利用GRACE观测资料分析日本M_W9.0地震前区域重力变化特征

STUDY ON REGIONAL GRAVITY CHANGES BEFORE THE M_W9.0 JAPAN EARTHQUAKE DETECTED BY GRACE

利用GRACE重力卫星观测资料,获得了日本MW9.0地震前的区域重力变化;获取了TSKB、DAEJ、SUIY和HLAR站GPS连续观测垂直位移。地震前区域重力变化主要经历2个过程:1)2002年8月至2007或2008年底的先上升后下降的小幅变化过程,该过程可用二次多项式拟合;2)第1个过程后的重力快速上升后下降的较大幅度变化。2008年黑龙江至乌苏里区域重力下降明显,2009年和2010年在太平洋俯冲带的广大区域重力呈显著上升,并且正、负重力变化区域都向SW方向迁移。2010年与2009年相比,重力变化沿板块边界带呈东、西部分的正、负分布特征;东部区域点位2010年初开始重力快速下降至地震发生;而西部点位在此期间仍然为重力正变化高值,地震发生后开始下降变化。远场GPS垂直位移具有2006年中开始的隆升变化,2009年初开始转向。经过与GLDAS陆地水储量改变引起的重力变化比较,认为区域重力变化主要由地表水储量改变引起。但位于海洋侧的P01和P03位置类似下降变化也非常明显,这表明区域重力的异常变化很可能与地下深部的地幔物质运移和热引起的区域气候改变有关。这种异常变化特征与地震孕育的关系有待今后深入研究。

Regional gravity changes before the Mw 9.0 Japan earthquake are obtained from the observations of GRACE satellites. At the same time, time series of vertical displacements at GPS fiducial stations of TSKB, DAEJ, SUIY and HLAR are acquired in the ITRF 2000 reference frame. There are mainly two stages of the pre-earthquake regional gravity changes. The first is a process with slightly gravity change behaved as gravity rise and then decline during August 2002 to the end of 2007 or 2008, which can be fitted with quadratic polynomial. The second stage is showed as gravity increasing rapidly and then decreasing after the first process. Significant gravity decrease appeared in Heilongjiang and Ussuri and its surrounding region in 2008. The gravity increased significantly in vast region along the Pacific subduction zone in 2009 and 2010. And the areas of positive and negative gravity changes migrated towards the southwest. Comparing with 2009, the positive and negative gravity change areas are divided into west and east distribution along the plate boundary. Gravity decreased rapidly at locations of the eastern part from the beginning of 2010 until the occurring of the earthquake. While gravity changes at locations in the western part were still positive with high value, which began to decline after the earthquake. Far-field GPS vertical displacements showed an uplift beginning from the middle of 2006, and this trend turned in early 2009. Comparison with the gravity changes caused by variation of surface water contents shows that the regional gravity changes are mainly caused by changes of surface water contents. But gravity changes at locations P01 and P03 in the ocean side have the same characteristic. This suggests that regional gravity anomalies may be related to underground mantle migration and heat which caused regional climate changes. The relationship between these abnormal variations and seismogenesis needs to be studied in the future.