Abstract: The Gansu Xiahe M5.7 earthquake on 28 October 2019 is located between the Lintan-Tanchang Fault and the northern Xiqinling Fault. The earthquake sequence offered significant information for interpreting the focal mechanism solution and analyzing the seismotectonics, which is helpful for the estimation of the level of future seismic risk in this region.
There are 180 aftershocks recorded by Gansu seismic network within 650 hours after the Xiahe M5.7 main shock up to November 24, 2019. The waveforms recorded by 20 seismic stations that have a distance less than 200km to the Xiahe M5.7 main shock are collected. HZT station, the nearest station to the main shock has an epicenter distance of 19km. Locations of 94 earthquakes including the main shock are obtained after locating the earthquake sequence using HYPO2000 method, each earthquake is with more than 3 recognizable Pg phases during location. The result of location shows that the aftershock epicenters are concentrated and mainly distributed in an area of 20km×20km near the epicenter of the main shock, the hypocentral locations are distributed in the depth of 1~16km.
After locating the Xiahe M5.7 main shock and the aftershock sequence, we calculated the focal mechanism of the main shock using gCAP and P waveform polarity method, and relocated the main shock and aftershocks by the method of double difference algorithm.
The results show that the parameters of the focal mechanism solution are as follows: For the nodal plane Ⅰ, strike is 185°, dip is 56°, and rake is 127°; for the nodal plane Ⅱ, strike is 312°, dip is 48°, and rake is 48°.The relocation results show the occurrence features of the seismogenic fault that dips northeast with dip angles about 47°~54°, which is near to nodal plane Ⅱ of the focal mechanism solution.
The epicenter of Xiahe M5.7 earthquake is closer to the western Lintan-Tanchang Fault, which has a complex geometry structure composed of several parallel and oblique secondary faults. Before Xiahe M5.7 earthquake, the activity of the western segment of the fault was weak. The Lintan-Tanchang main fault and its secondary faults are part of the transition zone of structural transformation between the Eastern Kunlun Fault and the north margin of West Qinling Fault, which caused several moderate earthquakes occurring near the Lintan-Tanchang Fault since 2003. The Minxian M_S6.6 earthquake 2013, which is the largest earthquake of these earthquakes, has a seismogenic fault which is a secondary fault of Lintan-Tanchang Fault, this fault merges into the Lintan-Tanchang main fault in the deep part.
The result of the relocation shows that the hypocentral location of Xiahe M5.7 is not on Lintan-Tanchang main fault but on a branch fault, since the epicenter of the earthquake has a certain distance from the western segment of Lintan-Tanchang main fault. Previous studies are lacking about seismogenic fault of Xiahe M5.7 earthquake. In this paper, based on the focal mechanism of Xiahe M5.7 earthquake, which is same to the mechanisms of the moderate strong earthquakes occurring near the Lintan-Tanchang Fault in the past twenty years and the feature of Lintan-Tanchang main fault with its secondary faults, we speculated that the seismogenic fault of Xiahe M5.7 earthquake is one of the secondary faults of Lintan-Tanchang Fault, and it has the same structure as other secondary faults that the fault merges into the Lintan-Tanchang main fault in the deep part. The seismogenic fault of Xiahe M5.7 earthquake has a strike angle of about 312° and a dip angle of about 48°, the strike angle is consistent with the intensity distribution from the post-earthquake investigations.

Key words: Xiahe M5.7 earthquake, focal mechanism, double difference relocation, seismogenic fault

摘要： 甘肃夏河M5.7地震的震中位于临潭-宕昌断裂与西秦岭北缘断裂之间。 文中采用gCAP方法及P波初动方法求解了主震的震源机制解, 并利用双差定位方法对夏河地震及其余震序列进行了重定位, 分析了地震的发震断层。 结果显示: 节面Ⅰ的走向、 倾角和滑动角分别为185°、 56°和127°; 节面Ⅱ的走向、 倾角和滑动角分别为312°、 48°和48°。 重定位后, 余震序列的震源呈NE倾、 宽2~4km的条带状分布, 深度集中于3~9km。 余震的分布特征与主震震源机制解的节面Ⅱ相符, 由此推断夏河M5.7地震的发震断裂为临潭-宕昌断裂的次级断裂, 该断裂的走向为312°, 倾向NE。 最后, 初步探讨了该次地震的发震模式。

关键词: 夏河M5.7地震, 震源机制解, 双差重定位, 发震断裂