龙门山断裂周边区域在汶川Ms8.0地震和芦山Ms7.0级地震前的地壳形变特征对比研究

STUDY ON CRUSTAL DEFORMATION FIELD BEFORE THE “4.20” Ms7 EARTHQUAKE IN LUSHAN,SICHUAN

基于龙门山断裂带周边多年的GPS观测数据,分析了"5.12"汶川Ms8.0地震和"4.20"芦山7.0级强烈地震前龙门山断裂带周边的地壳形变特征。结果表明:1)两次强震都发生在挤压缩短的典型应变率"低值区"断层段;2)由于汶川Ms8.0地震对周边区域震后地壳变形的调整作用,龙门山断裂带南段的水平应变率明显增大,但对比周边区域,芦山Ms7.0地震发生位置为一个相对的"低值区",其挤压缩短的典型应变率为30 nanostrain/a,而其南侧和北侧的挤压缩短的典型应变率量值至少在60 nanostrain/a以上。其原因可能是即将发生大地震的孕震区域,其应力已处于岩石破裂的临界状态,岩石的弹性变形积累也已趋于极限状态,因此其地壳形变的增量或应变率将趋缓或保持低值。

The characteristics of crustal deformation in the area around Longmenshan fault zone before Lushan M7.0 earthquake and Wenchuan Ms8.0 earthquake were analyzed with the data from GPS stations located in the Longmenshan fault zone.The initial result indicates that:(1) Both of the two seimic events occured the fault segment with low strain rate where was compressed and shortened;(2) Due to adjustment of crustal deformation after Wenchuan Ms8.0 earthquake,the horizontal strain rate in the southern segment of Longmenshan fault zone increased obviously.However,the location where Lushan M7.0 earthquake occured still was a low value area with a strain rate of30 nanostrain /a,while the strain rate in the surrounding area was more than 60 nanostrain /a.The reason formed this phenomenon could be the area where large earthquake will occur was in critical state of breaking,and the accumulation of plastic deformation reaches limit state.Therefor,the increment of crustal deformation or strain rare in the area would keep low value.