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.

The Significance of Crust Structure and Continental Dynamics Inferred from Receiver Functions in West Yunnan

In our study we collected the teleseismic record of 31 broadband stations and 9 PASSCAL stations in West Yunnan, as well as extracted more than a million receiver functions. Using the waveform model and stacking techniques, we calculated the earth crust thicknesses and Vp/Vs ratios below the stations and obtained 35 valid data points. At the same time, we evenly stacked the receiver functions at the same station and superimposed the two profiles＇ cross sections of the main tectonic units. The results show a clear difference between the crust thicknesses of different tectonic units. Because of the magma underplatting and delimanition of the lower crust in the role of deep process, the West Yunnan＇s crust can be divided two kinds-mafic-ultramafic and feldspathic crusts. The research also shows that the mafic-ultramafic crust corresponds to a good background of mineralization. The delamination of the lower crust is one of the leading causes for moderate to strong earthquake prone in central Yunnan. The thinner crust and high velocity ratio as well as the multimodal structure of Ps in the Tengchong volcanic area confirms existence of a deep process of the strong magma underplating. Due to the basic crust structure and nature, it is believed that the Honghe fault is a main suture of the Gondwana and Eurasia continents.

Upper crust structure of eastern A’nyemaqên suture zone: Results of Barkam-Luqu-Gulang deep seismic sounding profile

Barkam-Luqu-Gulang deep seismic sounding profile runs from north of Sichuan Province to south of Gansu Province. It is located at the northeastern edge of Tibetan Plateau and crosses eastern A＇nyemaqên suture zone. The upper crust structures around eastern A＇nyemaqên suture zone and its adjacent area are reconstructed based on the arrival times of refracted Pg and Sg waves by using finite difference method, ray tracing inversion, time-term method and travel-time curve analysis. The results show that the depth variation of basement along profile is very strong as indicated by Pg and Sg waves. The basement rose in Zoigê basin and depressed in eastern A＇nyemaqên suture zone, and it gradually rose again northward and then depressed. The results also indicate that eastern A＇nyemaqên suture zone behaves as inhomogeneous low velocity structures in the upper crust and is inclined to- ward the south. Hoh Sai Hu-Maqên fault, Wudu-Diebu fault and Zhouqu-Liangdang fault are characterized by low velocity distributions with various scales. The distinct variation in basement depth occurred near Hoh Sai Hu-Maqên fault and Zhouqu-Liangdang fault, which are main tectonic boundaries of A＇nyemaqên suture zone. Wudu-Diebu fault, located at the depth variation zone of the basement, possibly has the same deep tectonic background with Zhouqu-Liangdang fault. The strongly depressed basement characterized by low velocity distribution and lateral inhomogeneity in A＇nyemaqên suture zone implies crushed zone features under pinching action.

3-D shear wave velocity structure in the shallow crust of the Tan-Lu fault zone in Lujiang,Anhui,and adjacent areas,and its tectonic implications

The Tan-Lu fault zone is a large NNE-trending fault zone in eastern China.Investigations of the structures of the fault zone and its surrounding areas have attracted much attention.In this study,we used dense-array ambient noise tomography to construct a threedimensional shear wave velocity model of shallow crust in an area about 80km×70km in Lujiang,Anhui Province,eastern China.For approximately one month we collected continuous ambient noise signals recorded by 90 short-period seismographs in the region,and obtained the short-period Rayleigh wave empirical Green's functions between stations by the cross-correlation method;we also extracted 0.5–8 s fundamental mode Rayleigh wave group velocity and phase velocity dispersion curves.Based on the direct surface wave tomography method,we jointly inverted the group velocity and phase velocity dispersion data of all paths and obtained the 3-D shear wave velocity structure in the depth range of 0–5 km.The results revealed important geological structural features of the study area.In the north region,the sedimentary center of the Hefei Basin—the southwestern part of the Chaohu Lake—shows a significant low-velocity anomaly to a depth of at least 5 km.The southwestern and southeastern regions of the array are the eastern margin of the Dabie orogenic belt and the intrusion area of Luzong volcanic rocks,respectively,and both show obvious high-speed anomalies;the sedimentary area within the Tan-Lu fault zone(about 10 km wide)shows low-velocity anomalies.However,the volcanic rock intrusion area in the fault zone is shown as high velocity.Our shallow crustal imaging results reflect the characteristics of different structures in the study area,especially the high-speed intrusive rocks in the Tan-Lu fault zone,which were probably partially derived from the magmatic activity of Luzong volcanic basin.From the Late Cretaceous to Early Tertiary,the Tan-Lu fault zone was in a period of extensional activity;the special stress environment and the fractured fault zone morphology provided conditions for magma in the Luzong volcanic basin to intrude into the Tan-Lu fault zone in the west.Our 3-D model can also provide important information for deep resource exploration and earthquake strong ground motion simulation.

