Deep seismic sounding investigation into the deep structure of the magma system in Changbaishan-Tianchi volcanic region

The magma system of Changbaishan-Tianchi Volcanic region is studied with three-dimensional deep seismic sounding (DSS) technique. The results show that the magma system of Changbaishan-Tianchi volcanic region, mainly characterized by low velocity of P wave, can be divided into three parts in terms of depth. At the depth range of 9-15 km, the distribution of the magma system is characterized by extensiveness, large scale and near-SN orientation. This layer is the major place for magma storage. From the depth of 15 km down to the lower crust, it is characterized by small lateral scale, which indicates the 'trace' of magma intrusion from the upper mantle into the crust and also implies that the magma system most probably extends to the upper mantle, or even deeper.(less than 8-9 km deep), the range of magma distribution is even smaller, centering on an SN-oriented area just north of the Tianchi crater. If low velocity of P wave is related to the magma system, it then reflects that the magma here is still in a state of relatively high temperature. In this sense, the magma system of Changbaishan-Tianchi volcanic region is at least not 'remains', in other words, it is in an 'active' state.

Crustal Structure of the Chuan-Dian Block Revealed by Deep Seismic Sounding and its Implications for the Outward Expansion of the East Tibetan Plateau

The Chuan-Dian Block(CDB)is located in the southeastern margin of the Tibetan Plateau,with a complex geological structure and active regional faults.The present tectonic condition with strong crustal deformation is closely related to the ongoing collision of the India and Eurasia plates since 65 Ma.The study of the crustal structure of this area is key to revealing the evolution and deep geodynamics of the lateral collision zone of the Tibetan Plateau.Deep seismic sounding is the most efficient method with which to unravel the velocity structure of the whole crust.Since the 1980s,19 deep seismic sounding profiles have been captured within the CDB area.In this study,we systematically integrate the research results of the 19 profiles in this area,then image the 3D crustal velocity,by sampling with a 5 km spacing and 2D/3D Kriging interpolation.The results show the following.(1)The Moho depth in the study area deepens from 30 km in the south to 66 km in the north,whereas there is no apparent variation from west to east.The Pn wave velocity is higher in stable tectonic units,such as 7.95 km/s in the Lanping-Simao block and 7.94 km/s in the western margin of the Yangtze block,than in active or mobile tectonic units,such as 7.81 km/s in the Baoshan block,7.72 km/s in the Tengchong block and 7.82 km/s in the Zhongdian block.(2)The crustal nature of the Tengchong block,the northern Lanping-Simao block and the Zhongdian block reflects a type of orogenic belt,having relatively strong tectonic activities,whereas the crustal nature of the central Lanping-Simao block and the western margin of the Yangtze block represents a type of platform.The different features of the upper-middle crust velocity,Moho depth and Pn wave velocity to both sides of the Red River fault zone and the Xianshuihe fault zone,reflect that they are clearly ultra-crustal.(3)Based on the distribution of the low velocity zones in the crust,the crustal material of the Tibetan Plateau is flowing in a NW–SE direction to the north of 26°N and to the west of 101°E,then diverting to flowing eastwards to the east of 101°E.

Basic Features of the Crustal Structure in the Lower Yangtze and Its Neighboring Area in the Chinese Mainland： Review of Deep Seismic Sounding Research

The Deep Seismic Sounding( DSS) projects carried out from the 1970 s in the lower Yangtze region and its neighboring area were reviewed in this paper,then the basic wave group features of those wide angle reflection / refraction record sections,and of the crustal structure are summarized. It shows that there were in total five clear wave groups on the record sections,which include the first arrival Pg,the reflection P1 from the bottom interface of the upper crust,the reflection P3 from the bottom interface of the middle crust,the strong reflection Pm from the Moho boundary,and the refraction Pn from uppermost mantle. In general,these phases are easily consistently traced and compared,despite some first arrivals being delayed or arriving earlier than normal due to the shallow sedimentary cover or bedrocks. In particular,in the Dabie Mountain region the seismic events of a few gathered shots always have weak reflection energy,are twisted,or exhibit disorganized waveforms, which could be attributed to the disruption variations of reflection depth,the broken Moho,and the discontinuity of the reflection boundary within crust. The regional crustal structures are composed of the upper,middle and lower crust,of which the middle and lower layers can be divided into two weak reflection ones. The crustal thickness of the North China and Yangtze platform are 30km- 36 km,and the Moho exhibits a flat geometry despite some local uplifts. The average pressure velocity in lower crust beneath this two tectonic area is 6. 7 ± 0. 3km / s. Nevertheless,beneath the Dabieshan area the crustal thickness is 32km- 41 km,the Moho bends down sharply andtakes an abrupt 4km- 7km dislocation in the vertical direction. The average pressure velocity in the lower crust beneath the Dabieshan area is 6. 8 ± 0. 2km / s.

