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Focal Mechanism Solution Analysis of M_L≥4.0 Earthquakes in the 2010 Yushu Earthquake Sequence

SHI Xue-lu;YAO Jia-jun;ZHANG Bo;Lanzhou Institute of Seismology,CEA;Geophysical Exploration Center,China Earthquake Administration;  
In this study,we inverse 19 focal mechanism solutions of the foreshock,mainshock and aftershock events in the MS7.1Yushu earthquake sequence by the method of CAP.The results show that the 19 focal mechanism solutions are mainly strike-slip types,and the fore shock is similar to the main shock which reflects that the intimate relationship between the two events.The focal depth concentrates on about 7~12km,and two aftershocks with the most shallow(4.5km)and deepest(34km)facal depth significant thrusting and exhibited obviously boundary feature.The Epicenters of 19 events distrbute in the Yushu-Longbao fracture which is the northern branch of Ganzi-Yushu fault and it has been proven that the Yushu-Longbao fracture is the seismogenic structure of Yushu earthquake.Following the SE-NW direction along Yushu-Longbao fault pull a cross-section to observe the change of focal depth along the profile,we can see that the focal depth of Northwest section of Yushu-Longbao fault is greater than the southeast section,and this part is the main part in the late aftershock activity,so that the aftershocks have extended deep crust in the late seismic activity and the fracture-accumulated strain can be further released.The dominant distribution of P axis azimuth is 220°~230°while T axis azimuth is 310°~320°,and the two distributions are vertical to each other.It is found that the regional stress field is relatively uniform by analyzing the data of the focal mechanism solutions of historical earthquakes and GPS,which shows that the M_S7.1 Yushu earthquake is the normal release of the energy in the Earth.
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