Earthquake relocation and 3-dimensional crustal structure of P-wave velocity in cen-tral-western China

采用中国中西部地区(2l°～36°N，98°～112°E)193个地震台在1992～1999年间记录到的9988次地震的Pg和Sg震相走时的读数资料，用Roecker的SPHYPIT90程序，反演了该地区三维地壳P波速度结构，并用SPHREL3D90程序进行了地震的重新定位．反演结果揭示了中国中西部地区地震P波速度结构明显的横向不均匀性，这些不同深度上波速的横向变化多以该地区的活动断裂为分界线．可以看出活动断裂两侧存在明显的速度反差．通过重新定位，得到了6459次地震的震源参数，这些精确定位的地震震中明显沿该区活动断裂呈现条带状分布，其范围和尺度清晰地表示了这一地区地震活动与活动断裂的紧密关系．其中，82％重新精确定位的事件的震源深度在20km以内．这一结果与笔者用双差地震定位法得到的重新定位的震源深度分布相一致．

3-D velocity structure in the central-eastern part of Qilianshan

The 3-D velocity tomography image of the central-eastern part of Qilianshan is obtained by the joint inversion of 3-D velocity structure and focal parameters based on the S-P data of micro-earthquakes recorded by the digital seismic network set up for a Sino-French cooperation program since 1996. The inversed velocity structure does primarily reflect some important features of the deep structure in the region and provide the scientific background for the further study of active tectonic structure and the calculation of earthquake parameters.

3D S-Wave Velocity Structure of the Crust and Upper Mantle

In this paper, 238 Rayleigh wave path data are selected and processed by the matched-filtering frequency-time analysis technique and the grid dispersion inversion method to obtain the 3D S-wave velocity structure of China mainland and its adjacent sea regions. The results show that the velocity structure relates to geotectonic division, Bouguer gravity anomaly is basically controlled by the relief of Moho discontinuity, the buried depth of LVL in upper mantle concerns the surface heat flow deeply. In this paper, authors indicate the main characteristics of the velocity structure in tectonic active and stable regions.

Joint Inversion of the 3D P Wave Velocity Structure of the Crust and Upper Mantle under the Southeastern Margin of the Tibetan Plateau Using Regional Earthquake and Teleseismic Data

The special seismic tectonic environment and frequent seismicity in the southeastern margin of the Qinghai-Tibet Plateau show that this area is an ideal location to study the present tectonic movement and background of strong earthquakes in China's Mainland and to predict future strong earthquake risk zones. Studies of the structural environment and physical characteristics of the deep structure in this area are helpful to explore deep dynamic effects and deformation field characteristics, to strengthen our understanding of the roles of anisotropy and tectonic deformation and to study the deep tectonic background of the seismic origin of the block＇s interior. In this paper, the three-dimensional （3D） P-wave velocity structure of the crust and upper mantle under the southeastern margin of the Qinghai-Tibet Plateau is obtained via observational data from 224 permanent seismic stations in the regional digital seismic network of Yunnan and Sichuan Provinces and from 356 mobile China seismic arrays in the southern section of the north-south seismic belt using a joint inversion method of the regional earthquake and teleseismic data. The results indicate that the spatial distribution of the P-wave velocity anomalies in the shallow upper crust is closely related to the surface geological structure, terrain and lithology. Baoxing and Kangding, with their basic volcanic rocks and volcanic clastic rocks, present obvious high-velocity anomalies. The Chengdu Basin shows low-velocity anomalies associated with the Quaternary sediments. The Xichang Mesozoic Basin and the Butuo Basin are characterised by low- velocity anomalies related to very thick sedimentary layers. The upper and middle crust beneath the Chuan-Dian and Songpan-Ganzi Blocks has apparent lateral heterogeneities, including low-velocity zones of different sizes. There is a large range of low-velocity layers in the Songpan-Ganzi Block and the sub-block northwest of Sichuan Province, showing that the middle and lower crust is relatively weak. The Sichuan Basin, which is located in the western margin of the Yangtze platform, shows high-velocity characteristics. The results also reveal that there are continuous low-velocity layer distributions in the middle and lower crust of the Daliangshan Block and that the distribution direction of the low-velocity anomaly is nearly SN, which is consistent with the trend of the Daliangshan fault. The existence of the low-velocity layer in the crust also provides a deep source for the deep dynamic deformation and seismic activity of the Daliangshan Block and its boundary faults. The results of the 3D P-wave velocity structure show that an anomalous distribution of high-density, strong-magnetic and high-wave velocity exists inside the crust in the Panxi region. This is likely related to late Paleozoic mantle plume activity that led to a large number of mafic and ultra-mafic intrusions into the crust. In the crustal doming process, the massive intrusion of mantle-derived material enhanced the mechanical strength of the crustal medium. The P-wave velocity structure also revealed that the upper mantle contains a low-velocity layer at a depth of 80-120 km in the Panxi region. The existence of deep faults in the Panxi region, which provide conditions for transporting mantle thermal material into the crust, is the deep tectonic background for the area＇s strong earthquake activity.

