GRACE观测的区域重力场变化与大地震关系探讨

利用GRACE重力卫星观测资料, 系统地分析了喜马拉雅俯冲带及周边几次7级以上大地震前后的区域重力场变化特征。 研究发现, 2005年巴基斯坦M_S7.8、 2008年汶川M_S8.0、 2011年缅甸M_S7.2、 2013年芦山M_S7.0和2015年尼泊尔M_S8.1等大地震都发生在重力显著变化区域及周边, 地震前几年开始区域重力在原变化趋势下呈现快速减小特征, 而后, 减小后的重力恢复增加, 大地震均发生在这种重力变化过程中。 大地震前的这种重力变化过程与苏门答腊M_W9.3和东日本M_W9.0地震的同震和震后区域重力变化过程类似, 只是幅度要远远小于这两个地震的同震和震后变化, 而且重力减小恢复较快。 综合分析认为喜马拉雅俯冲带及周边区域几次7级以上大地震前后的区域重力变化, 都与巨大地震过程中, 板块活动调整的弹性变形、 区域地下深部地幔物质运移和热引起的区域气候改变有关。 据此, 为区域重力变化的共同特征提出一种物理过程模式解释。 苏门答腊M_W9.3及其巨大余震过程中, 喜马拉雅俯冲带局部区域出现地壳的拉伸、 下沉, 一段时间后, 转变为挤压、 抬升。

Regional Gravity Changes Associated with Several Large Earthquakes Observed by GRACE

Based on GRACE observation data, we calculate and analyze the gravity change in the Himalaya subduction zone and its surrounding areas in recent years. The characteristics of gravity change show that the large earthquakes, such as 2005 Pakistan M_S7.8, 2008 Wenchuan M_S8.0, 2011 Myanmar M_S7.2 and 2015 Nepal M_S8.1 all occurred in the regions with significant gravity changes. There has a common progress containing mainly two stages for the abnormal gravity changes associated with these earthquakes. Firstly, the regional gravity decreases rapidly a few years before the earthquake. And then, the gravity recovers with rapid increase. This progress of abnormal gravity change can be fitted with quadratic polynomial. All the large earthquakes occurred after or during this progress. This common progress of abnormal gravity changes is similar to the co- and post-seismic gravity changes of the Sumatra M_W9.3 and Tohoku-Oki M_W9.0 earthquakes. But the amplitude of the gravity change is much smaller, and the decrease of gravity restored soon with no permanent deformation. Combining with vertical displacement observed by GPS, precipitation and groundwater reserves, and thermal anomaly of the region, we proposed a physical process model to explain the common characteristics of regional gravity changes. During the Sumatra M_W9.3 and its huge aftershocks, the extension and subsidence of crust occurred in some regions around the Himalaya subduction zone, and then, it changed to compression and uplift.