维西-乔后断裂带上连续地震的应力降和两种不同的孕震机制

Stress drops of continuous earthquakes on the Weixi-Qiaohou fault zone and two distinct seismogenic mechanisms

2021年5月21日云南省维西-乔后断裂带上发生MS6.4漾濞地震,造成了大量的人员伤亡和财产损失.该断裂带上还曾先后发生过2013年MW5.3和2017年MW4.91两次洱源地震.本文反演了维西-乔后断裂中南段2013年、2017年与2021年三次地震震群的震源机制解,使用谱比法计算了主震和较大的前震、余震共17个事件的拐角频率与应力降.结果表明:2013年MW5.3主震与MW4.96余震均为正断层事件,随后的地震序列皆为右旋走滑事件,2017年MW4.91主震与MW4.89前震均为右旋走滑事件.发生在东南侧15 km左右的2021年漾濞MW6.2主震也为右旋走滑事件,但前震和余震震群中包括约70%的走滑事件和30%的正断层事件,推测发震断层为维西-乔后断裂带的次级断裂;2013年MW5.3主震的拐角频率为0.68±0.03 Hz,应力降为11.98-1.52/+1.66 MPa, 2017年MW4.91主震的拐角频率为1.59±0.05 Hz,应力降为39.84-3.64/+3.88 MPa, 2021年MW6.2主震的拐角频率为0.415±0.01 Hz,应力降为65.35-4.61/+4.84 MPa;17个地震应力降反映出走滑事件的自相似性,而正断层事件同震断层面上可能的流体扩散引起了自相似性破缺.两种地震事件在拐角频率与应力降方面存在明显差异,可能源于断层受构造形变、结构和岩性的差异的影响,在横向与纵向上的摩擦系数、破裂尺度、裂隙流体等特性存在较大差异,因而可能存在两种不同的孕震机制.从潜在的地震风险来说,走滑的孕震过程可能触发更大的破裂,具有更大的潜在破坏性.

On May 21, 2021, the Yangbi MS6.4 Earthquake occurred on Weixi-Qiaohou Fault in Yunnan Province of China, which caused a large number of casualties and property losses. The 2013 MW5.3 earthquake and the 2017 MW4.91 earthquake have also occurred on the same fault. We inversed the focal mechanism of three earthquake sequences by CAP(Cut and Paste) method and calculated the corner frequency and stress drop by spectral ratio method. The results show that the MW5.3 mainshock and MW4.96 aftershock in 2013 are both normal type, and the subsequent aftershock sequences are dextral strike-slip type. The MW4.91 mainshock and MW4.89 foreshock in 2017 are dextral strike-slip type, and the Yangbi MW6.2 mainshock in 2021, which occurred about 15 km southeast of the previous events, is also dextral strike-slip type, with about 70% of strike-slip events and 30% of normal events in its fore-and aftershock sequence, we speculated that the seismogenic fault is the secondary fault of Weixi-Qiaohou Fault;The corner frequencies of 2013, 2017, 2021 mainshock are 0.68±0.03 Hz, 1.59±0.05 Hz and 0.415±0.01 Hz respectively, and the stress drops are 11.98-1.52/+1.66 MPa, 39.84-3.64/+3.88 MPa, 65.35-4.61/+4.84 MPa. The stress drops of 17 relatively strong earthquakes reflects the self-similarity of strike-slip events, while the possible fluid diffusion on the coseismic fault plane caused the self-similarity breakdown of the normal events. It may be due to the diversity of structural deformation, structure and lithology on the fault, which caused great differences in friction coefficient, fracture scale, fracture fluid and other properties in transversal and longitudinal fault plane. In terms of potential earthquake risk, the seismogenic process of strike-slip may trigger greater rupture and have greater potential damage.