用CAP方法反演2010年6月5日山西阳曲M_S4.6地震震源机制解

利用CAP方法反演了2010年6月5日阳曲MS4.6地震震源机制解,得到震级MW为4.5,节面I走向213°、倾角47°、滑动角-161°,节面II走向109°,倾角76°,滑动角-44°,属于倾滑型;精确定位显示震中处于石岭关隆起区,CAP反演和精定位结果推断本次地震的震源深度为17～20km。震源机制解节面参数与震中附近的山根底断裂和系舟山西麓断裂产状存在差异,这两条断裂不是阳曲地震的发震断裂,由于现场野外地质考察未发现地表断裂,不排除本次地震为隐伏断层活动的结果。

Focal Mechanism of the M_S4.6 Yangqu Earthquake of June 5,2010,Determined from the CAP Method

On June 5,2010,a magnitude MS4.6 earthquake occurred in the Yangqu region,which was located on the Shilingguan uplift between the Taiyuan basin and the Xinding basin in the Shanxi rift.This was a moderately strong earthquake felt in Yangqu and surrounding areas.The earthquake ruptured on the Shangendi fault,which controlled the tectonic activities in the east side of the Taiyuan basin.Based on the seismicity distribution,it appears that this earthquake was also located in the seismic gap of the Taiyuanbasin.To better understand the seismotectonics of this region,we relocated this event and further studied its focal mechanism based on the local broadband waveform data recorded by the regional seismic network in the Shanxi Province.In this work,the earthquake was initially relocated by means of the absolute relocating method,and then the focal mechanism and focal depth were determined by the"Cut and Paste" (CAP)method.We inversed the focal mechanism using waveforms in seven stations near the epicenter with high SNR ratios and clear P wave polarization,and the stations were evenly azimuthally distributed.The results of the focal mechanism and focal depth were testified by comparing the observed records and the synthetic waveforms.The results indicate that the Yangqu earthquake was a normal fault earthquake with a small percentage of strike-slip components.The best double couple solution is 213°,47°,and -161°for strike,dip,and slip angles,respectively,for one nodal plane,and 109°,76°,-44°for another.The moment magnitude was MW4.5,and the estimated focal depth was 17~20km,as determined by the CAP method and absolute relocation method,which suggests that this event was a shallow earthquake.The focal depth of the Yangqu earthquake was consistent with the relocation of small to moderate earthquakes in the Shanxi rift.The Yangqu earthquake occurred about 6km away from the Shangendi fault,which is the eastern boundary fault of the Taiyuan basin and is about 9.7km away from the Xizhoushan-Xilu fault,which is the western boundary fault of the Xinding basin.The focal mechanism of the Yangqu earthquake was dramatically different with the tectonic settings of the Shangendi fault and the Xizhoushan-Xilu fault near the epicenter.Based on the relocated location,the Yangqu earthquake should occur in the Shilingguan uplift.However,there is no evidence to show that the Shangendi fault and the Xizhoushan xilu fault extend into the uplift region;furthermore,there is no fracture that is consistent with the mechanism solution of the Yangqu earthquake.Thus,combining previous studies and our results,we suggest that the Shangendi fault and the Xizhoushan-Xilu fault are not the seismogenic fault of the Yangqu earthquake.Because the energy is too weak to transport the rupture to the surface,no surface rupture has been found in the field survey.Therefore,we suggest that a buried fault may be the seismogenic structure of this event.This conclusion still needs further investigationwith ambient noise imaging,active fault detecting,a dense network of high dynamicrange seismometers,and geological field work in Yangqu and its surrounding areas.