| The mechanism of tectonic earthquake formation is an important issue inseismology. A mass of studies have been carried out, and the conclusion may bedrawn as that the markedly heterogeneous of the crustal structure and medium maycause the stress concentration and strain accumulated which may be the places oflarge earthquakes occurrence (epicenter areas). The seismogenic process is the resultof reciprocity between the crustal movement of circumjacent regions and inelasticdeformation of epicenter areas, under the control of regional tectonic stress field. Andit is the result and representation of the crustal movement. So, studies on the regionaltectonic stress fields, crustal structures and material types are important contents ofthe research on the seismogenic process.In this research, we try to get the underground medium information by studyingthe physical characteristics of the lithosphere and upper mantle, such as the regionalP-wave velocity structure, spatial distribution of S-wave attenuation, spatialdistribution of Qc value of coda wave. And we try to get the underground dynamicsinformation by studying the regional stress field which is partitioned in horizontalorientation and detached at vertical direction, as the crustal S-wave splitting, theseismic mechanism of middle and large earthquakes, the strain field of GPS, theanisotropy of Pn-wave, the upper mantle SKS splitting, and so on. Then, with thedynamic models, the coupling extent of different spheres has been analyzed, and thecognition of lithosphere structure, physical characteristics of medium, stress and strainfield has been increased, and the dynamic environment of seismogenic progress hasbeen made certain further.We have selected about 60,000 arrival times recorded by 205 regional stations,with the seismic tomography theory and three dimensional ray tracing method, todetermine a detailed three-dimensional (3-D) P wave velocity structure of thelithosphere in southwest China. Then we figure out the medium structure and tectoniccharacteristics of different depth which are reflected by the 3-D P wave velocityimage in this area, associating with the previous geological features and geophysical data. The results suggest that, 1) low-velocity layers exist far and wide in the middleand deep crust under the large fault zones around the Sichuan-Yunnan block, whichcan be taken as the decoupling layer adjusting the faults and blocks movement; 2)some deep structures related with paleo-block boundaries, and the trends andextending depths of primary active faults, can be distinguished from the P wavevelocity image; 3) alpine mountainous regions present as negative anomalies, andsome negative anomalies even extending to the deep crust or the upper mantle, whichreflects the stong uplifting of the neotectonic orogens and the concomitant gravityisostasy.Using the P wave travel times recorded at Yunnan and Sichuan local networksdate from April, 2000 to March, 2006, we implemented a joint inversion ofearthquake location and velocity structure, and got the new locations of 6642 smalland medium earthquakes at Sichuan-Yunnan region. The results suggest, 1) there is anobvious relationship between the focal depth and the magnitude: the bigger theearthquake, the deeper the focal depth, but all the focal depths are shallower than25km; 2) there are few earthquakes at depth larger than 15km at regions west ofRuili-Longling, Lijiang-Xiaojin and Longmenshan faults; 3) small and mediumearthquakes have the similar seismogenic background as large earthquakes. At thetransition zone of high velocity and low velocity, most of the earthquakes happened atthe higher velocity side, and usually there is a low velocity zone below whereearthquakes happened.We have selected 27,530 ML amplitude records from 5,897 events recorded by149 stations, as reported in the Annual Bulletin of Chinese Earthquakes (ABCE) andregional seismic network of Yunnan province and Sichuan province, and have usedtomographic imaging to estimate the lateral variations of the quality factor Q0 (Q at1Hz) beneath the crust of Sichuan-Yunnan and its adjacent regions. Estimated Q0values vary from 200 to 600 with an average of 400. Q0 value is consistent withtectonic and topographic structure. Q0 is low in the active tectonic regions with manyfaults, such as the Haiyuan-Qilian, Fenhe-Weihe, East Kunlun zones, Western Sichuan-Northwestern Yunnan-Baoshan area and Joint of Yunnan and Guizhou-Kunming -Simao area, and high in the stable regions, such as Sichuan Basin, Markem block,Western Guangxi-Eastern Yunnan block, Ordos Craton and Qinling-Dabie area.The coda-wave attenuation quality factor Qc of the areas in Yunnan