重庆荣昌及邻区中强震前后构造应力场变化特征

Variation of the tectonic stress field in Chongqing Rongchang and its adjacent areas before and after moderate earthquakes

分布于某一区域内的多个地震的P波初动方向记录的总和包含着该区域的构造应力场的状态和地壳介质的某些信息,通过多个地震的P波初动方向求小震综合断层面解可以了解当地构造应力场的状况.本文依据2000年1月—2017年4月共69个地震台站记录的数字地震波形资料,选取具有清晰的P波初动的记录,利用格点尝试法计算重庆荣昌及邻区中强震前后不同时间段的小震综合断层面解,通过大量的不同时段的综合断层面解寻找在时间上稳定的空间规律性,由此推断出该区构造应力场的方向特征,从而分析其局部构造应力场特征.研究结果表明:研究区域内中强震孕震过程中的孕震应力场有增强趋势,利用P轴转向可以得到孕震信息.此外,地震后一段时间内求解得到的小震综合断层面解中的P轴方向一旦背离背景构造应力场,在这个应力场调整过程中会发生多次3～4级地震,但不会有4级以上的地震发生,而4级以上地震的发生要在应力场调整到与背景构造应力场方向一致以后才会重新开始孕育,这一规律能够为今后重庆地区中强地震的发生提取有用的地震前兆信息,在中强地震预测方面具有一定的参考价值.

The sum of P-wave initial motion records of multiple earthquakes distributed in a certain region contains the state of the tectonic stress field in the region and some information of the crust medium.The situation of the local tectonic stress field can be understood by looking for the fault integrated fault plane solutions through the initial motion of P-waves from multiple small earthquakes.With the digital seismic waveform data recorded by 69 seismic stations from 2000-01 to 2017-04 in rongchang and its adjacent areas before and after moderate earthquakes,we calculated and analyzed composite fault-plane solutions of small earthquakes using a grid test method.The spatial regularity of time stability is found through a large number of comprehensive fault plane solutions at different time intervals,from this,we infer the directional characteristics of the tectonic stress field in this area and analyze the characteristics of the local tectonic stress field.The result shows that seismic precursory stress field is strengthened before shocks in the study area,and we could extract some information of earthquake precursor by using P axis changing.In addition,several earthquakes(M<4)would occur after moderate earthquakes in the adjustment process of stress field when P axis deviated from the background tectonic stress field.A probable forthcoming earthquake(M>4)can only happen after stress orientation rotated back to that of the background tectonic stress field,this law can be used to extract useful earthquake precursory information for the occurrence of medium-strong earthquakes in Chongqing in the future,and it has certain reference value in the prediction of moderate-strong earthquakes.