Tomographic investigation of the upper crustal structure and seismotectonic environments in Yunnan Province

Investigation has been made for the upper crust structure and seismotectonic environments in Yunnan Province using the plentiful DSS data of the four profiles. The derived velocity model has a good relationship with the ex-posed basins, uplifts and faults. The low velocity anomaly corresponding to the volcano also has been revealed. There exists a prominent lateral inhomogeneity within the upper crust of Yunnan region. The depth of crystalline basement generally ranges from 0 km to 5 km, and the bedrocks are exposed on the ground directly in some places, nevertheless the thickness of sedimentary cover also can reach to 8 km or even 12 km at some large depressions. Although the Changning-Shuangjiang fault is a boundary between two first class tectonic units, its incision depth within the crust maybe shallow. On the other hand, known as the plates seam, the Honghe fault has a distinct evi-dence of extending into the mid-lower crust. The widely spread activity of the volcanoes in the geological era has a close relationship with the earthquakes occurrence nowadays. Despite of the ceasing of the volcanoes in some places on the ground, the material in the mid-lower crust is still active, and there still exists strong upward stress. As the ceasing of the volcanoes on the surface, most parts of the power from the lower crust and the upper mantle cannot be released; therefore it accumulates at some appropriate tectonic locations. Moreover, the saturation of the water from the basin, the action of other fluids, and the effects of the outer stress maybe another direct reason ac-count for the strong earthquakes occurrence in Yunnan region.

Receiver functions of CCDSN and crustal structure of Chinese mainland

The teleseismic receiver functions of 48 stations belonging to the CCDSN are used to invert the crustal structure beneath each station with the neighborhood algorithm. Thin layers with low velocity have been found beneath eight stations with ＂abnormal＂ observed receiver functions. Unreasonable results of few stations have been adjusted lightly with the trial-and-error method. The final result indicates that the crust in the western China is relatively thicker than the eastern China. The crust thickness beneath the Tibetan plateau is very large, which reaches 84 km at the station LSA. Double-crust structure exists below the stations LSA and CAD in Tibet, which might imply the collision between the Indian and Eurasian plates. A pronounced low velocity zone in the lower crust beneath the station TNC of Yunnan province might relate to the high temperature or emergence of partially molten material caused by Quaternary volcano, magma and geothermal activities in this area. The Moho is a transitional zone made up of thin layers instead of simple sharp discontinuity beneath several stations. The Conrad discontinuity is clearly identified beneath 20 stations mainly in the southeastern China, whereas it is blurry beneath 14 stations and uncertain beneath remaining stations.

Crust-Mantle Structure and Coupling Effects on Mineralization : An Example from Jiaodong Gold Ore Deposits Concentrating Area, China

Based on the results of two dimension velo city structure, 1∶100 000 aeromagnetic anomaly, 1∶200 000 bouguer gravity anom aly and seismic anisotropy of Jiaodong and neighboring region in Shandong, China , the information of geophysical field was divided into two parts: deep and sh allow focus fields. And then, the information of two different fields was c ombined with that of deep seated geology and ore deposit features. The syntheti c result was adopted to analyze three dimension structure, to probe into crust mantle coupling effects of mineralization and dynamics of ore formation system .

Study on crust-mantle tectonics and its velocity structure along the Beijing-Huailai-Fengzhen profile

In order to investigate the interrelations of crust and upper mantle tectonics and its velocity distribution as well as seismicity in the Yanhuai basin and its surrounding area, a nearly EW trending Beijing Huailai Fengzhen wide angle reflection/refraction profile, which obliquely passes through seismic zone of Zhangjiakou Bohai Sea and coincides with a deep reflection profile in the Yanhuai basin, was completed recently. The results show: The crust presents layered structures and its thickness gradually increases from 35.0 km in Shunyi to 42.0 km in the west end of the profile; the interior crustal interfaces appear approximately horizontal or slowly sloping down from east to west; In the Yanhuai basin, the crust presents the characteristics of higher velocities alternating with the lower ones and the low velocity bodies obviously exist in the lower part of upper crust. Moreover, there are two deep crustal fault zones which stretch to the Moho discontinuity, are closely related with the seismicity in the Yanhuai area.

