Abstract: Based on the broadband records of the digital seismic networks of Qinghai, the focal mechanisms of the Dingqing, Xizang earthquakes(M_S≥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 M_S5.5 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 M_S5.1 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 M_S5.5 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 M_S5.1 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 M_S5.1 earthquake on January 25, 2020, there were also earthquakes with M_S5.5 on May 11, 2016 and M_S4.5 on October 12, 2017, while only a few earthquakes with M_S3.4 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 M_S5.5 earthquake in 2016 and M_S5.1 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 M_S5.5 earthquakes in 2016 and M_S5.1 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.

Key words: Dingqing, relocation, focal mechanism, normal fault, seismogenic structure

摘要： 文中利用青海省地震台网的宽频带数字记录, 通过CAP反演等方法获取了西藏丁青8次M_S≥3.0地震的震源机制解(1次地震的震源机制解来自USGS)。 结果显示, 7次地震以正断破裂为主, 兼具少量右旋走滑分量, 断层优势走向为NNE, P轴的优势方位为SWW, T轴的优势方位为SEE。 同时, 利用双差相对定位法获得了217个地震的重定位结果。 重定位后, 余震沿NE-SW向展布, 与震源机制解的走向基本吻合, 但与区域内主要走滑型断裂近EW的走向不一致。 2015—2018年发生的地震主要分布在5~15km深度范围, 2018—2020年震源深度范围缩小至7~12km, 2018年以后震源深度的分布范围明显收窄。 丁青地震发生在羌塘块体中部, 域内既受到SN向印度洋板块与亚欧板块的强烈碰撞挤压作用, 也存在EW向伸展构造活动。 综合分析重定位、 震源机制结果及地质构造背景等资料, 认为2016年M_S5.5、 2020年M_S5.1地震的发震构造可能是同一条NE走向的正断型断裂, 发震断层面可能为节面I, 即走向、 倾角和滑动角分别为12°、 58°、 -103°与9°、 57°、 -101°的节面。 由于丁青地区地质资料匮乏, 无法明确具体的发震断裂。

关键词: 丁青, 重定位, 震源机制, 正断层, 发震构造