2021年云南漾濞6.4级地震发震构造分析

doi:10.3969/j.issn.0253-4967.2021.03.015

摘要/Abstract

摘要： 2021年5月21日的漾濞6.4级地震发生在青藏高原东南缘川滇块体的西边界, 震中地区分布有多条活动断裂, 构造较为复杂。震源机制解和地震波形反演显示该地震为右旋走滑型, 破裂面走向NW, 与川滇块体西边界的维西-乔后、红河断裂带的运动性质和走向一致。在野外调查中没有发现明显的地震地表破裂, 只在漾濞县城西侧震中区一带发育一条NW向地表裂缝带, 分为间隔约6km的北西和南东2段, 长度分别为2.5~3km和3~3.5km, 分析认为此裂缝带为本次地震发震断裂活动在地表的表现。而维西-乔后断裂在漾濞6.4级地震中未见同震变形, 说明该断裂在此次地震中没有活动。根据野外地质调查结果, 并结合他人关于震源机制解、余震精定位、 InSAR和GNSS形变观测等结果进行综合分析, 认为维西-乔后断裂西侧的一条NW向次级断裂是此次漾濞6.4级地震的发震构造, 该断裂走向310°~320°, 长约30km, 以右旋走滑运动为主, 断裂面陡立, 略向SW倾, 可能为川滇块体西边界向W扩展中形成的较新构造。

关键词: 漾濞6.4级地震, 发震构造, 地表裂缝带, 维西-乔后断裂

Abstract: The May 21, 2021 Yangbi M_S6.4 earthquake occurred at the western boundary of the Chuandian tectonic block in southeast Tibetan plateau. The structural background is complex, with multiple active faults distributed around the epicenter area. Focal mechanism and seismic waveform inversion reveal that this earthquake is right-lateral strike-slip type with a NW-trending rupture plane. This accords with the strike and motion directions of the Weixi-Qiaohou and Red River faults along the western boundary of the Chuandian block.
We made a careful field investigation along the Weixi-Qiaohou Fault and around the epicenter area, and did not find any obvious earthquake surface rupture. But we observed a NW-trending ground fissure zone near the epicenter area to the west of the Yangbi County. This zone is divided into two sections, the Yangkechang-Paoshuitian section in the northwest and the Xiquewo-Shahe section in the southwest. These sections have a length of 2.5~3km and 3~3.5km, respectively, and are separated by a ~6km gap. They are characterized by NW-trending ground fissures with a width of several meters to tens meters. The formation of these fissures is inferred to be related to the tectonic movement under the ground, and the fissures have the following features: 1)they are not affected by the topography and cut the slope and range upward; 2)they are continuous and concentrated in a zone with a strike of NW 310°~320°, which is consistent with the belt of aftershocks and differs from the gravity fissures that usually have no regular strikes; 3)they usually have a plane dipping towards upslope(southwest), opposite to the valley; 4)they present shear property, not tensional. This zone thus is interpreted to be the surficial expression of the seismogenic fault of the Yangbi M_S6.4 earthquake.
Moreover, satellite image and field observation suggest that a~30km long linear structure with a NW strike traverses the epicenter area, which may suggest an undiscovered fault. Relocation of small earthquakes shows that the aftershocks are concentrated in a NW-trending belt that is consistent with the linear structure. Furthermore, the fissure zone lies in the northeast side of the aftershock belt, which suggests that the earthquake fault dips SW. Such a dip direction coincides with that of the observed fissure plane, and also agrees with the results from the focal mechanism and InSAR inversion. Both the focal mechanism and the waveform inversion result suggest that the Yangbi earthquake is a right-lateral strike-slip type, which is consistent with the type of the observed ground fissures. No displacement is observed on the fissures, with is also consistent with the InSAR inversion results that suggest the rupture did not break the surface. In addition, there is no coseismic deformation observed along the Weixi-Qiaohou Fault, which may indicate this fault did not move during this earthquake.
Based on our field investigation, in combination with the focal mechanism, aftershock distribution, and InSAR and GNSS inversion results, the seismogenic fault for this Yangbi M_S6.4 earthquake is believed to be a NW-trending(310°~320°)fault with a length of~30km, named as the Yangkechang-Shahe Fault. According to the location, size, and motion of the fault, it is suggested that the Yangkechang-Shahe Fault is a secondary fault of the Weixi-Qiaohou fault system. This fault has a slightly SW-dipping plane, and is dominated by right-lateral strike-slip motion, which may be a younger fault developed during the westward expansion of the western boundary of the Chuandian block.

Key words: Yangbi M_S6.4 earthquake, the seismogenic fault, ground fissure zone, the Weixi-Qiaohou Fault

中图分类号:

P315.2

李传友, 张金玉, 王伟, 孙凯, 单新建. 2021年云南漾濞6.4级地震发震构造分析[J]. 地震地质, 2021, 43(3): 706-721.

LI Chuan-you, ZHANG Jin-yu, WANG Wei, SUN Kai, SHAN Xin-jian. THE SEISMOGENIC FAULT OF THE 2021 YUNNAN YANGBI M_S6.4 EARTHQUAKE[J]. SEISMOLOGY AND GEOLOGY, 2021, 43(3): 706-721.

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