DETECTION OF COSEISMIC CHANGES OF GREAT EARTHQUAKES IN GRACE TIME-VARIABLE GRAVITY FIELD WITH EMPIRICAL ORTHOGONAL FUNCTIONS：A CASE STUDY OF THE MW9.0 TOHOKU-OKI EARTHQUAKE

doi:10.3969/j.issn.0253-4967.2014.03.017

Abstract

Abstract:

The Tohoku-Oki M_W9.0 earthquake of 11 March, 2011 has caused eastward movement and subsidence of the Japanese Islands as well as mass redistribution. The temporal-spatial features of mass redistribution were discussed by using the monthly GRACE time-variable gravity field, which would compensate the inefficiency for the undersea focal region where GPS, InSAR measurements are not available. The coseismic gravity changes were computed through least-square fitting and empirical orthogonal function(EOF) from the time series on 0.5°×0.5° grids, and through dislocation model as well. A dipole distribution of the coseismic changes appears in back-arc region and trench with maximum decrement and increment of～6μgal and～3μgal, respectively. The results suggest that EOF method avoids a priori knowledge, such as event time, as used in least squares fitting. Nevertheless, the gravity signal derived from GRACE satellites is an integral of many different geophysical processes, thus the reliability and exact physical sources are likely varying due to the event scale and observation time span, etc. In this study, most of seasonal changes are eliminated through PCs 2, 3, 4; and the coseismic gravity changes extracted from the first principle component of EOF, whose distribution is spatially coherent, are much closer to the result from dislocation model than the least square result, therefore can really reflect the changes resulting from the earthquake.

Key words: GRACE gravity field, empirical orthogonal function, least squares fitting, Tohoku-Oki M_W9.0 earthquake, coseismic change, non-tectonic factors

摘要：

2011年3月11日日本东海发生M_W9.0地震，造成日本岛整体东移、下沉并伴随巨大的质量重新分布。对于海底地震的震中区域，空间观测的GRACE卫星重力数据很好地弥补了GPS、InSAR等形变资料的缺失。利用GRACE卫星月重力场数据计算了地面0.5°×0.5°网格点上的重力变化时间序列，采用最小二乘拟合、经验正交函数（EOF）2种方法，提取了同震重力变化，结果显示震中两侧区域的重力变化呈两极分布，其中弧后区域重力下降，最大降幅约6μgal，海沟区域重力增加，最大增幅约3μgal。EOF方法避免了最小二乘拟合方法所需的地震发生时刻等先验信息，但卫星重力信号是由多种地球物理过程引起的重力变化的叠加，EOF结果的可靠性及其反映的真实物理来源往往随着事件的规模、观测时间的长短等而改变。文中第2，3，4主成分主要反映了非构造因素的影响，通过第1主成分空间一致性提取的同震重力变化与位错理论模型计算结果较为接近，因此较真实地反映了地震引起的重力变化特征。

关键词: GRACE卫星重力场, 经验正交函数, 最小二乘拟合, 日本M_W9.0地震, 同震变化, 非构造因素

CLC Number:

P313

ZHANG Ke-liang, GAN Wei-jun, ZHOU Xin. DETECTION OF COSEISMIC CHANGES OF GREAT EARTHQUAKES IN GRACE TIME-VARIABLE GRAVITY FIELD WITH EMPIRICAL ORTHOGONAL FUNCTIONS：A CASE STUDY OF THE M_W9.0 TOHOKU-OKI EARTHQUAKE[J]. SEISMOLOGY AND GEOLOGY, 2014, 36(3): 763-774.

张克亮, 甘卫军, 周新. GRACE卫星重力场同震变化的经验正交函数分解：以日本M_W9.0地震为例[J]. 地震地质, 2014, 36(3): 763-774.

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