Abstract:

The occurrence of earthquakes is closely related to the crustal tectonic movement and the migration of earth mass, which consequently cause the changes of the earth‘s gravitational field. Global time-varying gravity field data obtained by GRACE gravity satellite can be used to detect pre-seismic gravity anomalies. For example, gravity signals caused by several large earthquakes, such as the 2005 M_W8.6 Indonesia earthquake, the 2010 M_W8.8 Chile earthquake and the 2011 M_W9.0 Japan earthquake, have been successfully extracted using GRACE data. However, previous studies on GRACE satellite-based seismic gravity changes focused more on the dynamics of the co-seismic gravity field than on the pre-seismic gravity anomalies which are of great significance for the early warning of earthquakes. Moreover, the commonly adopted difference disposal of the gravity field with the gravity field of adjacent months or the average gravity field of many years when obtaining gravity anomalies cannot effectively remove the inherent north-south stripe noise in GRACE data. On the contrary, it is more likely to cause the annihilation of the medium-high order information in GRACE gravity field model, which results in the loss of some gravity information related to tectonic activities. To explore the pre-seismic gravity anomalies in a more refined way, this study proposes a method of characterizing gravity variation based on the maximum shear strain of gravity, inspired by the concept of crustal strain. In other words, the gravity strain tensor is obtained by further calculating the second-order gradient of the increment of disturbance potential after the removal of hydrological disturbance, and then the maximum shear strain of gravity is ultimately generated to characterize the pre-earthquake tectonic activities. Then, to better understand the seismogenic process of the fault zone by further extracting the pre-earthquake anomalous changes, the data of the maximum shear strain time series are analyzed in this study by means of the offset index K to describe the gravity anomaly. Because the maximum shear strain is calculated by the second-order gradient of GRACE gravity field, this method can suppress the stripe noise better than the difference disposal, thus effectively improving the sensitivity of gravity anomaly detection. The exploratory experiments are carried out in the Tibetan plateau and its surrounding area, which locates among the Pacific Ocean, the Indian Ocean and Eurasia, with the highest altitude, most complex topography and frequent strong earthquakes. Ultimately, the Wenchuan earthquake and Nepal earthquake were used as an example to complete the extraction of pre-earthquake gravity anomaly information by the above method, and the pre-earthquake tectonic activity of the fault zones was analyzed. The results show that a large area of gravity anomalies consistent with the spatial distribution of the fault zone appeared on the Longmenshan fault zone during the half a year before the earthquake, and the maximum anomalous value appeared within 50km from the epicenter, while no anomalies appeared during the non-earthquake period. In addition, compared with the traditional methods, the proposed method has a better ability to extract anomaly information of gravity field, which provides a new idea for understanding the dynamic mechanism of large earthquakes using GRACE data.

Key words: GRACE gravity satellite, pre-earthquake gravity anomaly, maximum shear strain, Wenchuan earthquake, Nepal earthquake

摘要：

地震的发生与地壳构造运动密切相关, 而地壳构造运动会造成地下介质密度的改变, 从而导致地球重力场发生变化。GRACE重力卫星所提供的全球时变重力场数据可用于发现大地震的震前重力异常。然而, GRACE数据中存在的南北条带噪声严重干扰震前异常信息的提取。目前常用的高斯滤波等处理方法在抑制噪声的同时, 也削弱了有价值的重力异常信号。为此, 文中提出了一种基于最大切应变的震前重力异常信息提取方法。该方法通过计算扰动引力位的2阶梯度对南北条带噪声进行压制, 并基于地壳形变理论得到最大切应变以获取重力的变化信息。最后, 利用最大切应变的偏移指数K进一步探究震前重力异常的时空演变规律。文中以汶川地震和尼泊尔地震为例, 采用上述方法完成震前重力异常信息提取, 并进行了断裂带的震前构造活动分析。结果显示, 在震前半年内, 发震断裂带上出现了与断裂带空间展布一致的大面积重力异常区, 且最大异常值出现在震中距50km范围内, 而在非震期并未出现异常现象。此外, 经分析发现, 与传统方法相比, 本文方法对重力场的异常信息提取能力更强, 这为利用GRACE数据认识大地震的动力机制提供了新思路。

关键词: GRACE重力卫星, 震前重力异常, 最大切应变, 汶川地震, 尼泊尔地震