华南地区流动重力变化异常指标分析

Anomaly Index Analysis of Mobile Gravity Changes in South China

2013年以来华南地区先后发生2013年9月4日福建仙游M_(S)4.8、2017年7月15日广西南丹M_(S)4.0、2018年3月20日广东阳西M_(L)4.2、2019年3月5日广东雷州M_(L)4.1、2019年8月20日海南三亚M_(S)4.2、2019年10月12日广西北流M_(S)5.2、2019年11月25日广西靖西M_(S)5.2与2021年8月4日广西德保M_(S)4.8等中强地震。通过系统梳理以上中强震例震前不同时空尺度流动重力变化特征,总结提炼地震孕育与重力变化异常时空演化特征之间的关系,分析利用流动重力地震观测资料开展地震预测的“时、空、强”定量依据。结果表明:华南地区多次4、5级左右地震震前出现局部重力场变化,且地震易发生在与构造活动有关联的重力变化四象限中心地带或正、负异常区过渡的高梯度带附近;华南地区4、5级左右地震前重力变化时变距和量级分析表明:震级为4级时,重力变化异常范围约100 km,重力变化异常量级约40µGal,震级为5级时,重力变化异常范围约150 km,重力变化异常量级约50µGal;目前流动重力观测多为1~2期/年的复测周期,未能对震前重力场演化过程进行较详细跟踪,在时间上流动重力以中期地震预测为主。

Since 2013,there have been several moderate-strong earthquakes in South China,such as Xianyou M_(S)4.8 earthquake in Fujian on September 4th,2013,Nandan M_(S)4.0 earthquake in Guangxi on July 15th,2017,Yangxi M_(L)4.2 earthquake in Guangdong on March 20th,2018,Leizhou M_(L)4.1 earthquake in Guangdong on March 5th,2019,Sanya M_(S)4.2 earthquake in Hainan on August 20th,2019,Beiliu M_(S)5.2 earthquake in Guangxi on October 12th,2019,Jingxi M_(S)5.2 earthquake in Guangxi on November 25th,2019 and Debao M_(S)4.8 earthquake in Guangxi on August 4th,2021.This paper systematically sorts out the characteristics of mobile gravity changes at different temporal and spatial scales before the above moderate and strong earthquakes,summarizes and refines the relationship between earthquake preparation and the temporal and spatial evolution characteristics of gravity variation anomalies,and analyzes the quantitative basis of"time,space and strength"for earthquake prediction using mobile gravity seismic observation data.The results show that:There are local gravity field changes before many M4 and M5 earthquakes in South China,and earthquakes are easy to occur in the center of the four quadrants of gravity change related to tectonic activities or near the high gradient zone of the transition between positive and negative anomaly areas.The time-varying distance and magnitude analysis of gravity changes before M4 and M5 earthquakes in South China show that:When the magnitude is 4,the anomaly range of gravity changes is about 100 km,and the anomaly magnitude of the gravity changes is about 40µGal;when the magnitude is 5,the anomaly range of gravity changes is about 150 km,and the anomaly magnitude of the gravity change is about 50µGal.At present,the re-measurement cycles of the mobile gravity observations are usually 1-2 periods/year and the evolution process of the gravity field before the earthquake is not be tracked in detail,so that mobile gravity monitoring is often used for the mid-term earthquake prediction.