Imaging 3-D crustal P-wave velocity structure of western Yunnan with bulletin data

Western Yunnan is a region with intensive tectonic activity and serious earthquake risk. It is of significant importance to study three dimensional crustal structure of this region to understand the tectonic setting and disaster mechanism. Densification and digitalization of seismic networks in this region provides an opportunity to study the velocity structure with bulletin data. In this study, we collect P-wave data of 10 403 regional earthquakes recorded by 79 seismic stations from January 2008 to December 2010. In addition to first arrivals data （Pg with epieentral distance less than 200 km and Pn）, the Pg （or P） data with epicentral distance more than 200 km are also considered as later direct arrivals in the tomographic inversion. We also compare the quantity and the quality of the seismic data before 2010 and after 2010. The test results show that adding the follow-up Pg phase can effectively improve the inversion ability of crustal imaging, and quantity and the data quality are significantly improved since 2010. The tomographie results show that： （1） The Honghe fault zone, which is the major fault systems in this region, may cut through the entire crust, and the velocity contrasts between two sides at lower crust beneath the Honghe fault are estimated at higher than 10%, while the velocity difference below Nujiang fault zone extends only in the upper crust; （2） Most of the earthquakes in the region occurred at the interface of high-velocity media and low-velocity media, i.e., the areas with high velocity gradient, which has been validated in other areas.

A study on 3-D velocity structure of crust and upper mantle in Sichuan -Yunnan region, China

Based on the first arrival P and S data of 4 625 regional earthquakes recorded at 174 stations dispersed in the Yunnan and Sichuan Provinces, the 3-D velocity structure of crust and upper mantle in the region is determined, incorporating with previous deep geophysical data. In the upper crust, a positive anomaly velocity zone exists in the Sichuan basin, whereas a negative anomaly velocity zone exists in the western Sichuan plateau. The boundary between the positive and negative anomaly zones is the Longmenshan fault zone. The images of lower crust and upper mantle in the Longmenshan fault, Xianshuihe fault, Honghe fault and others show the characteristic of tectonic boundary, indicating that the faults likely penetrate the Moho discontinuity. The negative velocity anomalies at the depth of 50 km in the Tengchong volcanic area and the Panxi tectonic zone appear to be associated with the temperature and composition variations in the upper mantle. The overall features of the crustal and the upper mantle structures in the SichuanYunnan region are the lower average velocity in both crust and uppermost mantle, the large crustal thickness variations, and the existence of high conductivity layer in the crust or/and upper mantle, and higher geothermal value. All these features are closely related to the collision between the India and the Asia plates. The crustal velocity in the SichuanYunnan rhombic block generally shows normal value or positive anomaly, while the negative anomaly exists in the area along the large strike-slip faults as the block boundary. It is conducive to the crustal block side-pressing out along the faults. In the major seismic zones, the seismicity is relative to the negative anomaly velocity. Most strong earthquakes occurred in the upper-mid crust with positive anomaly or normal velocity, where the negative anomaly zone generally exists below.

