2017年3月渤海地震序列微震检测与发震构造分析

Microseismic Detection and Seismogenic Structure Analysis for Bohai Earthquake Sequence in March 2017

地震序列发震构造研究是区域地震活动性和地震危险性分析的重要基础。2017年3月渤海海域发生地震序列活动,该序列发生在郯城-庐江断裂带与张家口-渤海地震带的交汇部位,区域构造较为复杂。然而在渤海海域,连续运行的固定地震监测仪器难以布设,导致地震监测能力相对较弱。本文首先采用模板匹配方法对序列遗漏地震进行检测,再使用波形互相关震相检测进行震相校正,基于校正后的震相到时数据对序列进行精定位,并计算序列中2次最大地震的震源机制解。通过计算共检测到目录遗漏地震32个,约为台网目录中地震数量的1.8倍。根据波形互相关聚类分析发现渤海地震序列可分为2组,一组为M_L4.4地震及其余震序列,一组为最大震级M_L3.5的震群,另有一个M_L1.6地震与其他地震波形相似度较低,可能为一个孤立的地震事件。精定位和震源机制结果显示,2组地震均为NE走向,M_L4.4地震发生在低倾角正断层,M_L3.5地震发生在高倾角走滑断层。最后结合区域地质构造相关研究成果,认为M_L4.4地震及其余震序列发震构造为渤中凹陷内NE向低倾角的伸展性正断层,M_L3.5震群发震构造为NE向倾角较陡的次级走滑断层。

The study of seismogenic structure of earthquake sequence is an important basis for regional seismic activity and seismic risk analysis.In March 2017,an earthquake sequence occurred occurred at the intersection of Tancheng-Lujiang fault zone and Zhangjiakou-Bohai seismic zone in Bohai Sea.The regional structure was relatively complex.It is difficult to deploy instruments in Bohai Sea which make the seismic monitoring capability relatively weak.In this paper,we firstly use the matched filter technique to detect the missing earthquakes in the sequence,and then use the waveform cross-correlation phase detection method to correct the P and S phase arrival time.Based on the corrected phase arrival time data,the sequence is precisely relocated.The focal mechanism of two largest earthquakes in the sequence are calculated.A total of 32 missing earthquakes were detected in the catalog,which is about 1.8 times of the number of earthquakes given in the catalog.According to the waveform cross-correlation clustering analysis,it is found that the Bohai earthquake sequence can be divided into two groups,one is mainshock-aftershock sequence of M_(L)4.4 earthquake,another is earthquake swarm of the largest earthquake M_(L)3.5.Moreover,there is a M_(L)1.6 earthquake that has low similarity with other seismic waveforms,which may be an isolated earthquake event.The results of earthquake relocation and focal mechanism show that the two groups of earthquakes are north-east strikes,the M_(L)4.4 earthquake occurred on the low-dip normal fault,and the M_(L)3.5 earthquake occurred on the high-dip strike-slip fault.Finally,combined with relevant research results of geological structure in the study area,it is considered that seismogenic structure of the M_(L)4.4 aftershock sequence is an extensional normal fault with a north-east low dip angle in Bozhong depression,and the seismogenic structure of the M_(L)3.5 earthquake swarm is a secondary strike-slip fault with a north-east steep dip angle.