Abstract:

Using the cataclastic analysis method, this paper tries to make an analysis on the focal mechanism data of 486 small earthquakes that occurred at the epicenter of the Wenchuan earthquake and its surrounding areas in more than three years before the 2008 Wenchuan earthquake. The result shows that obvious stress change occurred at the seismic source and its surrounding areas around June 2007 before the Wenchuan earthquake, manifested in two high numerical value areas of abnormal stress state. Meanwhile, the formation process of the above areas was accompanied by the drop of stress level of the Longmenshan central fault. The ultimate strong earthquake occurred on the stress gradient belt between the high stress area and the low stress area. The evolution process of stress level before the Wenchuan earthquake indicates that earthquake nucleation phenomenon turned up before the strong earthquake. One result can be inferred that there was an abnormal process of accelerated movement of the whole Bayankala block before the Wenchuan strong earthquake.

Key words: focal mechanism, the cataclastic analysis method, Wenchuan strong earthquake, stress state

摘要：

我们收集了紫坪铺水库区域台网记录到的2004年8月16日至2008年5月12日汶川地震前发生在震中区及附近区域的486个小震震源机制资料,运用碎裂分析法对这些震源机制数据进行分析,获得了汶川地震前3年多时间内震源区应力水平随时间的演化过程。结果显示,强震发生前震源及附近区域约在2007年6月发生了应力突变,出现了2个应力异常高值区,其形成过程同时伴随着龙门山中央断裂的应力水平降低,最终强震发生在高应力区和低应力区之间的应力梯度带上。汶川地震前应力状态的演化过程,类似于实验研究中出现的非均匀断层失稳前的地震成核现象,符合地震成核的宏观表现。巴颜喀拉块体的南边界-鲜水河断裂的短基线测量也显示出,在同一时间段鲜水河断裂南段由左旋滑动转为异常的右旋错动,反映了汶川强震前整个巴颜喀拉块体有向东加速运动的异常过程。

关键词: 震源机制, 碎裂分析方法, 汶川强震, 震前应力异常