于田M_s7.3地震震前重力扰动信号研究

2008年3月21日和2014年2月12日,新疆和田地区于田县先后发生两次M_s7.3地震.为分析两次地震前是否存在重力短临异常信号,应用Tsoft软件对原始数据进行理论固体潮改正和气压改正,并基于小波分解去除其低频趋势,有效提取了两次地震前国家连续重力观测网多台g-Phone重力仪的高频扰动信息.结果表明两次地震前都记录到高频扰动信号.2008年地震前牡丹江、沈阳台和漳州台三个台站记录到的高频扰动信号,主频分布在0.2~0.4 Hz.2014年地震前8个台站记录到两组不同频段的扰动信号.第一组信号集中在震前6天的东北台站,第二组扰动信号出现在震前3天的东南台站.两组扰动信号振幅呈现从北到南、从东到西逐步衰减的时空分布规律·同时期的气象资料显示局部强降雨与第二组扰动异常存在良好的相关性.对高频重力信号的时频分析结果表明震前第一组扰动信号存在异常频段0·13~0.18 Hz.应用扰动源扫描算法对2014年两组扰动信号进行空间定.位,结果显示第一组信号扰动源位置与于田震中相距约1935 km,不在于田地震的孕震区内;第二组信号扰动源来自海上.2008年和2014年于田地震第二组震前扰动信号与强降雨以及海浪影响相关.2014年第一组信号与于田地震的关系仍待进一步研究.

Study on gravity disturbance before the Yutian MS7.3 earthquakes

On March 21, 2008 and February 12, 2014, two powerful earthquakes of magnitude 7.3 occurred in Yutian County, Xinjiang, respectively. To explore the pre-seismic gravity anomalies prior to the earthquakes, 8-day-long 1 Hz sampling records from 18 g-Phone gravimeters in mainland China are obtained and analyzed. Firstly, based on wavelet decomposition, the low-frequency solid tides are removed from the preprocessed tidal gravity data and non-tidal gravity data are extracted as a result. Secondly, the non-tidal gravity data are analyzed using short time Fourier transformation (STFT) to determine their frequencies and spectral characteristics. Finally, the pre-seismic anomalies are located using the source scanning algorithm. The results show that (1) obvious high-frequency disturbance signals were observed before the two earthquakes. Before the 2008 earthquake, Mudanjiang, Shenyang and Zhangzhou stations have recorded such disturbance information. Two sets of pre-seismic anomalies can be clearly identified from the non-tidal gravity record before the 2014 earthquake. While the first set of anomalies appeared and increased three days before the occurrence of the earthquake, the second set of signals increased six days before the earthquake. The amplitude of the former type of anomalies decreased from north to south, while that of the latter type showed a gradual decrease from east to west. The anomalies show specific temporal and spatial distribution characteristics. (2) According to the evidence provided in many studies, the frequency of 0.125~0.18 Hz (6~8 s) maybe related to pre-seismic anomalies. The results of a high frequency of time-frequency analysis show that frequency anomalies existed at some stations prior to the 2014 earthquake. (3) Large-scale rain events covering a wide range occurred and continued for a long time, which shows that the temporal and spatial distributions have a better consistency with the second set of anomalies. (4) Two sets of disturbance signals are spatially located with a source scanning algorithm. The results show that the first set of anomalies originated in the northern areas, while the second from the eastern coastal areas. The emergence of the second set of seismic precursory may be associated with a wide range of rainfall events and sea waves. Whether the first set of anomalies contained seismic precursory information before the Yutian earthquake needs to be further studied. As pre-seismic gravity disturbance phenomena and the influencing factors are complex, it is necessary to conduct more observations of multi-parameters, and more research on its mechanism.