基于改进两步差法的玛多M_(S)7.4地震微波异常研究

Microwave anomaly of Maduo M_(S)7.4 earthquake derived by improved two-step difference method

针对2011年提出的两步差法在地震微波异常提取中的不足,提出了改进两步差法,该方法通过使用层次聚类和小波分析两种算法,克服了原方法中未考虑不同地物类型引起的亮温差异、亮温背景场不够稳健等问题,从而使异常结果在空间形态和异常幅度上更加合理,并将该方法应用于2021年5月22日青海省玛多县M_(S)7.4地震的微波异常提取。结果显示,玛多地震前三个月出现了一条长约900 km、增温幅值达12 K的NE向亮温增温条带,与2010年4月14日MS7.1玉树地震前异常特征在空间分布和演化特征上均有很高的相似性。此外,异常出现的时间和幅度与震级有一定相关性,震级较大的玛多地震出现异常时间较早,幅度较大;而震级较小的玉树地震出现异常时间较晚,幅度较小,且两次地震的微波亮温增温幅值均达8 K以上。初步的机理分析认为,地震前的微波异常条带与该区域的NE向张性断裂和张性断陷带在空间上分布一致,异常的出现可能与地震前该断陷带的地热活动以及地表岩石的发射率变化有关。

In response to the shortcomings of the two-step difference method for extracting seis-mic microwave anomalies proposed in 2011,an improved two-step difference method which incorporates hierarchical clustering and wavelet analysis is proposed to overcome the problems resulted from ignoring the brightness temperature differences caused by ground cover types and the lack of robustness of the brightness temperature background field in the original method,thus making the anomaly results more reasonable in terms of spatial pattern and anomaly amp- litude. The method is applied to the microwave anomaly extraction of the MS7.4 earthquake in Maduo County,Qinghai Province on May 22,2021。The results show that a NE-trending brightness temperature increase strip with a length of about 900 km and an amplitude of 12 K appeared three months before the Maduo earthquake,which has high similarity in spatial distribution and evolution characteristics with the anomaly characteristics of the MS7.1 Yushu earthquake on April 14,2010. Furthermore,the appearing time and magnitude of the anom- alies are related to the magnitude of the earthquakes. The anomalies of MS7.4 Maduo earth- quake appeared earlier than MS7.1 Yushu earthquake with larger scale,and the temperature-increase of microwave brightness due to both earthquakes reached more than 8 K. Preliminary mechanism analysis suggests that the pre-earthquake microwave anomaly stripes are consistent with the spatial distribution of NEward tensional fractures and extensional fracture zones in the region,and the appearance of the anomalies may be related to the geothermal activity of the fracture zones and the emissivity changes of surface rocks before the earthquakes.