Genetic algorithm in seismic waveform inversion and its application in deep seismic sounding data interpretation

A genetic algorithm of body waveform inversion is presented for better understanding of crustal and upper mantle structures with deep seismic sounding （DSS） waveform data. General reflection and transmission synthetic seismogram algorithm, which is capable of calculating the response of thin alternating high and low velocity layers, is applied as a solution for forward modeling, and the genetic algorithm is used to find the optimal solution of the inverse problem. Numerical tests suggest that the method has the capability of resolving low-velocity layers, thin alternating high and low velocity layers, and noise suppression. Waveform inversion using P-wave records from Zeku, Xiahe and Lintao shots in the seismic wide-angle reflection/refraction survey along northeastern Qinghai-Xizang （Tibeteau） Plateau has revealed fine structures of the bottom of the upper crust and alternating layers in the middle/lower crust and topmost upper mantle.

Multiple phases analysis of deep seismic sounding records in North China Craton

The North China Craton(NCC)is a key region to study the destruction of the ancient craton.Two groups of phases(denoted as"Pw1"and"Pw2"),which are parallel to the PmP phase reflected from the Moho discontinuity and the PLP phase reflected from the Lithosphere and Asthenosphere Boundary(LAB)respectively,are found on the record section of the Rongcheng-XinzhouAlxa long-range deep seismic sounding profile.The nature of the two phases is still unclear,although they are clearly observable and reverberant.In this paper,we use travel time inversion and amplitude forward modelling to fit the reflected and refracted phases in the lithosphere.The results show:(1)the Pw1 is a multiple reflected phase which is successively reflected by the crystalline basement,the surface,the Moho and then finally received on the surface;(2)the Pw2 phase is also a multiple reflected phase successively reflected by the crystalline basement,the surface,the LAB interface and then received on the surface.We conclude that the significant velocity difference between the thick sedimentary cover and the crystalline basement in the North China rifted basin may be the main reason for generating the multiple reflections.Furthermore,the two multiple reflections provide potent constraints on the lithospheric velocity model,and constitute seismological evidence for the lithospheric thinning in the eastern NCC.

Differences in lithospheric structures between two sides of Taihang Mountain obtained from the Zhucheng-Yichuan deep seismic sounding profile

A 2-D model of lithospheric velocity structures in the southern part of the North China Craton was obtained using data from the Zhucheng-Yichuan deep seismic sounding profile.Results show that there are great differences in lithospheric structures between two sides of Taihang Mountain.In the eastern region,the lithosphere is thinner,with a thickness of about 70-80 km,while in the western region,the thickness is 85-120 km.There is a jump of the lithospheric thickness across Taihang Mountain gravity anomaly belt with a magnitude of about 30 km.P wave velocities of the lithospheric mantle and lower crust are lower in the eastern region and higher in the western region.In the eastern region,there are low velocity bodies in the middle and lower crust,while none were found in the western region.These differences indicate that the Taihang Mountain gravity anomaly belt is a belt with a abrupt change of lithospheric thickness and lithological composition.According to the Pm waveform,it can be deduced that the Moho in the eastern region is not a sharp discontinuity,but a complex transitional zone.From a preliminary analysis,it is found that the geothermal mechanical-chemical erosion could be the main mechanism causing the thinning and destruction of the lithosphere beneath the eastern side of Taihang Mountain.In addition,subduction of the Pacific Plate is an important factor which changes the properties of the lithospheric mantle of the North China Craton.

