Abstract:

The Yangbi M_S6.4 earthquake in Yunnan Province and Maduo M_S7.4 earthquake in Qinghai Province occurred in western China on May 21 and May 22, 2021, respectively, which caused huge loss of life and property. Gravity changes in the epicentral area and its surroundings before the two earthquakes can provide important reference for studying the seismogenic environment and background. The ground gravity observation is relatively sparse in western China and satellite gravity can supplement this deficiency. GRACE(Gravity Recovery and Climate Experiment)(from March 2002 to June 2017)and GRACE-FO(GRACE Follow-on)(from May 2018 to March 2021)can produce the wide-area space, quasi-real-time, long-term and near-continuous observation data, which will provide large-scale background information for the ground gravity research.

In this paper, the epicenters of the two earthquakes and their surrounding area were taken as the study area(18°~45°N, 83°~115°E). We used GRACE, GRACE-FO and GLDAS(Global Land Data Assimilation System)data to calculate long-term gravity spatial-temporal distribution in the study area with 300km fan filter. We presented the gravity rate, cumulative gravity changes, differential gravity changes in the study area for about 20 years, and the gravity time series of Maduo earthquake and Yangbi earthquake. We simulated the theoretical co-seismic gravity variation of Maduo earthquake and evaluated the possibility of detecting the co-seismic gravity signal for GRACE-FO. The research results showed that:

(1)Long-term gravity changes in the study area were mainly characterized by positive-negative-positive-negative spatial layout in four quadrants. Gravity increased in Qinghai-Tibet block and South China block, and gravity decreased in Indian block and North China block. However, the North-South seismic belt and Bayankala block were located at the low-value areas in four quadrants and their gravity changes were relatively small. This was the large-scale gravity seismogenic background in western China.

(2)The epicenters of Maduo earthquake and Yangbi earthquake were both located in the center of the four quadrants and also at the corner of the high gradient zone of satellite gravity change. And their gravity changes were very small in the last 20 years, which was consistent with the basic characteristics of the ground gravity location prediction. After a year of continuous increase in the last two years before the Maduo earthquake, the gravity in Maduo area experienced a four-month period of decrease, then it increased again. This was similar to the process of gravity change before the Tangshan earthquake.

(3)M_S≥7.0 Earthquakes in the study area since 2002, such as Wenchuan M_S8.0 earthquake, Yushu M_S7.1 earthquake, Lushan M_S7.0 earthquake, Jiuzhaigou M_S7.0 earthquake, Maduo M_S7.4 earthquake and Nepal M_S8.1 earthquake, basically occurred in the central area of the four quadrants or at the corner of the tectonic-related high gradient zone, which was consistent with the earthquake case results of earthquake prediction based on the ground gravity observations. This study provided more earthquake cases for ground gravity prediction.

(4)Based on dislocation theory simulation, the magnitude of co-seismic gravity change of Maduo earthquake in Qinghai Province reached -40~151μGal. It is difficult for GRACE-FO to detect the co-seismic gravity change of Maduo earthquake with the current accuracy. And it could be possible only when the time-variable gravity accuracy of gravity satellite was improved by 1-2 orders of magnitude. This research provided earthquake case supports for the demand demonstration of gravity satellites in China in the future.

In this study, the temporal and spatial evolution of gravity in the western region of China and its surrounding areas from March 2002 to March 2021, which coverd the epicenters of Yangbi and Maduo earthquakes, was obtained by using the satellite gravity and global hydrological data after considering the influence of periodic signals. The theoretical coseismic effects of the Maduo earthquake on the local gravity field were analyzed and the accuracy of gravity satellite to detect this seismic signal was evaluated. This study provided the important background information of large-scale gravity field for the study of Maduo earthquake in Qinghai Province and Yangbi earthquake in Yunnan Province, and also provided valuable material for the seismic demand analysis of gravity satellite in China.

Key words: gravity satellite, dislocation theory, gravity variation, earthquake, GLDAS

摘要：

文中基于2002-2021年卫星重力观测数据及全球水文模型, 计算了青海玛多及云南漾濞地震震中区及周边(18°~45°N, 83°~115°E)范围的重力变化趋势、 累积重力变化、 差分重力变化及重力时变序列, 并模拟青海玛多地震同震重力变化, 以探索涵盖此次地震震中的大尺度重力变化背景及可能存在的孕震信息。 研究结果表明: 1)研究区域长期重力变化趋势的范围为-1~1μGal·a^-1, 重力变化长期呈现出负-正-负-正的大四象限空间布局; 2)云南漾濞地震及青海玛多地震均位于重力变化四象限分布的低值区及高梯度带的拐角处, 这与地表重力地点预测的基本特征一致; 3)2002年以来的M_S≥7.0地震, 如汶川地震、 玉树地震、 芦山地震、 九寨沟地震、 玛多地震、 尼泊尔地震等都发生在该四象限分布的中心区域或与构造相关的高梯度带的拐角处, 这与地面重力地震预测的震例经验吻合; 4)基于位错理论模拟的青海玛多地震的同震重力变化可达-40~151μGal, 重力卫星精度需要提高1~2个量级方能探测到这一变化。 重力卫星星座的搭建将为提高卫星重力地震监测能力提供可能。

关键词: 重力卫星, 位错理论, 重力变化, 地震, GLDAS