Analysis of the Focal Mechanism and Tectonic Stress of Minxian M_S6.6 Earthquake

The Minxian MS 6.6 earthquake,the largest destructive earthquake to affect Gansu province,occurred on July 22,2013.Its epicenter at N34.54°,E104.21°was located between the northern fringe fault of western segment of Qinling Mountain and the Lintan-Tanchang fault. After the earthquake occurred,we solved the focal mechanism of the main shock with P wave polarity from the waveforms obtained from the Gansu province seismic network.We then compared the solution with the focal mechanisms published by Harvard and the United States Geological Survey(USGS). During the 10years prior to the Minxian earthquake,several earthquakes stronger than Mb 4.5occurred in this region.We solved the focal mechanisms of these earthquakes with P wave polarity data,and we compared the focal mechanisms with those of the Minxian earthquake.Similar morphologies were revealed,and each contained a larger thrust component. The focal mechanism of a small earthquake is more difficult to solve than that of a large earthquake.However,because small earthquakes occur more often than large earthquakes,it is desirable to refer to the tectonic stress by the focal mechanisms of multiple small earthquakes to decrease the effects of local structures and to highlight the features of the region tectonic stress.Previous methods used to determine tectonic stress from multiple focal mechanisms include grid search.However,because the smallest search grid is 5°,the search grid was too large to meet the exact solution of the tectonic stress.Other methods ignore the data accuracy or cannot give the error of parameters.The newest method obtains the optimal solution of tectonic stress using finer grid parameters of 1°×1°and gives the error of parameters.In this paper,we solve the tectonic stress using this method. A broadband mobile seismometer observation array was conducted in southeastern Gansu in 2010-2011,in which abundant near shock waveforms were obtained.We examined small earthquakes in close proximity to the Lintan-Tanchang fault,which was the triggering seismic fault of the Minxian earthquake,at a distance of less than 50km.To maintain the reasonableness of the focal mechanism,after identifying the P wave polarity of selected earthquakes,we identified 31earthquakes in which the P wave polarity was more than 9and had a good envelope to the source.We solved their focal mechanisms,which we used to solve the tectonic stress in this region.The results show that the direction of the main pressure was 70.12°,which is very close to the results of previous studies.The main pressure was NEE in this region because of the combined action of the Qinghai-Tibetan Plateau,Ordos,and South China blocks.After the Minxian earthquake,a series of aftershocks occurred,from which and we expect to obtain additional data for analyzing the changes of tectonic stress.