Characteristics Of Focal Mechanisms And Stress Field Of The Sichuan-yunnan Rhombic Block And Its Adjacent Regions

The mechanism of tectonic earthquake is severely related to the crustal stress，thefaulting mechanism, and crustal rheology, etc. And the seismologic process isregarded as the results of the reciprocity between the crustal movements ofcircumjacent and tectonic stress field. During the process of crustal motion, localtectonic stress probably deviates from continental stress filed, due to the existence oflarge fault zones, density and intensity changes of faults and crustal thicknessmutation. The Sichuan-Yunnan rhombic block is located in the southeastern margin ofthe Tibetan Plateau. On the east side, the Sichuan-Yunnan rhombic block adjoins tostable South China.While,on the west side, the Sichuan-Yunnan rhombic block isaffected by the Tibet Plateau material eastward escape generated by the collisionbetween Indian block and Eurasia block, constantly northward pushing of Indianblock,as well as Assam vertex wedging action. Therefore, tectonic framework of theSichuan-Yunnan rhombic block is complex with huge active faults and lots ofdevasting earthquake.Special geological location,as well as strong crustaldeformation and fault movements,result in the frequently occurrences of moderateand large earthquakes and the potential risk of future strong earthquakes.Consequently, researches and discussion on the tectonic stress filed, the way of faultdeformation and movements of the blocks are becoming the focus topics in recentyears. The P axis, T axis and N axis obtained from the focal mechanism reflects thechanges of tectonic strss before and after earthquake of the seismic source, and theycould be regarded as reflection of tectonic stress of the focus zone. Tectonic stressdirections in varied layers could be estimated from P axis, T axis in the correspondinglayers.In this paper, based on the waveforms from86permanent broadband regionalnetwork stations and20temporary stations in the ANH-ZMH faults, focal mechanismsolutions together with the focal depths of75M3.5earthquakes occurred in theSichuan-Yunnan rhombic block and its adjacent region from Aug.1st,2007toApri.21th,2013was obtained by applying CAP method. Combining the results withthe historical focal mechanism solutions before July.2007from Harvard CMT, thetectonic stress field is estimated by FMSI method. Then, based on the existingresearches of faulting, sub-block motions, crustal magnetotelluric measurement andcrustal rheology, the characteristics of the spatial distribution of the focal mechanismsand the tectonic stress is analyzed with higher resolution. Main conclusions are drawnas follows.(1) Spatial distribution of focal mechanisms shows existing local features, which isconsistant with the tectonic background. The reverse focal mechanisms, thenormal mechanisms and the strike mechanisms are located in the Sichuan basinadjacent regions, the Northwest Sichuan block and Dianzhong block accordingly.By analysising these features based on researches of geogoly, deformationmeasurement and geodynamics, we get a conclusion that recently faulting and earthquake mechanism of the Sichuan-Yunnan rhombic block and its adjacentregions is controlled by different faulting and interaction between sub-blocks.(2) Our high spatial resolution of the stress field image reveals that the easternboundaries of Dianzhong block, ANH-ZMH-XJ faults, could be regarded as thestress boundary of large-scale continent tectonic stress in the Sichuan-Yunnanrhombic block. On the eastern side of this boundary, the tectonic stress source ismainly governed by the relative motion between the Philippine Plate and theEurasian Plate. While, on the western region of the eastern boundary, the stressfield may be associated with the interaction between India block and Tibetanblock.(3) Apart from being a tectonic stress boundary, the ANH-ZMH-XJ faults showdifferent tectonic stress characteristics on different section. The maximumprincipal stress axis of ANH-ZMH faults tends steep，however, the maximumprincipal stress axis of XJ faults shows more horizontal.This phenomenon isprobably due to the impacts on local tectonic stress which is caused by changes inthe media during fault movements.Also changes of the maximum principal stressdirection indicate that the ZMH faults and the north of HH faults probably is stressconverted regions.(4) LJ-XJH faults is the internal tectonic stress boundry of the Sichuan-Yunnanrhombic block，and tectonic stress fileds on the central and southern section alongthis fault are different.We hypothesize that this different characteristics is thereflection of the shielding and absorbing actions by LJ-XJH faults towards theprocess of the Tibetan Plateau material escape. The impact of the Tibet Plateaumaterial escape is separated into two parts by the LJ-XJH faults.One of these twoparts spreads to Dianzhong block, and forces the Dianzhong block to movetorwards the southeastern direction. Another part flows to the Baoshansub-block.The analysises of western Yunnan show that the effect of the latter partweakens in Tengchong-Longling region and is less significant than the draggingby Burma plate.(5) Focal depths of the events distributed in the Sichuan-Yunan rhombic block and itsadjacent regions are lower than15km, mainly distributed in the depth of5to15km, suggesting that brittle seismic layer beneath the Sichuan-Yunnan rhombicblock is in the middle and upper crust.