Abstract:An MS6.6 earthquake occurred at the junction of Minxian and Zhangxian on July 22,2013,at 7:45in the northern section of the North-South seismic zone in the southeast region of Gansu Province.This area is a moderate-strong earthquake-prone region in which the Minxian MS5.2 earthquake and Minxian-Zhuoni MS5.0 earthquake also occurred in 2003and 2004,respectively.In this study,we analyze teleseismic data recorded in 2008-2009 by Minxian station,which is the permanent station in closest proximity to the Minxian-Zhangxian earthquake.Receiver functions aligned by azimuth indicate obvious heterogeneity beneath the Minxian station;observed receiver functions of Minxian station vary according to azimuth.That is,receiver functions of the seismic source area and adjacent areas show obvious differences.In the seismic source area,the receiver function is clearly apositive signal of approximately 1.5s,which indicates that a high velocity zone is present in the middle-lower crust.In the adjacent areas,the receiver function is clearly a negative signal of approximately 1.5s,indicating the presence of a lower velocity zone in the middle-lower crust.This phenomenon shows obvious crustal structure differences between the seismic source area and adjacent areas.In addition,we used the H-kstacking method of receiver functions to determine the crustal thickness and wave velocity ratio of the two areas.The results show crust thicknesses of 45km and 47km and wave velocity ratios of 1.76and 1.69 in the source area and adjacent area,respectively.Therefore,obvious differences in crustal velocity structure appear between the two areas.In the seismic source area,the Ps,PpSs/PsPs clearly indicates crustal discontinuity.Therefore,the H-k stacking method of receiver functions was used to determine the depth of the discontinuity surface and the wave ratio velocity.The H-k stacking results showed an upper crustal thickness of 15km and a wave velocity ratio of 1.62.We estimated the middle-lower crustal thickness and wave velocity ratio to be 30km and 1.84,respectively.Therefore,the soft middle-lower crust is covered by the hard upper crust at the top in the source area.This type of small-scale material inhomogeneity may be an important part of the deep structure of the Minxian-Zhangxian earthquake background.We estimate that the rupture of the Minxian-Zhangxian earthquake may have been attributed to lower crustal flow leading to rupture of the upper crust.
