2017年西藏米林M_S6.9地震余震序列重定位和b值时空分布特征

Relocation of the aftershock sequence of Milin M_S6.9 earthquake in 2007 and spatio-temporal variation characteristics of b-value

本文采用双差定位方法对2017年西藏米林M_S6.9地震的余震序列进行重定位,获得了较准确的余震空间分布。在此基础上,计算了米林地震震源区的b值空间分布,并结合前人资料研究了米林地震前后的b值变化。重定位结果显示:米林地震余震序列呈NW-SE向分布,主要分布在雅鲁藏布江大拐弯缝合带和比鲁构造岩片的北东边界,震源深度集中于3—20 km范围内。b值时间分布显示:米林地震震前震源区的b值降低,表明震前震源区存在较强的应力积累;地震后震源区的b值呈现跳跃式上升,反映出震源区应力释放,同时随时间的推移,b值逐渐趋于稳定。米林地震震源区的b值在0.52—1.35范围内变化,15 km深度以上b值在东、西方向上存在差异,15 km以下在东北、南西方向上存在差异,由此推测不同的b值分布与地下的结构特征有关,浅层的b值变化与震源区的破裂程度相关,深部变化则反映了不同构造单元的岩性差异。

The aftershock sequence of the Milin MS6.9 earthquake in Tibet in 2017 are relocated by double-difference algorithm in this paper. Based on the relocated results,the spatial distribution of b-value in the source region of Milin earthquake is obtained. Combining historical b-value data from previous researches,the temporal distribution of b-value in the studied area is also obtained. The relocation results show that the aftershock sequence of the Milin earthquake are distributed in NE-SE direction along the NE boundary of Indus-Yarlung Tsangpo big-turned suture and Bilu tectonic slice,most aftershocks are concentrated in 3-20 km depth. The temporal distribution of b-value shows before Milin earthquake b-value is lower than that in the year of 2008,indicating that the stress accumulation in the studied area is strong before Milin earthquake. After Milin earthquake,there was a step-rise of b-value,showing the releases of stress,and b-value tended to stable. b-value ranges in 0.52-1.35 in the source region of Milin earthquake,the b-value variation above the depth of 15 km is distributed along the east-west direction,and below 15 km along the NE-SW direction. Therefore,it is deduced that different bvalue distribution represents different structure characteristics,i.e.,the b-value variation of the shallow area is related to the degree of rupture in the source region,and the b-value variation of deep area reflects the lithology difference of different tectonic units.