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.

Relationship between Crustal 3D Density Structure and the Earthquakes in the Longmenshan Range and Adjacent Areas

This paper presents the 3D density structure of crust in the Longmenshan range and adjacent areas, with constraints from seismic and density data. The density structure of crust shows that the immense boundary plane of density distribution in relation to the Longmeshan fault belt is extended downward to -80 km deep. This density boundary plane dips towards the northwest and crosses the Moho. With the proximity to the Longmenshan fault belt, it has a larger magnitude of undulation in the upper and middle crust levels. Density changes abruptly across Longmeshan fault belt. Seismic data show that most of the earthquakes in the Longmenshan area after the 2008 Ms8.0 Wenchuan Earthquake occurred within the upper to middle crust. These earthquakes are clearly distributed in the uplifted region of the basement. A few of them occurs in the transitional zone between the uplifted and subsided areas. But most of the earthquakes distributes in transitional zone from subsided to uplifted areas in the upper and middle crust where relatively large density changes occurr The 3D density structure of crust in the Longmenshan and adjacent areas can thus help us to understand the pattern of overthrusting from the standpoint of deep crust and where the earthquakes occurred.

Study on fine structure of crust-mantle transi-tion zone in Yanqing-Hailai basin based on CDP and DSS data

The fine structure of crust mantle transition zone in Yanqing Hailai(Yan Huai basin) basin has been analyzed and discussed by using the data of Yanqing Hailai deep seismic reflection profile and Beijing Huailai Fengzhen deep seismic sounding profile obtained respectively in 1992 and in 1993. The primary model is established based on CDP stacking profile. The phases of seismic refraction waves and wide angle reflection waves are analyzed, travel time inversion is carried out and 2 D ray tracing is computed. Synthetic seismograms are completed by using re flectivity method for waveform fitting of phases P M and P M′, they are reflection waves from both the upper and the lower boundaries of the crust mantle transition zone in this basin. The results show that the P M′ reflection waves are stronger at some points and come from the lower boundary of Moho discontinuity. It is confirmed and inter preted that the Moho discontinuity in Yanqing Hailai basin consists of a group of thin layers with alternatively higher and lower velocities.

3-D seismic tomography for velocity and interface structure of the crust and upper mantle(theoreticalpart)

A method of three dimensional (3-D) model parameterization is presented that makes forward and inverse problems become easy. The velocity and interface structure of crust and upper mantle are described by a set of highly smoothed functions. Shooting ray tracing method is chosen to calculate the ray paths for both forward and inverse problems. The partial derivatives of traveltime with respect to parameters of the model grids are calculated analytically while rays are being traced. Because velocity and interface functions have second-order continuous partial derivatives, the geometrical shadow zones at the surface caused by scattering and focusing of ray paths can be prevented. After ray tracing, an equation consisting of matrix and vectors for inverse problem is obtained. We use singular value decomposition method with damped factor to solve the equation. A synthetic data set which consists of several in-line profiles is used to test the methods. The results show that the methods are robust. Compared with the two dimensional method, the 3-D inversion method can give the right position of interfaces and the velocity structure when the crustal model is complicated.

Prilimary result of temperature distribution and associated thermal stress in crust in Tianshui, China

The heat flow in crust and the thermal stress generated by the flow play a very important role in earthquake occurrence. Different crustal structure has different effect on heat distribution and associated thermal stress. In all of typical seismogenic crustal structure models, including the bulge of Moho surface, the deep-large fault in the mid-lower crust, low-velocity and high-conductive layer in the middle crust, and the typical crustal structure in mid-upper crust, the thermal stress produced by deep heat disturbance may move up to the mid-upper crest. This leads to upper brittle part of crust break and hence, strong earthquakes. This result is constructive in enhancing our understanding of the role of deep fieat flow in curst in development of earthquake and its generation, as well as the generation mechanism of the shallow flowing fluid.

Anomalies of geophysical field in deep crustand earthquakes in Central-South Shanxi

Comprehensive studies on the crustal structures, the anomalies of geophysical field and the seismicity in Central-South Shanxi are conducted. The research results show that there are deep tectonic background for occurring moderate earthquakes in Lingshi-Jiexiu region between Linfen and Taiyuan. From now on, the region should be attached closely.

