CAP方法反演2014年山东乳山M4.2、M4.0地震震源机制解

Inversion of the Focal Mechanisms of the 2014 M4.2 and M4.0 Earthquakes in Rushan，Shandong Using the CAP Method

摘要:

选用山东数字地震台网波形资料，利用CAP方法反演乳山2014年1月7日M4.2及4月4日M4.0地震震源机制解。反演结果显示，两次地震均在矩心深度为6 km处获得最佳震源机制解，其节面参数基本一致，属于近走滑型地震。震中附近流动台站的震源距计算表明地震序列震源深度应略小于7 km，与CAP方法反演结果具有一致性，表明地震的主体破裂发生在上地壳。

Abstract:

There have been many intensive small earthquakes since the M3.8 earthquake occurred in Rushan，Shandong on October 1，2013. Two earthquakes（M4.2 and M4.0）occurred separately on January 7 and April 4，2014.There have never been so many earthquakes in Rushan.A focal mechanism describes the nature and source rupture process and is an important foundation to understand the stress state of the source region，earthquake faults, and for analyzing the causes of a seismic event.Therefore，seismic interpretation and post earthquake seismogenic stress distribution mechanisms play very important roles in determining earthquake focal mechanisms.In this study，using waveforms from the Shandong Seismological Network and comparing the synthetic and the observed seismograms，the focal mechanism of the Rushan M4.2 and M4.0 earthquakes in 2014 are inverted using the CAP（Cut and paste）method.The results show that the two earthquakes get their best focal mechanism at 6 km depth，and that the parameters of the nodal planes are similar，belonging to the strike-slip type.At a depth of 6 km，the correlation coefficients of 40 phases of the M4.2 earthquake greater than 0.7 that were recorded by eight stations are 37，which accounts for 93%.The correlation coefficients greater than 0.9 are 28，accounting for about 70%. Similarly，at a depth of 6 km，the correlation coefficients of 40 phases of the M4.0 earthquake that were recorded by the eight stations are all greater than 0.7 and the correlation coefficients greater than 0.9 are 31，accounting for about 78%.Therefore，the synthetic seismograms of the two earthquakes fit the observed seismograms well demonstrating that the inversion results are credible. The focal mechanism shows that the moment magnitude of the M4.2 earthquake was MW =4.3；one nodal plane has a strike of 290°，rake of 84°，and dip of 22° and the other nodal plane has a strike of 189°，rake of 68°，and dip of 174°. The moment magnitude of the M4.0 earthquake is MW=4.2，one nodal plane has a strike of 289°，rake of 90°，and dip of 27° and the other nodal plane has a strike of 199°，rake of 63°，and a dip of 180°，The hypocentral distance was calculated by the mobile seismic station near the epicenter.The results show that the focal depth is slightly less than 7 km，which is consistent with the inversion results of the CAP method，and therefore，the main rupture of the earthquake was in the upper crust.The parameters of the Rushan fault nearest to the epicenter （strike of 10° normal fault）are not consistent with the focal mechanisms of the two earthquakes，which show that the Rushan fault is not the seismic fault and that the seismic faults may be concealed faults.