3-D velocity structure in the central-eastern part of Qilianshan

The 3-D velocity tomography image of the central-eastern part of Qilianshan is obtained by the joint inversion of 3-D velocity structure and focal parameters based on the S-P data of micro-earthquakes recorded by the digital seismic network set up for a Sino-French cooperation program since 1996. The inversed velocity structure does primarily reflect some important features of the deep structure in the region and provide the scientific background for the further study of active tectonic structure and the calculation of earthquake parameters.

3D Velocity Structure and Its Tectonic Implications in the East China Sea and Yellow Sea

3D structure of the crust and upper mantle in the studied area has been analyzed from surface wave tomography. The velocity distribution in the uppermost crust is symmetrical on two sides of the central line of the sea, and coincides with the structure of crystalline basement. The essential difference in tectonics between the East China Sea and the Yellow Sea mainly lies in that the velocity structures of their lower crust and upper mantle are identical to those of South China and North China respectively. In the upper mantle there exists a high-velocity zone with a nearly EW strike from the Hangzhou Bay, China, to the Tokara Channel, Japan, along about the latitude of 30°N. It is found that between the East China Sea and the Yellow Sea there are systematical differences in geomorphology, geology, seismicity, heat flow, quality factor and gravity and aeromagnetic anomalies, which is related to both left-lateral shear dislocation and right-lateral tear of the Benioff zone from the Hangzhou Bay to the Tokara Channel.It is inferred that the East China Sea was formed by Cenozoic back-arc extension. The boundary between the North China and South China crustal blocks stretches along the southern piedmont of Mts. Daba-Dabie-Hangzhou Bay-Tokara Channel, and the subduction zone at the Okinawa trench is the eastern boundary of the South China crustal block. The movements of the Pacific plate, Indian plate and upper mantle rather than the Philippine plate subduction have played a dominant role for the modern tectonic movements in East Asia.

Three-dimensional P-wave velocity structure of the crust beneath Hainan Island and its adjacent regions,China

Using over 3 500 first P arrival times recorded by nine digital seismic stations from Hainan Digital Seismic Net-work during 1999～2005,a 3-D P-wave velocity model of the crust under Hainan Island and adjacent regions has been determined. The results show that the pattern of velocity anomalies in the shallower upper crust is somewhat associated with the surface geological tectonics in the region. A relative low-velocity anomaly appears north of the Wangwu-Wenjiao fault zone and a relative high-velocity anomaly appears south of the Wangwu-Wenjiao fault zone,corresponding to the depressed areas in north Hainan Island,where many volcanoes are frequently active and geothermal values are relatively higher,and the uplifted and stable regions in central and south of the Hainan Is-land. In the middle and lower crust velocities are relatively lower in east Hainan than those in west Hainan,possi-bly suggesting the existence of the upwelling of hot materials from the mantle in east Hainan. The pattern of veloc-ity anomalies also indicates that NW faults,i.e.,the Puqian-Qinglan fault,may be shallower,while the E-W Wangwu-Wenjiao fault may be deeper,which perhaps extends down to Moho depth or deeper.

Earthquake relocation and 3-dimensional crustal structure of P-wave velocity in central-western China

A simultaneous inversion of earthquake relocation and three-dimensional crustal structure of P-wave velocity in central-western China (21癗~36癗, 98癊~112癊) were performed in this paper. The crustal P-wave velocity model and earthquake relocation for this region are obtained using Pg and Sg phase readings of 9 988 earthquakes from 1992 to 1999 recorded at 193 seismic stations within central-western China by SPHYPIT90 and SPHREL3D90 programs. A lateral inhomogeneous structure of P-wave velocity in this region was obtained. Ob-vious contrast of P-wave velocities was revealed on both sides of active fault zones. Relocated epicenters of 6 459 events show clear lineation along active faults, which indicated a close correlation between seismicity and the active faults in this region. Focal depths of 82% relocated events ranged from 0 to 20 km, which is in good agreement with that from double-difference earthquake location algorithm.