Crustal velocity structure of the Shaowu-Nanping-Pingtan transect through Fujian from deep seismic sounding-tectonic implications

The Shaowu-Nanping-Pingtan deep seismic sounding profile is located in northern Fujian Province. High-quality seismic sounding data were acquired by five large explosive blasts received by 133 digital seismic instruments along the profile. Based on seismic facies analysis and travel-time picking on shot record sections, a model of the velocity structure of upper crust was developed by finite-difference tomography of the first breaks; the 2-D P-wave velocity structure and tectonic characteristics of the crust were interpreted further by fitting of waveforms and seismic travel times. The results show that the top of the crystal- line basement is buried at depths of 2.0-4.0 kin, with the deepest buried up to 4.0 km within the Fuzhou Basin. The Moho in- terface was found to be deeper in the west and shallower in the east （i.e., 30.0 km near the coast, increasing to 33.0 km north- westward）. The lower crust on the east side of the Zhenghe-Haifeng Fault Zone has a smoothly varying gradient structure, whereas on the west side it has two distinct layers with a boundary between those layers at a depth of 23 km. Seismic velocities on the west side are generally lower than on the east side; a low velocity layer is observed with a lowest speed of 6.25 km/s at a depth of 22 km on the west side, which may consist of partially molten material. The Zhenghe-Haifeng Fault is a deep crustal fault, and should be a channel for deep material upwelling; it has a direct relationship with multiple stages of continental tectonic movements in Southern China and with multiple magmatic events that started in the Proterozoic and ended in the of late Tertiary in Fujian.

Deep Background of Wenchuan Earthquake and the Upper Crust Structure beneath the Longmen Shan and Adjacent Areas

By analyzing the deep seismic sounding profiles across the Longmen Shan, this paper focuses on the study of the relationship between the upper crust structure of the Longmen Shan area and the Wenchuan earthquake. The Longmen Shan thrust belt marks not only the topographical change, but also the lateral velocity variation between the eastern Tibetan Plateau and the Sichuan Basin. A low-velocity layer has consistently been found in the crust beneath the eastern edge of the Tibetan Plateau, and ends beneath the western Sichuan Basin. The low-velocity layer at a depth of -20 km beneath the eastern edge of the Tibetan Plateau has been considered as the deep condition for favoring energy accumulation that formed the great Wenchuan earthquake.

2-D P-wave velocity structure of lithosphere in the North China tectonic zone: Constraints from the Yancheng-Baotou deep seismic profile

We obtained the 2-D P-wave velocity structure of the lithosphere in the eastern North China Craton, Shanxi fault subsidence zone, and Yinchuan-Hetao fault subsidence zone by ray tracking technology based on six groups of clearly identified crustal phases and one group of lithospheric interface reflection phases from seismic recording sections of 21 shots along the 1300-km-long Yancheng-Baotou deep seismic wide-angle reflection/refraction profile. The results indicate significant differ- ences between the lithospheric structure east and west of the Taihang Mountains, which is a gravity-gradient zone as well as a zone of abrupt change in lithospheric thickness and a separation zone of different rock components. East of the Taihang Mountains, the Mesozoic and Cenozoic lithospheric structure of the North China Craton has undergone strong reformation and destruction, resulting in the lithosphere thickness decreasing to 70-80 km. The North China Basin has a very thick Cenozoic sedimentary cover and the deepest point of crystalline basement is about 7.0 kin, with the crustal thickness decreasing to about 31.0 kin. The crystalline basement of the Luxi uplift zone is relatively shallow with a depth of 1.0-2.0 km and crustal thickness of 33.0-35.0 km. The Subei Basin has a thicker Cenozoic sedimentary cover and the bottom of its crystalline basement is at about 5.0-6.0 km with a crustal thickness of 31.0-32.0 km. The Tanlu fault is a deep fracture which cuts the lithosphere with a significant velocity structure difference on either side of the fault. The Tanlu fault plays an important role in the lithospheric destruction in the eastern part of the North China Craton. West of the Taihang Mountains, the crustal thickness increases sig- nificantly. The crust thickness beneath the Shanxi fault depression zone is about 46 km, and there is a low-velocity structure with a velocity of less than 6.1 km s-~ in the upper part of the middle crust. Combined with other geophysical study results, our data shows that the lithospheric destruction at the Shaanxi-Shanxi fault depression zone and the Yinchuan-Hetao rift surround- ing the Ordos block is non-uniform. The lithosphere thickness is about 80-90 km in the Datong-Baotou area, 75-137 km at the Dingxiang-Shenmu region, and about 80-120 km in the Anyang-Yichuan area. The non-uniform lithospheric destruction may be related to the ancient tectonic zone surrounding the Ordos block. This zone experienced multi-period tectonic events in the long-term process of its tectonic evolution and was repeatedly transformed and weakened. The weakening level is related to the interactions with the Ordos block. The continental collision between the Cenozoic India and Eurasia plates and N-E thrust- ing by the Qinghai Tibet Plateau block is causing further reformation and reduction of the lithosphere.

