2010年玉树7.1级地震震源区横波分裂的变化特征

本文对2010年玉树M_s7.1级地震序列的横波分裂的时空变化现象进行了研究.通过在横波窗内S波质点运动图的分析,从位于甘孜-玉树断裂带台站的地震记录中提取了S波分裂的快波偏振方向和快慢波之间的时间延迟.断裂带上3个台站的S波分裂的结果显示了NEE向快波偏振方向,与区域内最大主压应力方向基本一致.观测到了快、慢波到时延迟的趋势性变化,空间上,位于震源区甘孜-玉树断裂带的北支断裂上L6304和YUS台的到时延迟大于位于南支断裂上的L6303的结果,同时YUS台的到时延迟又大于L6304台的;时间上,YUS台的到时延迟在主震后迅速增大,持续一段时间后又减小.横波分裂现象主要由台站下方岩层中随应力分布排列的微裂隙控制,本文结果揭示了主震和余震过程中区域应力场的变化,玉树地震主震区的应力在主震后得到了释放,地壳应力向东南转移,导致了YUS台的到时延迟大于L6304台的;玉树地震的发震断裂是甘孜-玉树断裂的北支断裂,而南支断裂对其影响较小;YUS台附近的地壳应力在主震后得到了增强,随着余震活动导致的应力释放,后期应力减小.

Variations of shear wave splitting in the 2010 Yushu Ms7.1 earthquake region

In this paper, variation of shear wave splitting in the 2010 Yushu M_s7.1 earthquake sequence was 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 Gare-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 spatial distribution, in the focal region, the delay times at stations L6304 and YUS in the north branch fault of the Gare-Yushu fault belt are greater than that of station L6303 in the south branch fault, meanwhile, the delay time at station YUS is larger than L6304. In the time domain, after the main shock, the delay times at station YUS increased rapidly, and then reduced 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 during the main-shock and the aftershocks. The stress in the main-shock region was released by the main-shock, the crustal stresses transferred to the southeastern part of the zone, and caused the delay time at station YUS is being greater than L6304. The north branch fault of the Gare-Yushu fault belt is the causative fault of the 2010 Yushu M_s7.1 earthquake, whereas the south branch fault has little influence. The stress at station YUS increased after the main-shock, and with the stress release caused by the aftershocks, the stress reduced in the late stage.