巴颜喀拉块体东缘形变及九寨沟地震孕震环境数值分析

2017年8月8日四川发生九寨沟M7.0地震,是继2008年汶川M8.0地震后发生在巴颜喀拉块体东部的又一强震.现今GPS速度观测数据显示,2008年汶川地震前后的1999-2007年和2011-2016年两个时间段内巴颜喀拉块体东部地表速度场存在明显的差异.本文以实际GPS速度观测资料为约束,构建三维有限元地球动力学模型,分别计算分析了两个时段内震源区及周边现今地壳形变、弹性应变能和应力积累特征,进一步探讨汶川地震的发生对九寨沟地区变形及应力的影响.数值模拟结果显示,汶川地震之后(2011-2016年)巴颜喀拉块体东部的地壳形变、弹性应变能积累及应力积累速率均明显大于震前,增加量值达1.5-3倍;九寨沟地震发震断裂上库仑应力增长率在1999-2007年约为0.7 kPa·a~(-1),2011-2016年间增至1.2 kPa·a~(-1).上述结果表明,现今巴颜喀拉块体东部地壳应力积累过程有利于左旋走滑型九寨沟地震的发生,汶川地震的发生调整了区域应力状态,加速了九寨沟地震的孕育过程.

Numerical modeling of crustal deformation in the eastern margin of the Bayan Har block and analysis of seismogenic environment of the 2017 Jiuzhaigou earthquake

The M7.0 Jiuzhaigou earthquake occurred on August 8, 2017 in the eastern Bayan Har block of the Tibetan plateau, which is another major event in this region since the 2008 Wenchuan M8.0 earthquake. The GPS velocities in two time periods, 1999-2007 and 2011-2016, suggest that crustal deformation changed remarkably after the 2008 M8.0 Wenchuan earthquake in this area, which is possibly attributed to the occurrence of great Wenchuan shock. To analyze the tectonic setting of the Jiuzhaigou earthquake, we conduct 3-D FEM numerical modeling constrained by the GPS velocities and calculate the changes of crustal deformation, strain energy and stress in the eastern Bayan Har block. Results show that there are significant increases in the accumulation rates of crustal strain, strain energy and stress after the Wenchuan earthquake, which are approximately 1.5~3 times of those before the Wenchuan earthquake. The Coulomb stress on the seismogenic fault of the Jiuzhaigou earthquake was 0.7 kPa·a^-1 during 1999-2007, and increased to 1.2 kPa·a^-1 during 2011-2016. So the current accumulation of tectonic stress is favorable for the nucleation of strike-slip earthquakes in this area. Therefore, we consider that 2008 Wnehcuan event has adjusted the stress state of the eastern Bayan Har block, which has accelerated the nucleation process of the 2017 Jiuzhaigou earthquake to some extent.