Abstract: The 2008 Wenchuan earthquake started at the intersection of the south and central segments of the Longmenshan Fault. Its aftershocks extended northward to the whole central and north segments of the Longmenshan Fault. The seismicity of the aftershocks along the central and north segments of Longmenshan Fault zone decreased very quickly. Within one year after the Wenchuan earthquake,the number of aftershocks per month with magnitude >3.5 is greater than 10,indicating that there was a strong tectonic stress adjustment around the epicenter region during this period. However,after April of 2009,the number of aftershocks per month with magnitude >3.5 is less than 10,indicating that the tectonic stress adjustment became more stable. Even after the Ya'an Lushan earthquake occurring in the south segment of the Longmenshan Fault in April,2013,the seismicity in the central and north segments does not increase obviously. In the present study,we firstly calculated focal mechanisms of micro-earthquakes from about one year of seismic observations recorded by 26 stations located around the main-shock epicenter. We then made an analysis on the tectonic stress changes before and after the main shock,based on our calculated focal mechanism solutions and previous results of focal mechanisms and in-situ stress measurements. The analysis results show that the maximum principal stress direction close to the epicenter changes from near E-W before the main shock to near N-S after the main shock. The focal mechanisms indicate that the Dujiangyan-Yingxiu-Gengda Fault,separating the middle segment with the south segment of the Longmenshan Fault zone,and the region to its north have a trend of northeastward movement. The northeastward movement may be one of the reasons triggering the Ya'an Lushan earthquake which occurred in the middle of the south segment of the Longmenshan Fault zone in 2013.

Key words: Wenchuan earthquake, micro-earthquake, focal mechanism, active fault, tectonic stress

摘要： 2008年汶川地震自龙门山断裂带中段和南段交界处开始，其余震向NE贯穿了整个龙门山断裂带中段和北段。汶川地震余震发生频次快速降低。主震发生之后的1年时间内，每月发生 >3.5级的余震超过10个，表明该阶段区域构造应力场发生了比较剧烈的调整。而2009年4月之后每月 >3.5级余震发生次数则少于10个，表明该区应力调整逐渐趋于平稳。即使是龙门山断裂带南段于2013年4月发生了雅安芦山地震后，龙门山断裂带中段和北段的汶川地震余震数量并没有明显增多。文中首先利用汶川地震震源区（映秀及周边）26个地震台记录到的约1年的余震观测数据计算了微地震震源机制解。然后综合参考前人获取的汶川地震前、后震源机制解和原地应力测量结果，对汶川地震震源区及其附近构造应力场在汶川地震前后的变化进行了分析。分析结果显示，汶川地震震源区水平最大主应力方向发生了明显变化，由汶川地震震前的近EW向变为震后的近SN向。余震震源机制结果显示，龙门山断裂带中、南段交界NWW走向的都江堰-映秀-耿达断裂带周边及其NE区域都存在向NE运动的趋势。这种NE向运动可能是诱发龙门山断裂带南段中部发生2013芦山地震的原因之一。

关键词: 汶川地震, 微地震, 震源机制, 活动构造, 构造应力