Abstract:

On May 21, 2021, a strong earthquake of magnitude 6.4 occurred in Yangbi County, Dali Prefecture, Yunnan Province. The focal depth of this earthquake is 8km. The earthquake broke the calm of magnitude 6 earthquake that had lasted for more than 6 years in Yunnan, and is a significant strong earthquake in the northwestern Yunnan region. Before the M_S6.4 Yangbi earthquake, the foreshock activity near the epicenter was frequent, and the maximum magnitude of foreshock is 5.6. After the M_S6.4 earthquake, another M_S5.2 earthquake, and many aftershocks of magnitude 3 and 4 occurred. The earthquake sequence was very rich. In order to further study the spatio-temporal distribution, source characteristics and seismogenic structure of the magnitude 6.4 earthquake sequence in Yangbi, in this paper more than 2 800 seismic events of the Yangbi earthquake sequence were relocated using the double-difference relative positioning method based on the seismic phase data from the Seismic Cataloging System of China Earthquake Networks Center, and finally 2 116 precise location results were obtained. At the same time, based on the broadband digital waveform data provided by the China Earthquake Networks Center, focal mechanism solutions 31 earthquakes of the sequence were obtained by MTINV program.

The results of the moment tensor inversion show that the moment magnitude of the Yangbi M_S6.4 earthquake is M_W6.0, the centroid depth is 10km, and the optimal double-couple solution is strike 135°, dip 81° and rake 176° for nodal plane I, and strike 226°, dip 86° and rake 9° for nodal plane Ⅱ. It is a strike-slip earthquake. Combining the strike of the fault in the earthquake source area and the distribution of aftershocks, it is inferred that the seismogenic fault is the nodal plane Ⅰ which strikes NW. Focal mechanism solutions of other 30 earthquakes of the sequence are mainly strike-slip type, which are consistent with the main shock. There are also a few events with mixed types. The focal mechanisms of several earthquakes close to the occurrence time of the M_S6.4 main earthquake are in good agreement with the main earthquake. The relocation results show obvious linear distribution characteristics of the sequence. The overall strike is in the NW direction and the dip to the SW direction. The depth profile sequence is horizontally linear along the strike. The dip angles of the fault planes in the south and north sections are different. The dip angles of the northern section are approximately vertical, and that of the southern section is about 45° or so. However, the sequence of the northern section is more concentrated along the fault plane than southern section. The dominant strike of the Yangbi earthquake sequence is NW-SE, the dip angles are concentrated between 70° and 90°, and the rakes are distributed around 180°, indicating that the Yangbi earthquake sequence is mainly characterized by strike-slip faulting. The dominant azimuth of the P-axis is SN and that of the T-axis is EW. The plunge of P-axis and T-axis are near horizontal. This indicates that the activities of the Yangbi earthquake sequence are mainly controlled by the regional SN-direction horizontal compression stress field. The dominant directions of the sequence’s fault planes and P-axis parameters are single, indicating that it is less likely that complex fault activity and large-scale stress adjustment will occur in the source area of this earthquake.

Integrating the results of relocations and focal mechanisms, it suggests that the seismogenic fault of Yangbi earthquake is a right-handed strike-slip active fault, striking northwest and dipping to the southwest, and the dip distribution is segmented. The dip angle of the northern segment is nearly vertical, and the dip angle of the southern segment is lower than that of the northern segment. There may exist rupture segmentation in the fault in the earthquake source area, and the structure morphology of local small areas may be more complicated.

Key words: Yangbi M6.4 earthquake, focal mechanism solutions, double-difference relocation, moment tensor inversion

摘要：

2021年5月21日云南省大理白族自治州漾濞县发生6.4级强烈地震。 为了深入研究漾濞6.4级地震序列的时空分布、 震源特性以及发震构造等, 文中利用双差法对漾濞地震序列中约2 000个地震进行重新定位, 并且利用MTINV程序包反演了地震序列中31个地震的震源机制解。 结果显示: 漾濞6.4级地震是一次右旋走滑型地震事件, 发震断层走向NW, 倾角近直立; 序列震源机制与主震的一致性较好, 以走滑型为主, 存在少量混合类型; 精定位结果显示序列呈明显的NW向展布特征, 震源深度主要集中在10km以浅; 与走向垂直的深度剖面显示, 南、 北段断层倾向均为SW, 但倾角不同; P轴的优势方位为SN向, 倾俯角近水平, 与区域应力场特征一致。 综合重定位和矩张量反演结果推测, 漾濞地震的发震断层是一条整体呈NW走向、 倾向SW、 倾角可能具有分段性的右旋走滑活动断裂, 震源区断裂破裂可能存在分段现象, 且局部小区域构造形态可能较为复杂。

关键词: 漾濞6.4级地震, 震源机制解, 双差定位, 矩张量反演