• 研究论文 •

### 丁青地区地震重定位、 震源机制及其发震构造初步分析

1. 青海省地震局, 西宁 630001
• 收稿日期:2020-04-14 修回日期:2020-10-19 出版日期:2021-02-20 发布日期:2021-05-06
• 通讯作者: *李玉丽, 女, 1981年生, 高级工程师, 主要从事地震监测和地震学研究工作。
• 作者简介:李启雷, 男, 1981年生, 2009年于四川大学获原子与分子物理学专业硕士学位, 工程师, 主要从事地震活动性与数字地震学研究, 电话: 0971-6150545, E-mail: 27263836@qq.com。
• 基金资助:
青海省重大科技专项(2019-ZJ-A10); 青海省2020年基础研究项目(2020-ZJ-752); 中国地震局地震科技星火计划项目(XH21047Y); 青海省地震科学基金(2020B03)共同资助

### THE RELOCATION, FOCAL MECHANISMS OF THE DINGQING EARTHQUAKES AND A PRELIMINARY STUDY OF ITS SEISMOGENIC STRUCTURE

1. Qinghai Earthquake Agency, Xining 630001, China
• Received:2020-04-14 Revised:2020-10-19 Online:2021-02-20 Published:2021-05-06

Abstract: Based on the broadband records of the digital seismic networks of Qinghai, the focal mechanisms of the Dingqing, Xizang earthquakes(MS≥3.0) are of the obtained with Cut-and-Paste(CAP)inversion method and from USGS, seven of them are normal fault type with a little strike-slip component. The dominant direction of the fault strikes is near SN, the dominant distribution of dip angles is 58°~69°, and the dominant distribution of rake angle is -81°~-103°. The dominant direction of P axis is SWW, and that of T axis is SEE. The best double couple solution of the MS5.5 earthquake in 2016 is 12°, 58° and -103° for strike, dip and rake angles, respectively, the second nodal plane solution is 216°, 34° and -70°, the centroid depth is 7.3km, and its moment magnitude is 5.3. For the MS5.1 earthquake in 2020, the solution is 9°, 57°, -101° for strike, dip and rake angles, respectively, the second nodal plane solution is 209°, 35° and -74°, the centroid depth is 6.8km, and its moment magnitude is 4.9.
The double difference relative positioning method(HypoDD)is used to relocate the Dingqing earthquakes from February 1, 2015 to March 5, 2020. Broadband data of 9 seismic stations of Qinghai seismic network, Tibet seismic network and scientific array within about 400km around the epicenter are used, and the relocation of 217 earthquakes is obtained. After relocation, the Dingqing earthquake sequence is more clustered than before, with zonal distribution along NE-SW direction, which is in agreement with the fault strike of focal mechanism solutions, but not consistent with the major strike-slip faults in the region. The focal depths of the Dingqing earthquakes are close to the normal distribution, 75 percents of them range from 8 to 12km. The focal depths of earthquakes in 2015-2018 are confined in the range of 5~15km, and that in 2018—2020 are mainly from 7km to 12km, the range of focal depths is significantly reduced after 2018. After the occurrence of MS5.5 earthquake in 2016, the earthquakes ruptured rapidly to the west and south, and most of the aftershocks were of magnitude 3 or below, and the sequence attenuation was fast, which may be because that the mainshock released most of the energy in the sequence. The aftershocks of the MS5.1 earthquake in 2020 mostly ruptured along the horizontal direction or to the deep. The earthquakes occurring from 2019 to March 2020 are located in the middle of the sequence in spatial distribution, and there are two dominant directions of NE-SW and SSE in the spatial distribution of epicenters, showing an L-shape distribution. The reason may be that the earthquake encountered obstacles in the rupture along the NE-SW direction, the strain energy was not fully released, and then turned to the SSE faults after stress adjustment to induce subsequent aftershocks. In the NE direction of the “L-shape”, in addition to the MS5.1 earthquake on January 25, 2020, there were also earthquakes with MS5.5 on May 11, 2016 and MS4.5 on October 12, 2017, while only a few earthquakes with MS3.4 and below occurred in SSE direction, indicating that the NE-trending faults are the dominant area of Dingqing earthquakes activity in recent years.
Since the focal mechanism solutions of MS5.5 earthquake in 2016 and MS5.1 earthquake in 2020 are both of normal fault type, the dominant distribution direction of aftershocks is NE, according to the analysis of relocation, focal mechanism and geological structure background, it is inferred that the seismogenic structure of MS5.5 earthquakes in 2016 and MS5.1 in 2020 may be of a same normal fault type with NE direction. The fault plane may be nodal plane Ⅰ, i.e. the nodal plane with strike of 12°, dip angle of 58°, rate angle of -103° and strike of 9°, dip angle of 57° and rate angle of -101°. Because the Dingqing earthquakes occurred in the hinterland of the Qinghai-Tibet Plateau, the related research data on the distribution and attitude of small-scale faults is very scarce, so it is difficult to determine the seismogenic faults of the Dingqing earthquakes.