Study on the characteristics of crust－mantle transition zone in Western Yunnan Province

In this paper the fine structure of crust mantle transition zone in Western Yunnan Province is analysed and discussed based on the reflection phases from Moho discontinuity in Project Western Yunnan 86～87. It shows that in two points: in the north 27.64 km from shot point Jinggu and south 58.74 km from shot point Zhiti, there are transition zones of group of thin layers with inverse velocity.These two reflection points are both situated in the lower velocity anomaly zone in the top of upper mantle. The crust of this region is more seismicity. Maybe the unusual structure of this transition zone is related with the characteristics of this region. This paper discusses the possible geological interpretation model for this transition zone, and also makes suggestion about its application in earthquake prediction.

Progress in the Study of Deep Profiles of Tibet and the Himalayas (INDEPTH)

This paper introduces 8 major discoveries and new understandings with regard to the deep structure and tectonics of the Himalayas and Tibetan Plateau obtained in Project INDEPTH, They are mainly as follows. (1) The upper crust, lower crust and mantle lithosphere beneath the blocks of the plateau form a 'sandwich' structure with a relatively rigid-brittle upper crust, a visco-plastic lower crust and a relatively rigid-ductile mantle lithosphere. This structure is completely different from that of monotonous, cold and more rigid oceanic plates. (2) In the process of north-directed collision-compression of the Indian subcontinent, the upper crust was attached to the foreland in the form of a gigantic foreland accretionary wedge. The interior of the accretionary wedge thickened in such tectonic manners as large-scale thrusting, backthrusting and folding, and magmatic masses and partially molten masses participated in the crustal thickening. Between the upper crust and lower crust lies a large detachment (e.g. the Main Himalayan Thrust in southern Tibet, 5-8 km thick) or a very thick shear-schistose zone (e.g. the Main Qiangtang Thrust-MQT in northern Tibet, up to 20 km thick), which causes the decoupling of the upper crust and lower crust and separation of tectonic activities. (3) During the collision-compression, the Indian mantle lithosphere was delaminated into two layers from where the crust thickened most rapidly (beneath the High Himalayas). The upper layer extends to 34.5°N and the lower layer to 33.5°. They have been underthrust to depths of 250-300 km into the asthenosphere. Meanwhile the Asian lithosphere (possibly the Qaidam terrane) has also been subducted southwards. Very thick mantle lithosphere does not exist beneath the plateau. (4) The oceanic lithosphere, in light of its lithology and dynamic behaviour, might be close to those of the continental lithosphere and its front might enter the asthenosphere before the continental lithosphere. (5) A 150-200 km deep low-velocity body below 35°N and a wide low-velocity zone below the area between 33.5° and 35°N dip north at very steep angles. Volcanism took place frequently in northern Tibet and anisotropy variations are prominent at depths, which might indicate a zone of large-scale eastward transfer of deep-seated materials.

Indirect-approach method for specified-end points seismic ray tracing in three dimen-sional inhomogeneous media

Generally speaking, the factors of both medium and tectonic that give rise to heterogeneity of the earth crust and mantle structures should be taken into account simultaneously in three dimensional seismic ray tracing. In this paper, the three dimensional structure models are constructed with the model similar to generation system in computer aid design and manufacturing (CAD/CAM). Based on the algorithm proposed by Cerveny et al . for complete seismic ray tracing in complex three dimensional structures, a new technique called the indirect approach method for two point seismic ray tracing in three dimensional laterally heterogeneous media has been put forward, and its numerical computing examples were given.

Gravity observation at continental borderlands (Russia, Primorie, Cape Shults)

The paper is focused on different kinds of gravity results obtained in Shults Cape Observatory for 2010 -2015. Gravity observation is interpreted together with GPS observation data which was obtained from 2012 to 2015 at the same station, The station is situated on Gamov peninsular （42.58° N, 131.15° E, Russia） at the coast of Japan Sea, This region constitutes the eastern boundary of Eurasia. This major continental tectonic feature is associated with a seismic activity, high heat flow and anomalous thickness of earth＇s crust. The goal of the observation was the investigation of gravity variation with time and seismicity situation monitoring. Gravity observation was developed at special basement by absolute gravimeter （GABL type） and by spring gravimeter （SCINREX CG-5and gPhone type）. Tidal models were tested by results of observation with spring gravimeters. Reduction task was solved, as the experimental data received from different points of Shults Cape Observatory was used. Applied reduction coefficient is 203.3 12Gal m l, and agrees with theoretical calculation. Next goal was studying structure of earth＇s crust by means of gravity models. Gravity anomaly varied from 30 mGal to 46 mGal, which also depend on difference reference system, Experimental results were used for testing of the structure of continental boundary, which also depends on the sea bottom flexion. Thickness of elastic layer was estimated from 12 km to 18 km by using different models.