3-D velocity structure of P wave in the upper mantle beneath southwestern China and its adjacent areas

3-D velocity structure of P wave in the upper mantle beneath southwestern China and its adjacent areas (10°N [similar to] 36°N, 70°E [similar to] 110°E) down to the depth of 400 km has been studied by using 80 974 P-wave first arrival times recorded at 165 stations from 7 053 events both within the studying areas, selected from the ISC bulletin and the Bulletin of China and NEIC fundamental seismic network. With a resolution of grid spacing of 2°×2°, the velocity heterogeneity on the horizontal profile is obvious though it attenuates with the depth increasing. On the vertical profiles of velocity along the latitude of 16°N and 24°N, the collision and extrusion of India plate to Eurasia plate is displayed, and a remarkable velocity difference between India plate and Eurasia plate is shown. In the vertical profile along the longitude of 90°E, the subducting of India plate northward beneath Eurasia plate (Tibet plateau) is also obvious. On the horizontal profile at the depth of 90 km, a slow velocity stripe from Myitkyina, Myanmar to Donghai, Vietnam seems to be related to Honghe fault belt. An illustration method of describing the resolution more directly and exactly has been proposed and utilized in this paper.

Three-dimensional P-wave Velocity Structure Modelling of the Middle and Lower Reaches of the Yangtze River Metallogenic Belt: Crustal Architecture and Metallogenic Implications

In this study,we compiled and analyzed 69310 P-wave travel-time data from 6639 earthquake events.These events(M≥2.0)occurred from 1980 s to June 2019 and were recorded at 319 seismic stations(Chinese Earthquake Networks Center)in the study area.We adopted the double-difference seismic tomographic method(tomo DD)to invert the 3-D P-wave velocity structure and constrain the crust-upper mantle architecture of the Middle and Lower Reaches of the Yangtze River Metallogenic Belt(MLYB).A 1-D initial model extracted from wide-angle seismic profiles was used in the seismic tomography,which greatly reduced the inversion residual.Our results indicate that reliable velocity structure of th e uppermost mantle can be obtained when Pn is involved in the tomography.Our results show that:(1)the pattern of the uppermost mantle velocity structure corresponds well with the geological partitioning:a nearly E-W-trending low-velocity zone is present beneath the Dabie Orogen,in contrast to the mainly NE-trending low-velocity anomalies beneath the Jiangnan Orogen.They suggest the presence of thickened lower crust beneath the orogens in the study area.In contrast,the Yangtze and Cathaysia blocks are characterized by relatively high-velocity anomalies;(2)both the ultra-high-pressure(UHP)metamorphic rocks in the Dabie Orogen and the low-pressure metamorphic rocks in the Zhangbaling dome are characterized by high-velocity anomalies.The upper crust in the Dabie Orogen is characterized by a low-velocity belt,sandwiched between two high velocity zones in a horizontal direction,with discontinuous low-velocity layers in the middle crust.The keel of the Dabie Orogen is mainly preserved beneath its northern section.We infer that the lower crustal delamination may have mainly occurred in the southern Dabie Orogen,which caused the mantle upwelling responsible for the formation of the granitic magmas emplaced in the middle crust as the low-velocity layers observed there.Continuous deep-level compression likely squeezed the granitic magma upward to intrude the upper crustal UHP metamorphic rocks,forming the'sandwich'velocity structure there;(3)high-velocity updoming is widespread in the crust-mantle transition zone beneath the MLYB.From the Anqing-Guichi ore field northeastward to the Luzong,Tongling,Ningwu and Ningzhen orefields,high-velocity anomalies in the crust-mantle transition zone increase rapidly in size and are widely distributed.The updoming also exists in the crust-mantle transition zone beneath the Jiurui and Edongnan orefields,but the high-velocity anomalies are mainly stellate distributed.The updoming high-velocity zone beneath the MLYB generally extends from the crust-mantle transition zone to the middle crust,different from the velocity structure in the upper crust.The upper crust beneath the Early Cretaceous extension-related Luzong and Ningwu volcanic basins is characterized by high velocity zones,in contrast to the low velocity anomalies beneath the Late Jurassic to Early Cretaceous compression-related Tongling ore field.The MLYB may have undergone a compressive-to-extensional transition during the Yanshanian(Jurassic-Cretaceous)period,during which extensive magmatism occurred.The near mantle-crustal boundary updoming was likely caused by asthenospheric underplating at the base of the lower crust.The magmas may have ascended through major crustal faults,undergoing AFC(assimilation and fractional crystallization)processes,became emplaced in the fault-bounded basins or Paleozoic sequences,eventually forming the many Cu-Fe polymetallic deposits there.

