Abstract:

Earthquake relocation and focal mechanism inversion can provide seismogenic structure information, especially in the source area without obvious fault trace on the surface, and further reveal the deep geometry of hidden faults. The Yangbi M_S6.4 earthquake sequence recorded by Yunnan regional seismic network from May 18 to June 4, 2021 is relocated by using the double-difference location method. A total of 3 233 events, from 4 days before and 14 days after the main shock, are relocated and the b-value in the Yangbi source region is calculated accordinly. Then, using the waveform data recorded by the Yunnan and Sichuan regional broadband seismic stations, the full moment tensor solutions of 10 earthquakes (M≥4.0), including the main earthquake, are obtained using the near-field full waveform inversion method, and further the tectonic stress field is retrieved. The high-precision relocation of earthquakes shows that there are significant differences between the foreshocks and the aftershocks in the tempo-spatial distribution. The foreshocks are primarily in a belt-like distribution along the NW-SE direction, whose epicenters are in a back-and-forth migration. The aftershocks mainly occurred on asymmetric conjugate faults along NW and NE directions, and multi-groups of aftershocks with different strikes were distributed in the south end of the NW-striking seismic zone, implying the complexity of the medium and fault geometry in the focal area. The temporal distribution of the b-value shows that the b-value has a rising trend before the main earthquake, indicating that the stress accumulation in the source area had begun to release gradually at that time, which may be related to the fact that the sequence is of the foreshock-mainshock-aftershock type. After the main shock, the variation range of b-value is large, which may reflect very strong seismicity of the aftershocks and large release of the stress. The focal mechanism solutions show that the moderate earthquakes are mainly of strike-slip with a normal component and a significant non-double-couple component, which may indicate the staggered distribution of the NW- and NE-trending faults in the source region, and the earthquake rupture is not simply the slip along the fault plane. Taking into account for the above-mentioned results as well as the compressional stress field environment in nearly NS direction and the extensional environment in nearly EW direction, the seismogenic structure of Yangbi M_S6.4 earthquake is a dextral strike-slip fault, NW striking with a high-dip angle, located in the Baoshan block, which may be a secondary fault parallel to the Weixi-Qiaohou-Weishan Fault and including multi-fault branches in NE direction in the southern segment. The tempo-spatial distribution characteristics of the earthquake sequence and the diversity of the fault plane rupture are controlled by the geometric complexity of fault system in the focal area.

Key words: Yangbi M_S6.4 earthquake, branch fault, b-value, full moment tensor, stress field

摘要：

地震精定位及震源机制反演能够提供发震构造信息, 特别在地表无明显断层迹线的震源区, 可有效揭示隐伏断裂的深部几何结构。 文中采用双差定位方法对2021年5月18日-6月4日期间云南区域台网记录到的漾濞M_S6.4地震序列进行精定位, 得到主震前4d和主震后14d共3 233次地震事件的空间位置, 并计算了漾濞震源区的b值; 进一步利用云南和四川区域台网记录的波形数据, 采用近震全波形反演方法得到了包括主震在内10个4.0级以上地震的全矩张量解, 并反演得到震源区构造应力场。 地震定位结果显示, 前震序列和余震序列在时空分布上存在明显差异: 前震主要在NW-SE方向呈条带状分布, 且震中位置存在往返迁移现象; 余震主要在NW和NE 2个方向分布, 呈不对称共轭断层分布形态, 在NW向地震带南端存在多组不同走向的余震分布, 暗示震源区介质及断层几何结构的复杂性。 b值随时间变化的结果显示, 主震前b值已有上升趋势, 表明震前震源区应力积累已开始逐步得到释放, 可能和本次地震为前震-主震-余震的序列类型有关; 而主震发生后b值的变化幅度较大, 可能反映余震活动较为剧烈, 应力释放变化较大。 震源机制结果显示地震以走滑型为主, 正断分量其次, 并包含显著的非双力偶成分, 可能表明震源区NW及NE向断裂交错分布, 地震破裂并非简单地沿断层平面滑动。 综合上述结果以及近SN向挤压、 EW向拉张的应力场环境, 我们认为漾濞M_S6.4地震序列的发震构造为位于宝山地块内部一条NW向的高倾角右旋走滑断裂, 可能是与维西-乔后-巍山断裂平行的次级断裂, 并且该断裂在南段包含多条NE向分支断裂。 文中结果表明漾濞震源区断层系统几何结构的复杂性控制了地震序列时空分布范围及震源破裂的多样性。

关键词: 漾濞M_S6.4地震, 分支断层, b值, 全矩张量, 应力场