Crustal structure in Xiaojiang fault zone and its vicinity

Based on the integrative interpretation of travel-time data and amplitude information obtained from the deep seismic sounding experiment on the Chuxiong-Luoping profile, eastern Yunnan province, carried out in January of 2005, we present a 2-D P wave velocity structure along the profile. The crustal structure shows remarkable contrasts between the two sides of the Xiaojiang fault zone, although the whole profile is situated within the Yangtze platform. The average P wave velocities of the crust on the west and east sides of the fault zone are 6.21 km/s and 6.32 km/s, respectively, and the crustal thicknesses are 41 km and 45 km, respectively. These results imply that the crust to the east of the Xiaojiang fault zone presents characteristics of crustal structure in a stable platform, while the crust to the west is complicated with a lower velocity zone in middle of the upper crust. The average velocity of 6.21 km/s is lower than the global continental crustal average （6.30 km/s）, indicating that the region is tectonically active. According to the lateral variation of velocity and depth of interfaces （including the Moho）, it is inferred that the Xiaojiang fault zone has cut through the whole crust. It is also deduced that existence of low velocity zone in middle of the upper crust is conducive to the south-southeastern sliding of the Sichuan- Yunnan （Chuan-Dian） rhombus block.

Crustal structure beneath the Songpan——Garze orogenic belt

The Benzilan-Tangke deep seismic sounding profile in the western Sichuan region passes through the Song-pan-Garze orogenic belt with trend of NNE. Based on the travel times and the related amplitudes of phases in the record sections, the 2-D P-wave crustal structure was ascertained in this paper. The velocity structure has quite strong lateral variation along the profile. The crust is divided into 5 layers, where the first, second and third layer belong to the upper crust, the forth and fifth layer belong to the lower crust. The low velocity anomaly zone gener-ally exists in the central part of the upper crust on the profile, and it integrates into the overlying low velocity basement in the area to the north of Ma'erkang. The crustal structure in the section can be divided into 4 parts: in the south of Garze-Litang fault, between Garze-Litang fault and Xianshuihe fault, between Xianshuihe fault and Longriba fault and in the north of Longriba fault, which are basically coincided with the regional tectonics division. The crustal thickness decreases from southwest to northeast along the profile, that is, from 62 km in the region of the Jinshajiang River to 52 km in the region of the Yellow River. The Moho discontinuity does not obviously change across the Xianshuihe fault based on the PmP phase analysis. The crustal average velocity along the profile is lower, about 6.30 km/s. The Benzilan-Tangke profile reveals that the crust in the study area is orogenic. The Xianshuihe fault belt is located in the central part of the profile, and the velocity is positive anomaly on the upper crust, and negative anomaly on the lower crust and upper mantle. It is considered as a deep tectonic setting in favor of strong earthquake's accumulation and occurrence.

Seismological study on the crustal structure of Tengchong volcanic-geothermal area

Based upon the deep seismic sounding profile conducted in the Tengchong volcanic-geothermal area, a two-dimensional crustal P velocity structure is obtained by use of the finite-difference inversion and the forward travel-time fitting method. The crustal model shows that there is a low velocity zone in upper crust in the Tengchong area, which may be related to the volcanic-geothermal activities, and two intracrustal faults (the LonglingRuili fault and Tengchong fault) exist on the profile, where the Tengchong fault may extend to the Moho discontinuity. Meanwhile, based on teleseismic data recorded by a temporary seismic network, we obtained the S-wave velocity structures beneath the RehaiRetian region in the Tengchong area, which show the low S-wave velocity anomaly in upper crust. The authors discuss the causes of Tengchong volcanic eruption based on the deep crustal structure. The crustal structure in the Tengchong volcanic-geothermal area is characterized by low P-wave and S-wave velocity, low resistivity, high heat-flow value and low Q value. The P-wave velocity in the upper mantle is also low. For this information, it can be induced that the magma in the crust is derived from the upper mantle, and the low velocity anomaly in upper crust in the Tengchong area may be related to the differentiation of magma. The Tengchong volcanoes are close to an active plate boundary and belong to plate boundary volcanoes.

