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Crustal Shear-wave Splitting And Earthquake Relocation In The Tien Shan Region

Posted on:2017-02-24Degree:MasterType:Thesis
Country:ChinaCandidate:Z W BaoFull Text:PDF
GTID:2180330488479388Subject:Geophysics
Abstract/Summary:PDF Full Text Request
There are widespread stress-aligned grain-boundary cracks in the crust, these cracks lead to splitting phenomenon where share-wave travels through anistropic media. Using SAM method(Systematic Analysis Method of Shear-wave Splitting) could acculate the directions of fast wave and time delay of slow wave, these two parameters could be used to discusse the directions of cracks、the compressive stress、the degree of anisotropy and regional structural complexity.The Tien Shan tectonic belt is located in northwest China, which is the typical lithosphere intercontinental shortening orogenic belt. The belt is under the north-south squeezing action from Tarim Basin and Junggar Basin, the compression direction is NS, but local stress is special. The structure of the study area is complex, which distributes some EW、NEE strtching faults in I zone; NE、NWW and NW strtching faults in zone II.In this study, earthquake which happened in the Tien Shan tectonic belt regions from January 2009 to December 2014, were relocated with the double difference algorithm. The results of relocation show that the earthquakes in the Tien Shan tectonic belt are almost concentrated in fault zone, and hypocenters distribute along the faults, and the main depth of small-medium earthquake hypocenter is above 20 km. In general, the seismic activity of the Tien Shan tectonic belt is strong, which provide favorable conditions for regional crustal anisotropy analysis.In this study, the parameters of anisotropic characteristics of 40 stations are computed with the systemic analysis method of shear-wave splitting based on the seismic data during January 2009 to December 2014, recorded by Xinjiang Regional Digital Seismic Network. The spatial characteristics of shear-wave splitting parameters demonstrate obvious zoning of anisotropic in the upper crust; anisotropic characteristics are related to local structures, crustal medium deformation and stress distribution. There are two polarizations of fast shear-waves in the Tien Shan tectonic belt, the first dominant polarization is generally parallel to the fault strike nearby the stations and the strike of the Tien Shan tectonic belt, which is related to the fracture of faults and stress; the second predominant polarization reflects directly effect of compressive stress. The shear-wave splitting parameters of the east bending part of North Tien Shan piedmont fault and local part of South Tien Shan piedmont fault, which are closely related to the two-way extrusion from Tarim Basin and Junggar Basin. The acute changes of predominant polarization indicate the local inhomogeneous deformation. The west side of Tarim Basin shows two directions, the one is NNE which is related to the northward compression of Pamirs caused by India-Eurasia collision, and the other one indicates the direction of the compressive stress of Tarim Basin is NW. The NE and NW polarizations of fast shear-wave in Junggar Basin are related to the faults. The time delay of shear-waves in Tien Shan tectonic belt is generally below the dime delay in Tarim Basin and Junggar Basin, and this result further demonstrates intense local deformation in Tien Shan tectonic belt.In this study, the double difference algorithm and shear-wave splitting analysis are separately completed, which discuss the regional seismic activity and anisotropy characteriscs. The results show that on the one hand, the seismic activity is strong and the spatial distributions of hypocenters concentrated along the faults; on the other hand, the regional principal compressive stress and the fault strike both influence the polarization direction of the fast shear-waves.
Keywords/Search Tags:Tien Shan tectonic belt, Shear-wave splitting, Polarization of fast shear-wave, Time delay of slow shear-wave, Stress, Crust, Fault, Double difference algorithm
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