利用双台站背景噪声分析2017年墨西哥MW7.1地震震源区的地震波速变化

背景噪声中包含有丰富的地球内部结构信息，可以用来监测地震波速度变化.基于墨西哥中部G.UNM和MX.TLIG两个地震台站2016年4月1日至2018年4月1日的连续波形资料，本文首先对其进行不同频带下的背景噪声互相关处理，然后采用小波变换对获取到的互相关函数进行滤波来解决台站间距过长所引起的互相关函数不稳定问题，最后分别通过Stretching方法和MWCS方法测量得到的走时偏移来提取地震波速度变化.结果表明：利用小波变换能有效提高互相关信噪比和结果精度；采用Stretching方法发现震源区浅部5~10 km的地震波速在地震中上升约0.07%，而在2~5 km的地震波速则有约0.05%的下降趋势；MWCS方法计算结果稳定性稍差，但变化趋势与Stretching结果相同.造成该地震波速变化的主要原因可能是2017年地震所引起的脆性岩层结构破裂.

Seismic wave velocity change in hypocentral region of 2017 MW7.1 Mexico earthquake from ambient noise of two stations

Ambient noise has abundant information of the internal structure of the earth, which can be used to monitor the change of seismic wave velocity. Here, continuous waveform records from two seismic stations (G.UNM and MX.TLIG) between April 1, 2016 and April 1, 2018 were used to study the seismic wave velocity change in hypocentral region of September 19, 2017 M_W7.1 Mexico earthquake. Firstly, ambient noise at different frequency range was cross-correlated to derive Green's functions. Then the functions were filtered by wavelet transform to reduce correlation instability caused by long distance between stations. Finally, regional seismic wave velocity changes were calculated with travel time offsets determined from MWCS and Stretching method, respectively. The results indicate that the wavelet transform can improve signal to noise ratio of the cross-correlation and accuracy of results. The seismic wave velocity in the shallow crust at 5~10 km of hypocentral region raised about 0.07% but dropped about 0.05% at 2~5 km revealed by stretching method. The results from MWCS method show slightly poor stability but same trend with Stretching method. This seismic wave velocity change may be related to the damage from fault rupture caused by the earthquake.