Study of crustal structure with S-wave data from Maqen-Jingbian profile

D crustal velocity structure and vP/vS are obtained by processing and interpretation of S-wave data from Maqen-Jingbian deep seismic sounding (DSS) profile. The result shows that there exist obvious differences in 2-D S-wave velocity structure and vP/vS ratio structure along the profile. The S-wave velocities are low and vP/vS ratio is high for the western section of the profile and Haiyuan region, while they are normal for the middle and eastern sections. The changes in lithologic characters of two major anomalous zones are discussed according to lateral variation of S-wave velocity structure and vP/vS ratio structure. It is concluded that the development and occurrence of the Haiyuan strong earthquake is not only related to tectonic activities, but also to lithologic characters of the region.

Study on the crust-mantle structure in the central and southern parts of Shanxi

Another comparative interpretation was conducted with respect to the data from 5 DSS profiles in the central and southern parts of Shanxi, leading to the conclusion that in Linxian, Linfen and Xingtai earthquake regions, through which the five profiles pass, there exist anomalous crust mantle structure and abyssal crustal faults extending to Moho, all being regarded as the deep indications for earthquake occurrence.

Investigation of plateau basin crustal structures and thickening mechanisms in the northeastern margin of the Tibetan plateau

This paper uses deep seismic sounding （DSS） data to contrast and analyze the crustal structures of three plateau basins （Songpan-Garze, Qaidam, Longzhong） in the northeastern margin of the Qinghai-Xizang （Tibetan） plateau, as well as two stable cratonic basins （Ordos, Sichuan） in its peripheral areas. Plateau basin crustal structures, lithological variations and crustal thickening mechanisms were investigated. The results show that, compared to the peripheral stable cratonic basins, the crystalline crusts of plateau basins in the northeastern margin are up to 10 15 km thicker, and the relative medium velocity difference is about 5% less. The medium velocity change in crustal layers of plateau basin indicates that the upper crust undergoes brittle deformation, whereas the lower crust deforms plastically with low velocity. The middle crust shows a brittle-to-plastic transition zone in this region. Thickening in the lower crust （about 5 10 km）, and rheological characteristics that show low- medium velocity （relatively reduced by 7%）, suggest that crustal thickening mainly takes place in lower crust in the northeastern margin of the Tibetan plateau. The crust along the northeastern margin shows evidence of wholesale block movement, and crustal shortening and thickening seem to be the main deformation features of this region. The GPS data show that the block motion modes and crustal thickening in the Tibetan plateau is closely related to the peripheral tectonic stress field and motion direction of the Indian plate. The Mani-Yushu- Xianshuihe fold belt along the boundary between the Qiangtang block and the Bayan Har block divides the different plateau thickening tectonic environments into the middle-western plateau, the northeastern margin and the southeastern plateau.

The depth of Moho in the mainland of China

In the last decade, deep seismic sounding (DSS) of profiles several ten thousand kilometers in length has beenconducted in the mainland of China. A lot of results published in Chinese were compiled and a map of Mohodepth was presented in this paper. Coverage of DSS profile is not uniform,and is lacking in some areas, especiaLly in the western part of China. In this case,the results from dispersions of seismic surface waves and other materials were considered. According to the Moho depth, the mainland of China can be divided into eight crustalblocks.The Moho depth is uniform and varies little inside the same block, while it changes abruptly from oneblock to another. The Tibetan Plateau is characterized by a thickest crust in the world. It may be due to thepenetration of indian crustal materials into the Tibetan crust or upper mantle. Deeper Moho in the neighbouringareas of Tibet may also be effected by the complicated collision processes between the two continents.Thin crusto f the North China Plain is considered to be related to the magmatic activities in the lower crust and uppermantle. While the crust of the eastern part of SOuth China may be due to the accretion of terrain from the oceanislands.

