Variations of Shear Wave Splitting in the 2010 Yushu Ms7.1 Earthquake Region

Abstract

AbstractIn this paper, variations of shear wave splitting in the 2010 Yushu Ms7.1 earthquake sequence were studied. By analyzing S‐wave particle motion of local events in the shear wave window, the polarization directions of fast shear waves and the delay times between fast and slow shear waves were derived from seismic recordings at 3 stations near the Garzê‐Yushu fault belt. The shear wave splitting results of 3 stations in the area of the fault belt reveal the NEE polarization directions of fast shear waves, which are consistent with the direction of regional principal compressive stress. Systematic changes of the time delays between two split shear waves were also observed. As for the spatial distribution, in focal region, the delay times at stations L6304 and YUS in the north branch fault of the Garzê‐Yushu fault belt are more than that of station L6303 in the south branch fault, meanwhile, the delay time at station YUS is more than that of station L6304. As for temporal changes in shear wave delay times, after the main shock, the delay times at station YUS increased rapidly, and decreased after a period of time. Seismic shear wave splitting is caused mostly by stress‐aligned microcracks in rock below the stations. The results demonstrate the changes of local stress field in the course of the main‐shock and aftershocks. After the main shock, the stress in the main‐shock region was released by the main‐shock, the crustal stress transferred to the southeastern part of the Garzê‐Yushu fault zone, and caused the delay time at station YUS to be greater than that of station L6304. The north branch fault of the Garzê‐Yushu fault belt is the causative fault of the 2010 Yushu Ms7.1 earthquake, whereas the south branch fault has a small impact on the earthquake. The stress at station YUS increased after the main‐shock, with the stress release caused by the aftershocks the stress reduced in the later stage.