Provincewere estimated using the single-scattering attenuation model of Sato from 5668 localseimic events recorded by a regional network of 22 digital stations from the latterhalf of 1999 to 2003. The used events were in epicentrai distances up to 50 km.According to the variation of Q0 and, we classified the quality factor Qc to two types.The classified results showed that there was regional distributing characteristic inmedium structure of the areas in Yunnan Province, and this characteristic could beelementarily interpreted by geological structures, seismic activity and heat flow.Generally speaking, the quality factor Qc of Dianzhong Block and its boundary areaare notably less than those of other regions, as the intensity of tectonic activity ofthese areas. According to seismic activity, there were large earthquakes in the areaswith lower Qc, and there are few or only some little earthquakes at the areas withhigher Qc. Besides, there is negative relation between quality factor Qc and heat flowof the areas in Yunnan Province, namely higher heat flow corresponding to lower Qcand lower heat flow corresponding to higher Qc.We collect 6 361 waveform data to calculate the shear wave splitting parametersfrom a regional seismic network of 22 digital stations in Yunnan and its adjacent areafrom July 1999 to June 2005. By using the cross-correlation method, 64 splittingevents of 16 stations are processed. We also collect the splitting results of 8earthquake sequences to present the characteristics of shear wave splitting in Yunnanand its adjacent areas. The orientations of maximum principal compressive stress of 3sub-regions in this area are derived from the CMT focal mechanism solutions of 43moderate-strong earthquakes provided by Harvard University by the P axisazimuth-averaging method. The principal strain rate at each observatory is deducedfrom the observations of Crustal Movement Observation Network of China during theperiod from 1999 to 2004. In addition, the data of Pn anisotropy and SKS splitting ofYunnan and its adjacent areas are also collected. We have discovered from this studythat the continental lithosphere, as a main seismogenic environment for strong earthquake, can be divided into blocks laterally; the mechanical behavior oflithosphere varies with depth and can be divided into different layers in the verticalorientation; the information of crustal deformation obtained from GPS might beaffected by the type of blocks, since there are different types of active blocks inYunnan and its adjacent areas; the shear wave splitting in this region might be affectedmainly by the upper crust or even the surface tectonics.Considering previous work of different people, and the new results of differentresearch, as active faulting, three dimensional velocity structure, geodesy, seismicity,we give a new three-dimensional dynamic model of Sichuan-Yunnan and its adjacentregions. From GPS data, we define the velocity of the finite element model's edge.From SKS's anisotropy data, we determine the flow pattern of the upper mantle'smaterial. We give out the crustal movement pattern and the stress field of differentlayers in Sichuan-Yunnan and its adjacent regions, and then we make certain how theboundary's condition and the force at the upper mantle effect in the research ofkinematics and dynamics in Sichuan-Yunnan and its adjacent regions. The resultsshow, 1) the phenomena that the material of Sichuan-Yunnan and its adjacent areasflows clockwise around Himalaya's east tectonic node is related to the specialboundary condition, and the stress field (seismic mechanism) in middle and uppercrust is also related to the special boundary condition; 2) there may exist drag force atthe upper mantle effect on the crust, and the orientation may be west at south of26°north latitude; 3) when we changed the lower crust from elastic material toviscoelastic material, the stress field of different crustal layers decoupling come forthin the Dianzhong block and its neighbor area (100.9°-103.4°E, 22.9°-27.6°N),but this phenomena can not be found in other areas. This shows that the differencebetween plane division in vertical stress field may be mainly caused by the flow traitdifference in landscape orientation of the lower crust.With the upwards systemic studies, we can get a whole cognition as that, themost important dynamic environment of seismogenic progress in Sichuan-Yunnan andits adjacent regions is the lithosphere asymmetry at the structure, physicalcharacteristics, dynamical operation, and so on, especially the flow trait of lower crust and the drag force of upper mantle acting on the crust.
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