Three-dimensional velocity structure and tectonic characteristics of earthquake area in Yibin

In this study,on the basis of absolute first-arrival times of 84756 P-and S-waves from 6085 earthquakes recorded at 56 fixed stations in Yibin and surrounding areas in China from January 2009 to January 2019,focal parameters and three-dimensional(3 D)body-wave high-resolution velocity structures at depths of 0–30 km were retrieved by double-difference tomography.Results show that there is a good correspondence between the spatial distribution of the relocated earthquakes and velocity structures,which were concentrated mainly in the high-velocity-anomaly region or edge of high-velocity region.Velocity structure of P-and S-waves in the Yibin area clearly shows lateral inhomogeneity.The distribution characteristics of the P-and S-waves near the surface are closely related to the geomorphology and geologic structure.The low-velocity anomaly appears at the depth of 15–25 km,which is affected by the lower crust current.The Junlian–Gongxian and Gongxian–Changning earthquake areas,which are the two most earthquake-prone areas in the Yibin region,clearly differ in earthquake distribution and tectonic characteristics.We analyzed the structural characteristics of the Junlian–Gongxian and Gongxian–Changning earthquake areas on the basis of the 3 D bodywave velocity structures in the Yibin region.We found that although most seismicity in the Yibin area is caused by fluid injection,the spatial position of seismicity is controlled by the velocity structures of the middle and upper crust and local geologic structure.Fine-scale 3 D velocity structures in the Yibin area provide important local reference information for further understanding the crustal medium,seismogenic structure,and seismicity.

Three-dimensional crustal velocity structure and activity characteristics of the Madoi Ms7.4 earthquake in 2021

In this paper,using natural earthquake P-wave arrival time data recorded by the seismic network in the surrounding area of Madoi,the three-dimensional fine P-wave crustal velocity structure at depths above 60 km in the epicenter of the Madoi Ms7.4 earthquake was inverted using the double-difference seismic tomography method.On the basis of the relocation of the source of the aftershock sequence,we summarized the strip-shaped distribution characteristics along the strike of the Jiangcuo fault,revealing the significant heterogeneity of the crustal velocity structure in the source area.Research has found that most of the Madoi Ms7.4 aftershocks were located in the weak area of the high-speed anomaly in the upper crust.The focal depth changed with the velocity structure,showing obvious fluctuation and segmentation characteristics.There was a good correspondence between the spatial distribution and the velocity structure.The high-velocity bodies of the upper crust in the hypocenter area provided a medium environment for earthquake rupture,the low-velocity bodies of the middle crust formed the deep material,and the migration channel and the undulating shape of the high-speed body in the lower crust corroborated the strong pushing action in the region.The results confirmed that under the continuous promotion of tectonic stress in the Madoi area,the high-speed body of the Jiangcuo fault blocked the migration of weak materials in the middle crust.When the stress accumulation exceeded the limit,the Madoi Ms7.4 earthquake occurred.Meanwhile,the nonuniform velocity structure near the fault plane determined the location of the main shock and the spatiotemporal distribution of the aftershock sequence.

