Abstract:

At 21:48 on May 21, 2021(Beijing time), the M_S6.4 earthquake occurred in western Town(25.700°N, 99.880°E), Yangbi County, Dali, Yunnan Province, with a focal depth of 10km(China Earthquake Networks Center). The Yangbi earthquake is a typical type of foreshock-mainshock-aftershock earthquake, which had a significant impact on the local residents and attracted great attention from society. To better understand the seismogenic structure and mechanism of this earthquake, the present study relocates the May 21, 2021 Yangbi M_S6.4 earthquake sequence, collected from the China Earthquake Networks Center from 2021 to June 18, 2022. Finally, 2681 precisely located events are obtained through the double-difference relocation algorithm. Our results show that the Yangbi earthquake sequence extended for about 32km, mainly along the NW-SE direction, and it is an overall echelon structure changing from narrow in the northwest to broad in the southeast. The dominant depth of the earthquake sequence is 5-10km. The foreshocks were mainly active in the northern section of this earthquake sequence, with the mainshock being a unilateral rupture. The aftershocks primarily extended in the southeast direction, but the southeast extension process was not simply a unilateral extension. Multiple secondary oblique activity sequences were derived on the west side of the sequence. With the continuous release of stress in the study area, only the main rupture continued to be active in the southeastern section of the sequence in the later stage of activity. Still, the secondary oblique ruptures that evolved was no longer active. The average location errors of these earthquakes are about 0.47km in the east-west direction, about 0.50km in the north-south direction, and 0.62km in the vertical direction, and the average RMS travel-time residual is 0.22s.

This study collects broadband digital seismic waveform data of earthquakes with M_S≥4.0 on the main fault of the earthquake sequence recorded by regional seismic networks in Yunnan, Sichuan, and other areas from the International Earthquake Science Data Center. The focal mechanism solutions of the major earthquake events are obtained using the gCAP full waveform inversion method. The results show that the focal mechanism solutions of earthquakes with M_S≥4.0 on the main fault all have an NW-SE oriented nodal plane I, consistent with the dominant distribution of the NW-SE oriented sequence. Except for the nodal plane I of the Yangbi M_S5.6 earthquake, which has a northeast dipping angle, all other focal mechanism solutions have a southwest dipping nodal plane I, which was consistent with the sequence orientation as shown in the vertical cross sections. According to the inclination angles of the P, B, and T axes, the inverted focal mechanism solutions all belong to a strike-slip type.

In this study, the parameters of the seismic fault plane are fitted in segments according to the distribution density of small-to-medium-sized earthquakes. The results show that the strike trending of the main fault plane varies between 126°-137° and gradually increases from north to south, dipping towards the southwest. The dip angle varies between 79°-87° gradually decreasing from north to south. There are four secondary oblique faults with variations in striking directions of 157°, 338°, 157° and 313° from north to south, corresponding to dip angles of 86°, 87°, 87°, and 86°, respectively.

Based on the above research results, combined with the background stress field and V_P/V_S tomographic results, it is inferred that the Yangbi earthquake occurred on the high-dip-angle and NW-SW strike-slip faults in the southwest mountainous areas of Yangbi County. These faults consist of a strike-slipping main fault and multiple secondary crisscrossing small faults, which may be jointly affected by regional stress and deep fluid activity.

Key words: Yangbi earthquake, earthquake sequence, double-difference relocation, focal mechanism solution, seismogenic tectonics

摘要：

文中基于2021年5月1日—2022年6月18日中国地震台网中心收集的2021年5月21日云南漾濞 M_S6.4 地震序列的相关数据, 利用双差定位法进行了重新定位, 得到了2 681个精定位地震事件。结果表明, 漾濞地震序列的扩展方向为NW-SE向, 长约32km, 但并非简单的单侧扩展, 而是呈西北窄、 东南宽的雁列状分布结构特征, 且发震优势深度为5~10km。前震有明显的空间迁移特征, 且主震南、 北两侧的地震活动差异性明显。这些地震在EW向、 SN向、 垂直向的平均定位误差约为0.47km、 0.50km和0.62km, 平均均方根走时残差约为0.22s。采用gCAP全波形反演方法获得该地震序列主断裂上M_S≥4.0主要地震事件的震源机制解, 结果显示出明显的右旋走滑分量。利用基于小地震反演断层面参数的方法, 通过拟合获得了发震断层面的参数, 发现主断层面走向在126°~137° 之间变化, 且由北至南走向逐渐增大, 倾向SW, 倾角为79°~87°, 由北至南倾角逐渐变小。结合该区域的构造应力场及前人的近震层析成像结果推测, 漾濞地震的发震构造为漾濞县城西南山地区域中NW-SE走向的高倾角走滑断裂系, 该断裂系由一条走滑主断层和多条次级斜交小断层组成, 且该地区的地震活动可能受区域应力与深部流体作用共同影响。

关键词: 漾濞地震, 地震序列, 双差定位, 震源机制解, 发震构造