Lithospheric structure and continental geodynamics

This paper briefly reviews main progress in the research on lithospheric structure and continental geodynamics made by Chinese geophysicists during last 4 years since 22nd IUGG general assembly in July 1999. The research mainly covers the following fields: investigations on regional lithospheric structure, DSS survey of crust and upper mantle velocity structure, study on present-day inner movement and deformation of Chinese mainland by analyzing GPS observations, geodynamics of Qingzang plateau, geophysical survey of the Dabie-Sulu ultra-high pressure metamorphic belt and probing into its formation mechanism, geophysical observations in sedimentary basins and study on their evolution process, and plate dynamics, etc.

Study and review on crust-mantle velocity structure in Bohai Bay and its vicinity

Observational data from some of the 10-odd deep seismic sounding profiles in Bohai Bay and its adjacent areas were processed with the methods of two-dimensional ray tracing, travel-time fitting and synthetic seismogram. The crust and upper-mantle velocity structure model in this area was built. The results show that the crust and upper mantle structures present obvious lateral and vertical inhomogeneity. The upper mantle uplifts near Yongqing of northeast Jizhong depression, in Bohai Bay of Huanghua depression and near Kenli of Jiyang depression, where crustal depths are about 31 km, 28 km and 29 km, respectively. According to the dynamic and kinetic characteristics of seismic waves as well as the seismic interfaces and velocity contour undulation in the 2-D velocity structure model, three deep crustal fault zones are inferred in the area. Low velocity (5.90~6.10 km/s) layers (bodies) exist on one or two sides of these deep crustal fault zones.

Three-Dimensional P-Wave Velocity Structure of the Crust of North China

Since the Xingtai （邢台） earthquake in 1966, China Earthquake Administration has carried out a survey campaign along more than thirty deep seismic sounding （DSS） profiles altogether about twenty thousand kilometers long in North China to study the velocity structure of the crust and the upper mantle in this region, and has obtained a great number of research findings. However, these researches have not provided a 3D velocity structure model of the crust of North China and cannot provide seismic evidence for the study of the deep tectonic characteristics of the crust of the whole region. Hence, based on the information from the published data of the DSS profiles, we have chosen 14 profiles to obtain a 3D velocity structure model of North China using the vectorization function of the GIS software （Arc/Info） and the Kriging data gridding method. With this velocity structure model, we have drawn the following conclusions： （1） The P-wave velocity of the uppermost crust of North China changes dramatically, exhibiting a complicated velocity structure in plane view. It can be divided into three velocity zones mainly trending towards north-west. In the research area, the lowest-velocity zones lie in the Haihe （海河） plain and Bohai （渤海） Bay. Although the geological structure of the sedimentary overburden in the study area is somewhat inherited by the upper crust, there are still several differences between them. （2） Generally, the P-wave velocity of the crust increases with depth in the study area, but there still exists local velocity reversion. In the east, low-velocity anomalies of the Haihe plain gradually disappear with increasing depth, and the Shanxi （山西） graben in the west is mainly characterized by relatively low velocity anomalies. Bounded by the Taihang （太行） Mountains, the eastern and western parts differ in structural trend of stratum above the crystalline basement. The structural trend of the Huanghuaihai （黄淮海） block in the east is mainly north-east, while that of the Shanxi block and the eastern edge of the Ordos block is mainly north-west. （3） According to the morphological features of Moho, the crust of the study area can be divided into six blocks. In the Shanxi block, Moho apppears like a nearly south-north trending depression belt with a large crustal thickness. In the southern edge of the Inner Mongolia block and the south of the Yanshan （燕山） block,the Moho exhibits a feature of fold belt, trending nearly towards east-west. In the eastern edge of the Ordos block, the structure of Moho is relatively complex, presenting a pattern of fold trending nearly towards north-west with alternating convexes and concaves. Beneath the Huanghuaihai block, the middle and northern parts of the North China rift zone, the Moho is the shallowest in the entire region, with alternating uplifts and depressions in its shape. For the anteclise zone in the west of Shandong （山东） Province, the Moho is discontinuous for the fault depression extending in the north-west direction along Zaozhuang （枣庄） -Qufu （曲阜）.

Crustal and upper mantle structure and deep tectonic genesis of large earthquakes in North China

From the 1960 s to 1970 s, 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.