending push of the northeastern margin of the Tibetan Plateau has caused NW-SE-trending lithospheric extension in the western and northern margins of the Ordos Block,and made the upper mantle flow southeastwards.This in turn has resulted in the alignment of the upper mantle peridotite lattice with the direction of material deformation.In the southern margin of the Ordos Block,the collision between the North China and Yangtze blocks resulted in the fast-wave direction running parallel to the collision boundary and the Qinling Orogen.Combining this with the APM and velocity structure of the Qinling Orogen,we propose that eastward-directed asthenospheric-mantle channel flow may have occurred beneath the Qinling Orogen.In the eastern margin of the Ordos Block,the complex anisotropic characteristics of the Fenhe Graben and Taihang Orogen may be caused by the interaction of western Pacific Plate subduction,regional extensional tectonics,and the orogeny.For station YCI,the apparent splitting parameters (the fast-wave directions range from 45° to 106° and the delay times range from 0.6 to 1.5 s) exhibit systematic variations as a function of incoming polarization with a periodicity of π/2.This variation can be best explained by a two-layer anisotropic model (φlower=132°,δtlower=0.8 s,φupper=83°,δtupper=0.5 s).The upper layer anisotropy beneath station YCI can again be attributed to "fossil" anisotropy frozen in the ancient North China Craton.The lower layer anisotropy is affected by the tectonic activity of the western Ordos Block.The NW-SE trending extension caused by the NE trending push of the northeastern margin of the Tibetan Plateau affected the deformation of the lower anisotropic layer beneath station YCI.By comparing the fast-wave directions with GPS velocity directions,we see that the crust and upper mantle possibly have vertically coherent deformation in the margins of the Ordos Block,whereas the internal deformation characteristics of the Ordos Block are complex and require further study.

Linhe Block in Ordos Basin

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Differences in both the structure and interaction of the crust and mantle on the eastern and western sides of the Ordos Block

The Ordos Block,the western part of the North China Craton(NCC),has preserved most of its cratonic lithospheric roots during the destruction and modification of the NCC,and nowconnects two distinctly different tectonic units,i.e.,the Tibetan Plateau(TP)to the west and the North China Plain to the east.It is important to studythe detailed crust-mantle structure of the Ordos Block and surroundings to understand the interactions between the Ordos and adjacent regions and the dynamics of the NCC evolution.In this study,the S-wave velocity structure at depths of 0–100 km along an E-W profile at around 36.5°N from the Qilian orogen(QLO)to the Trans-North China Orogen(TNCO)within the NCC was investigated by joint inversion of the receiver functions(RFs)and surface wave dispersion data from 104 broadband seismic stations deployed in the region under the ChinArray project(PhaseⅡand PhaseⅢ).The image of discontinuity structures along the profile was further constructed by common conversion point stacking of RFs.The main results are as follows:(1)The depth of the Moho in the study region gradually decreases from west to east,being the deepest in the QLO,the second in the Ordos Block,and the shallowest in the TNCO.Local depression and uplift of the Moho are also identified beneath the Haiyuan fault zone and the Shanxi-Shaanxi rift system(SSR),respectively.(2)The crust-mantle structure displays obvious lateral heterogeneities among tectonic regions.A pronounced low-velocity anomaly is observed at depths of 20–40 km beneath the QLO and becomes weakened and thinned to the east,indicating that thickening of the mid-to-lower crust probably occurred in the QLO during the growth and lateral extrusion of the northeastern TP under the resistance of the NCC.The lithosphere-asthenosphere boundary beneath the TNCO is imaged at~90 km depth,which is significantly shallower than that beneath the Ordos Block.This observation suggests that the lithosphere of the TNCO may have experienced a notable thinning,possibly by thermal erosion and modification due to the mantle upwelling associated with the western Pacific subduction in the Mesozoic.(3)The uplifted Moho under the SSR and its neighboring areas is spatially coincident with the thinned lithosphere in the TNCO,indicating a close relationship between the development of the SSR and the lithospheric thinning and modification in this region.

Characteristics of horizontal crustal movement in Weihe basin and adjacent zones by GPS observation

Based on the GPS data provided by CMONOC during 1999~2002 and observed by Seismological Bureau of Shaanxi Province during 2001~2002, we have obtained the crustal movement velocity field in Weihe basin and adjacent regions. The results show that the motion velocity in Weihe basin and adjacent zones is characterized by distinct NE-directional stripped variation. The south border of Ordos block seems to show an discontinuous anti-clockwise rotation on the whole with an average velocity of about 5.7 mma-1 relative to Yulin site on the Ordos block. There is a left-lateral shear belt on both sides of Binxian-Xi'an-Lantian zone in the central Weihe basin, and its northern area has a better corresponding relationship with the dense zone of small earthquakes in Tong-chuan-Jingyang-Lintong-Weinan.

Deep Deformation Processes in the Southern Ningxia Geomorphic Belt: Insight from Two-dimensional Particle-in-cell Finite Difference Numerical Modelling

The deep lateral growth process of the plateau is a hot topic in earth science.As the growth boundary zone of the northeastern Tibetan Plateau,southern Ningxia geomorphic zone is controlled by four major arcuate fault zones(Fig.1),namely Haiyuan fault(HYF),XiangshanTianjingshan fault(XTSF),Yantongshan fault(YTSF).