Velocity and Structural Modeling of Mesozoic Chiltan Limestone and Goru Formation for Hydrocarbon Evaluation in the Bitrisim Area, Lower Indus Basin, Pakistan

The present study focuses on building a workflow for structural interpretation and velocity modeling and implementing to Jurassic-Cretaceous succession （Chiltan Limestone and Massive sand of the Lower Goru Formation）. 2D-Migrated seismic sections of the area are used as data set and in order to confirm the presence of hydrocarbons in the study area, P and S-wave seismic velocities are estimated from single-component seismic data. Some specific issues in the use of seismic data for modeling and hydrocarbon evaluation need to deal with including distinguishing the reservoir and cap rocks, and the effects of faults, folds and presence of hydrocarbons on these rocks. This study has carried out the structural interpretation and modeling of the seismic data for the identification of traps. The results demonstrate existence of appropriate structural traps in the form of horst and grabens in the area. 2D and 3D velocity modeling of the horizons indicates the presence of high velocity zones in the eastern half of the study while relatively low velocity zones are encountered in the western half of the area. Two wells were drilled in the study area （i.e. Fateh-01 and Ichhri-01） and both are dry. Immature hydrocarbons migration is considered as a failure reason for Fateh-01 and Ichhri-01 well.

Upper crustal velocity and seismogenic environment of the M7.0 Jiuzhaigou earthquake region in Sichuan, China

On August 8,2017,a magnitude 7.0 earthquake occurred in Jiuzhaigou County,Sichuan Province,China.The deep seismogenic environment and potential seismic risk in the eastern margin of Tibetan Plateau have once again attracted the close attention of seismologists and scholars at home and abroad.The post-earthquake scientific investigation could not identify noticeable surface rupture zones in the affected area;the complex tectonic background and the reason(s)for the frequent seismicity in the Jiuzhaigou earthquake region are unclear.In order to reveal the characteristics of the deep medium and the seismogenic environment of the M7.0 Jiuzhaigou earthquake region,and to interpret the tectonic background and genesis of the seismicity comprehensively,in this paper,we have reviewed all available observation data recorded by the regional digital seismic networks and large-scale,dense mobile seismic array(China Array)for the northern section of the North-South Seismic Belt around Jiuzhaigou earthquake region.Using double-difference seismic tomography method to invert the three-dimensional P-wave velocity structure characteristics of the upper crust around the Jiuzhaigou earthquake region,we have analyzed and discussed such scientific questions as the relationship between the velocity structure characteristics and seismicity in the Jiuzhaigou earthquake region,its deep tectonic environment,and the ongoing seismic risk in this region.We report that:the P-wave velocity structure of the upper crust around the Jiuzhaigoug earthquake region exhibits obvious lateral inhomogeneity;the distribution characteristics of the shallow P-wave velocity structure are closely related to surface geological structure and formation lithology;the M7.0 Jiuzhaigou earthquake sequence is closely related to the velocity structure of the upper crust;the mainshock of the M7.0 earthquake occurred in the upper crust;the inhomogeneous variation of the velocity structure of the Jiuzhaigou earthquake area and its surrounding medium appears to be the deep structural factor controlling the spatial distribution of the mainshock and its sequence.The 3D P-wave velocity structure also suggests that the crustal low-velocity layer of northeastern SGB(Songpan-GarzêBlock)stretches into MSM(Minshan Mountain),and migrates to the northeast,but the tendency to emerge as a shallow layer is impeded by the high-velocity zone of Nanping Nappe tectonics and the Bikou Block.Our results reveal an uneven distribution of high-and low-velocity structures around the Tazang segment of the East Kunlun fault zone.Given that the rupture caused by the Jiuzhaigou earthquake has enhanced the stress fields at both ends of the seismogenic fault,it is very important to stay vigilant to possible seismic hazards in the large seismic gap at the Maqu-Maqên segment of the East Kunlun fault zone.

