2010年玉树地震序列M_L≥4.0事件震源机制解特征分析

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

采用CAP方法反演2010年玉树7.1级地震序列前震、主震及余震19个M_L≥4.0事件的震源机制解,19个结果以走滑类型为主,前震、主震的震源机制解十分接近,反映出前震、主震之间密切的联系;震源深度集中在7~12km,震源最浅(4.5km)与最深(34km)的两个余震事件具有明显的逆冲性质,表现出明显的边界特征;19个事件的震中分布在甘孜—玉树断裂北支玉树—隆宝断裂上,目前已经证明该断裂即为玉树地震的发震构造。自SE-NW沿玉树—隆宝断裂走向拉一剖面,观察震源深度沿剖面的变化情况,可看出玉树—隆宝断裂西北段震源深度要大于东南段,该段主要是余震活动的中后期,因此在地震活动的中后期,余震向地壳深部扩展,断裂累积的应变能得到更进一步的释放;P轴方位角优势分布集中在220°~230°,T轴方位优势分布集中在310°~320°,两个优势分布互相垂直性与单个事件的沙滩球应力轴一样,说明玉树地震的震源机制解类型较为简单;玉树周边地区应力场分布比较均匀,并不像汶川周边地区那么复杂,本次玉树地震为巴颜喀拉地块与羌塘块体边界处甘孜—玉树断裂应变能量的正常释放。

In this study, we inverse 19 focal mechanism solutions of the foreshock, mainshock and aftershock events in the Ms7.1 Yushu 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~ 12 km,and two aftershocks with the most shallow { 4.5km） and deepest （34 kin） facal depth significant thrusting and exhibited obviously boundary fea- ture.The Epicenters of 19 events distrbute in the Yushu--I.ongbao fracture which is the northern branch of Ganzi--Yushu fault and it has been proven that the Yushu--I.ongbao fracture is the seismogenic structure of Yushu earthquake. Following the SE-NW direction along Yushu--I.ong- bao 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--I.ongbao 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 earth- quakes and GPS, which shows that the Ms7.1 Yushu earthquake is the normal release of the en- ergy in the Earth.