Geochronology and geological significance of the strata of the Neoproterozoic Nanhua System,SW North China Craton

A set of low-grade clastic metamorphic and carbonate rocks,and greenschists outcropping in the southwestern(SW)margin of the North China Craton(NCC),was originally classified as the Paleoproterozoic Xiong’er Group according to stratigraphic correlation.To verify the age,this paper carried out detrital zircon U–Pb LA-ICP-MS dating of low-grade clastic metamorphic rocks exposed in the Changqing area at the SW margin of the Ordos Block in the SW part of the NCC.Results from detrital zircon dating indicate that the metamorphic and carbonate rocks can be classified into the Neoproterozoic Nanhua System,which is the only Nanhua System stratum in this block so far,and it probably could provide new clues to Rodinia break-up and Snowball Earth of the NCC.The nine peak ages of the low-grade clastic metamorphic rocks reflected its relatively complex provenance,and almost all major geological events experienced by the NCC basement since the Neoarchean,but some age peaks were difficult to correspond to that of the NCC,indicating that the southwestern part of the Ordos Block was also affected by the Qinling and Qiliang orogenic belts during Nanhua System of Neoproterozoic.Combined with provenance analysis,it was revealed that the current southwest boundary of the Ordos Block was the previous southwest boundary of the Ordos Block during the Qingbaikou-Nanhua Period of the Neoproterozoic.

Relationship between Landslides and Active Normal Faulting in the Epicentral Area of the AD 1556 M～8.5 Huaxian Earthquake,SE Weihe Graben(Central China)

In this paper, we focus on the characteristics of the landslides developed in the epicentral area of AD 1556 M^8.5 Huaxian Earthquake, and discuss their relations to the active normal faults in the SE Weihe Graben, Central China. The results from analyzing high-resolution remote-sensing imagery and digital elevation models（DEMs）, in combination with field survey, demonstrate that：（i） the landslides observed in the study area range from small-scale debris/rock falls to large-scale rock avalanches;（ii） the landslides are mostly developed upon steep slopes of ≥30°; and（iii） the step-like normal-fault scarps along the range-fronts of the Huashan Mountains as well as the thick loess sediments in the Weinan area may facilitate the occurrence of large landslides. The results presented in this study would be helpful to assess the potential landslide hazards in densely-populated areas affected by active normal faulting.

Crustal P-wave velocity structure in the northeastern margin of the Qinghai-Tibetan Plateau and insights into crustal deformation

The transitional area between the northeastern margin of the Qinghai-Tibetan Plateau, Ordos Block and Alxa Block, also being the northern segment of the North-South Seismic Belt, is characterized by considerably high seismicity level and high risk of strong earthquakes. In view of the special tectonic environment and deep tectonic setting in this area, this study used two seismic wide-angle reflection/refraction cross profiles for double constraining, so as to more reliably obtain the fine-scale velocity structure characteristics in both the shallow and deep crust of individual blocks and their boundaries in the study area, and further discuss the seismogenic environment in seismic zones with strong historical earthquakes. In this paper, the P-wave data from the two profiles are processed and interpreted, and two-dimensional crustal velocity structure models along the two profiles are constructed by travel time forward modeling. The results show that there are great differences in velocity structure, shape of intra-crustal interfaces and crustal thickness among different blocks sampled by the two seismic profiles. The crustal thickness along the Lanzhou-Huianbu-Yulin seismic sounding profile （L1） increases from -43 km in the western margin of Ordos Block to -56 km in the Qilian Block to the west. In the Ordos Block, the velocity contours vary gently, and the average velocity of the crust is about 6.30 km s^-1; On the other hand, the velocity structures in the crust of the Qilian Block and the arc-like tectonic zone vary dramatically, and the average crustal velocities in these areas are about 0.10 km s^-1 lower than that of the Ordos Block. In addition, discontinuous low-velocity bodies （LVZ1 and LVZ2） are identified in the crust of the Qilian Block and the arc-like tectonic zone, the velocity of which is 0.10-0.20 krn s^-1 lower than that of the surroundings. The average crustal thickness of the Ordos Block is consistently estimated to be around 43 km along both Profile L2 （Tongchuan-Huianbu-Alashan left banner seismic sounding profile） and Profile L1. In contrast to the gently varying intra-crustal interfaces and velocity contours in the Ordos Block along Profile L 1, which is a typical structure characteristic of stable cratons, the crustal structure in the Ordos Block along Profile L2 exhibits rather complex variations. This indicates the presence of significant structural differences in the crust within the Ordos Block. The crustal structure of the Helan Mountain Qilian Block and the Yinchuan Basin is featured by ＂uplift and depression＂ undulations, showing the characteristics of localized compressional deformation. Moreover, there are low-velocity zones with altemative high and low velocities in the middle and lower crust beneath the Helan Mountain, where the velocity is about 0.15-0.25 km s^-1 lower than that of the surrounding areas. The crustal thickness of the Alxa Block is about 49 kin, and the velocity contours in the upper and middle-lower crust of the block vary significantly. The complex crustal velocity structure images along the two seismic sounding profiles L1 and L2 reveal considerable structural differences among different tectonic blocks, their coupling relationships and velocity structural features in the seismic zones where strong historical earthquakes occurred. The imaging result of this study provides fine-scale crustal structure information for further understanding the seismogenic environment and mechanism in the study area.