Crustal structure in Dabieshan UHP metamorphic belt and its tectonic implication

The model of Dabieshan crustal structure has been obtained on the basis of the deep seismic sounding data in thisarea. The 2-D crustal structure shows the feature of the collision orogens and provides some deep geophysicalevidences of the ultra-high pressure (UHP) metamorphic belt. The 3-D upper-crustal velocity struCture reveals thatthe velocity distribution at 2 km deep obviously relates to the surface geological setting and the UHP metarnorphicbelt has the higher velocity at 5~10 km deep. The observed data of Bouguer gravity anomalies reveal a largerrange of negative anomalies in Dabieshan area while the positive anomalies in the UHP metamorphic belt is calculated from the 3-D upper-crustal velocity structure. The 2-D crustal model along the seismic profile shows thatthe 'root' beneath the orogen is only 4-5 km thick and the velocity in the uppermost mantle changes a little in thelateral direction. The inconsistency between the observed and calculated Bouguer gravity anomalies mainly resultsfrom the crust, and at least the middle-upper crust should yield the negative anomalies. The material density of thecrust in the UHP metamorphic belt should be lower than that in the surrounding areas. This material with lowerdensity relates to the collision processes in which Yangtze crust subducted nor'thward to 100 km deep and thenreturned to the crust.

Study on the Physical Process and Seismogenic Mechanism of the Yangbi MS 6.4 Earthquake in Dali,Yunnan Province

The Ms 6.4 earthquake occurred on May 21,2021 in Yangbi County,Dali Prefecture,Yunnan Province,which was the largest earthquake after the 2014 Jinggu Ms 6.6 earthquake,in western Yunnan.After the earthquake,the rapid field investigation and earthquake relocation reveal that there was no obvious surface rupture and the earthquake did not occur on pre-existing active fault,but on a buried fault on the west side of Weixi–Qiaohou–Weishan fault zone in the eastern boundary of Baoshan sub-block.Significant foreshocks appeared three days before the earthquake.These phenomena aroused scholars'intensive attention.What the physical process and seismogenic mechanism of the Yangbi Ms 6.4 earthquake are revealed by the foreshocks and aftershocks?These scientific questions need to be solved urgently.

CRUST AND UPPER STRUCTURE OF QINGHAI-TIBET PLATEAU AND ITS ADJACENT REGIONS FROM SURFACE WAVEFORM INVERSION

In this paper,218 long period Rayleigh wave records from 7 seismic station of CDSN are selected.We applied a partitioned waveform inversion to these data in order to construct a 3\|D model of shear velocity down to 400km depth in the crust and upper mantle of Qinghai\|Tibet plateau and Its Adjacent Regions (22°～44°N,70°～110°E).The first step of the waveform inversion used involved the matching of the waveforms of fundamental and highermost Ravleigh waves with waveforms synthesized from stratified models;in the second stage,the 3\|D model was constructed by solve linear constrains equation. The major structural features inferred from the surface waveform inversions can be summarized as follows:(1) There is a great contrast between surface waveform through Qinghai—Thibet plateau and the others.Main frequency of the former is lower than the latter, which indicate the crust depth of Qinghai—Tibet plateau is deeper than the others. In addition,the amplitude of about 30s period and 50s period is lower than both sides,which implied these exist lower velocity layer at about 25km depth and about 50km depth in Qinghai—Tibet plateau Crust.The former is common,the latter was argued because resolution of most method can not prove it.

Crustal and uppermost mantle structure of Caucasus and surrounding regions

A 3-D P-wave velocity model is developed for the crust and uppermost mantle of Caucasus and the surrounding area by applying the tomographic method of Zhao et al. using 300 000 high-quality P-wave first arrivals from 43 000 events between 1964 and 2005. This tomographic method can accommodate velocity discontinuities such as the Moho in addition to smooth velocity variations. The spatial resolution is 1°× 1° in the horizontal direction and 10 km in depth. The velocity images of the upper crust correspond well with the surface geology. Beneath the southern Caucasus high velocity anomalies are found in the middle crust and low velocity anomalies are found in the uppermost mantle. Relatively low Pn velocities are located under the Lesser Caucasus, eastern Turkey, and northern Iran. Higher Pn velocities occur under the eastern portion of the Black Sea and the southern Caspian Sea, and also extend into the eastern edge of Azerbaijan. Tomographic model significantly reduces the travel-time residuals.