汶川Ms8.0地震的前兆异常时空演化过程及其力学分析

摘要

为了研究2008年汶川Ms8.0地震前10年前兆异常的时空演化过程,采用地震破裂区的几何中心和余震区椭圆中心线为中心的两种方法计算前兆异常的震中距.结果表明:在汶川地震前2年左右前兆异常逐渐增多;近源区(震源2倍左右尺度)异常测项比大于25％,远源区中震源3～5倍尺度区域的异常测项比为17％～24％.前兆异常时空演化过程存在三个不同的变化阶段,即α阶段(包括α1和α2,震前700～3000 d)的异常主要分布在余震区以外的西南地区和西北地区,表现为远源区与近源区的前兆均存在向外扩展的特点;β阶段(震前300～700d)的异常分布在余震区西南部和北部地区,表现为大范围出现异常;γ阶段(包括γ1和γ2,震前300 d内)的异常分布范围较广,异常主要分布在余震区的西南部和东北部地区,表现为远源区的异常向震中收缩过程(γ1)和近源区的异常向外扩展的过程(γ2).地震孕育过程的实验研究和力学分析表明,地震孕育过程中前兆异常的三阶段特征可能是受孕震体的控制,是孕震过程中的一种表现形式,是一个地震孕育过程中的普遍性特点,对地震预测具有一定指导作用.%In order to study the spatio-temporal evolution of the precursory anomalies 10 years before the 2008 Wenchuan Ms8.0 earthquake,the epicentral distance of the precursory anomalies is calculated by using the geometric center of the rupture region and the elliptical centerline of the aftershock region.The result shows that precursor anomalies gradually increase about 2 years before the Wenchuan earthquake.The ratio of abnormal items is greater than 25％ in the near the source area (about 2 times the source scale).The ratio of abnormal items is 17～24％ (about 3～5 times the source scale) in the remote area.There are three different stages of spatio-temporal evolution of precursory anomalies:during the α stage (including α1 and α2,700 to 3000 days before the main earthquake) the anomalies are mainly distributed in the southwest and the northwest area of the Wenchuan aftershocks area.It is shown that the precursors of the far source region and the near source area have the characteristics of outward expansion.During the 3 stage (300 to 700 days before the main earthquake),the anomalies are distributed in the southwest and northern regions of the aftershock region,showing a large range of anomalies.During the γ stage (Including γ1 and γ2,300 days before the main earthquake),the range of anomaly distribution is wide,and the anomalies are distributed in the Southwest and Northeast of the aftershock area.The anomalies contract to the epicenter (γ1) in the far source region and extend outward (γ2) in the near source region.Through the experimental study and mechanical analysis of earthquake preparation process,the results indicate that the threestage characteristics of precursory anomalies in the process of earthquake preparation may be controlled by the seismogenic body,which is a form of expression in the process of earthquake preparation and a universal feature during the earthquake preparation process.