Characteristics of gravity field before the Lushan MS7.0 earthquake based on time variable gravity data from GRACE satellite
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摘要: 基于GRACE卫星时变重力数据， 利用小波多尺度分解方法， 分析研究了芦山MS7.0地震前重力场的时空动态演化特征. 结果表明, 雅安地区震前卫星时变重力场具有明显的重力变化信息. 卫星时变重力场在时间域表现为， 累积重力场和差分重力场存在较强的规律性动态变化， 以2003—2004年均值为基值的2009—2012年的变化特征与2005—2007年的变化特征具有较好的相似性， 以2009年均值为基值的2009—2012年即芦山地震前4年， 芦山震区的时变重力场呈现上升→减速上升→加速上升以及上升→轻微下降→快速上升的变化特征； 卫星重力场在空间域表现为， 震中位置由靠近重力场正变化的中心→接近重力场正变化边缘→接近重力正变化四象限分布中心的变化特征. 因此， 芦山地震的重力场动态变化信息和波场分离结果符合逆冲推覆兼走滑型地震引起的区域变形特征.Abstract: Based on monthly gravity data from GRACE satellite, this paper analyzes the characteristics of dynamic changes in gravity field before the Lushan MS7.0 earthquake by multi-scale wavelet decomposition method. Results show there is a regulation of the dynamic variations in the cumulative gravity field and differential ones. In the time domain, taking the annual average value of gravity field in 2003—2004 as the base value, the annual variation characteristics of gravity field in 2009—2012 are similar to those in 2005—2007. On the basis of the annual average value of gravity field in 2009， the annual changes in gravity field from 2009 to 2012 exhibit a process, i.e., increasing → decelerated increa-sing → accelerated increasing or increasing → slightly descending → rapidly increasing. In the spatial domain, the positive gravity variations are characterized by the process that the epicentral location moves from the center of positive gravity variations to its border, and then to the center of the four-quadrant distribution. So dynamic changes in the gravity field and wave field separation results of the Lushan MS7.0 earthquake are consistent to the regional deformation characteristics resulted from the Lushan earthquake as a thrust-nappe and strike-slip earthquake.
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图 1 庐山Ms7.0地震前重力场累积动态变化和差分动态变化图(单位：10-8 m/s2)
Figure 1. Annually cumulative gravity changes and differential gravity changes before the Lushan Ms7.0 earthquake (unit:10-8 m/s2)
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图 2 重力差分场小波多尺度分解结果
Figure 2. Results of multi-scale wavelet decomposition with differential gravity field
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图 3 芦山MS7.0地震震源区构造应力主轴反演结果(引自中国地震局地震预测研究所，2013b)
□表示最大应力轴反演结果，△表示最小主应力轴反演结果，○表示中间主应力轴反演结果； 最大主应力轴和中间主应力方向为水平向，而最小主压应力轴方向为近垂向
Figure 3. Tectonic stress axes inversion of the source region of Lushan earthquake (after Institute of Earthquake Science,China Earthquake Administration，2013b)
Open squares represent maximum principal stress axis，open triangles represent minimum principal stress axis and open circles represent medium principal stress axis. The maximum principal stress axis and the medium one are in horizontal direction,and the minimum one is in vertical direction
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