雷兴林,王志伟,马胜利,何昌荣. 2021. 关于2021年5月滇西漾濞MS6.4地震序列特征及成因的初步研究. 地震学报,43(3):261−286. doi: 10.11939/jass.20210100
引用本文: 雷兴林,王志伟,马胜利,何昌荣. 2021. 关于2021年5月滇西漾濞MS6.4地震序列特征及成因的初步研究. 地震学报,43(3):261−286. doi: 10.11939/jass.20210100
Lei X L,Wang Z W,Ma S L,He C R. 2021. A preliminary study on the characteristics and mechanism of the May 2021 MS6.4 Yangbi earthquake sequence,Yunnan,China. Acta Seismologica Sinica43(3):261−286. doi: 10.11939/jass.20210100
Citation: Lei X L,Wang Z W,Ma S L,He C R. 2021. A preliminary study on the characteristics and mechanism of the May 2021 MS6.4 Yangbi earthquake sequence,Yunnan,China. Acta Seismologica Sinica43(3):261−286. doi: 10.11939/jass.20210100

关于2021年5月滇西漾濞MS6.4地震序列特征及成因的初步研究

A preliminary study on the characteristics and mechanism of the May 2021 MS6.4 Yangbi earthquake sequence,Yunnan,China

  • 摘要: 2021年5月21日21时48分在滇西苍山西麓漾濞地区发生MS6.4 (MW6.1)强震,相关地震活动表现为一个典型的前震−主震−余震序列。本研究分别就该地震序列的构造背景、M1.0以上地震的双差定位、主要地震的矩张量反演和破裂传播方向、应力场反演及断层滑动趋势以及潮汐作用等方面进行了初步分析。矩张量反演结果表明,矩心深度为6.0 km。根据断层破裂传播方向分析结果及精定位余震分布判定,主震震源断层产状为走向137°,倾角75°,滑动角−167°,破裂沿南东向单侧扩展,右旋走滑含正断层分量。漾濞地震序列发生在红河断裂带北段延伸方向上的乔后—巍山断裂附近,但主震震源断层及主要余震的分布在走向和位置上均明显偏离已知的乔后—巍山断裂。地震序列受一个发育程度不高、含多级雁列构造的北西向为主、北东向为次的共轭走滑断层系统(本文称为“漾濞断层”)所控制,整体上沿北西向断层展布,主震与部分强余震为北西向断层活动所致,但中强前震和多数余震为北东向断层活动所致。中强震的断层破裂均为单侧扩展,北西向断层主要表现为南东向破裂扩展,而北东向断层沿两个方向破裂扩展,相邻地震还存在往返破裂现象。对截至5月23日所发生的M>4.0前震和余震进行了全矩张量反演。利用漾濞地震震中15 km范围内20多个MW>3.4余震的比较可靠的震源机制解反演了该区的应力场,结果显示:主应力形状比φ=(σ2σ3)/(σ1σ3)为0.46±0.17;最大主应力轴的方位角为188.0°±9.0°,倾伏角为12.4°±7.0°;中间主应力轴近直立,倾伏角为72.1°±11.3°;最小主应力轴的方位角为280.3°±7.0°,倾伏角为10.4°±12.0°。本文还对理论潮汐应变及应力进行了分析,结果表明,该地震序列受潮汐调制作用十分明显。5月18日18时及19日20时开始的两组前震群的首个主要地震以及5月21日晚发生的主震均发生在潮汐体应变和库仑应力的峰值附近,余震活动也与潮汐有明显的相关性。综合主要地震震源机制解、前震及余震分布、潮汐调制特征、基于应力场反演的断层滑动趋势分析以及滇西北地区以往类似地震活动研究结果,本文初步推断:漾濞地震受深部流体作用的影响明显,5月18日18时开始的第一次前震活动高潮从北西向断层的一个拉张性断层阶区开始,最大前震的震源断层为北东向断层,随后向北西方向迁移;19日20时开始的第二次前震活动高潮集中在主震震源附近。这些地震的触发及深部流体作用共同促进了北西向断层的活动,但主震的发生受深部流体作用为主。

     

    Abstract: The MS6.4 (MW6.1) earthquake occurred on May 21, 2021 in Yangbi county, Yunnan, China is the main shock of a typical foreshock-mainshock-aftershock sequence. This research carried out a preliminary study focusing on tectonic background, double-difference hypocenter location, moment tensor and rupture directivity of major earthquakes, stress field and fault slip-tendency, as well as tidal effect. According to the results of moment tensor inversion and rupture directivity of the main shock and the distribution of aftershocks, it is determined that the source faults of the main shock has strike 137°, dip 75°, rake −167°. The centroid depth of the main shock is 6.0 km, and the fault ruptured unilaterally toward the south-east direction, showing pure double-couple mechanism with dominant right-lateral strike-slip and minor normal components. Relocated hypocenters show that the Yangbi earthquake sequence is located near, but significantly departs from the well-known Qiaohou-Weishan fault in the northern segment of the Honghe fault zone, demonstrating an unmapped NW-trending strike-slip fault (we named “Yangbi fault” in this paper) with some small scale conjugate faults of NE-trending. The hypocenters are distributed along the NW-trending major fault, but also exhibit clusters along the NE-trending faults. The strongest foreshocks and most of the major aftershocks were caused by rupture of NE-trending source faults. All major earthquakes show a unilateral rupture directivity. We have obtained reliable mechanism solutions of more than 20 events with MW>3.4 in the focused area (within 15 km of the center of the Yangbi earthquake sequence), which enabled us to be able to invert the mean stress field in the area. The principal stress shape ratio φ=(σ2σ3)/(σ1σ3) is 0.46±0.17; the (azimuth, plunge) of the maximum, intermediate, and minimum principal stress axes are (188.0°±9.0°, 12.4°±7.0°), (50°±45°, 72.1°±11.3°), and (280.3°±7.0°, 10.4°±12.0°), respectively. Through theoretical tidal strain and stress analysis, it is found that this seismic sequence is significantly affected by tidal modulation. The first major earthquakes of the foreshock clusters that began at 18:00−20:00 on May 18 and 19 and the main shock occurred near the peaks of tidal volumetric strain and Coulomb failure stress. Based on the focal mechanism solutions of the main earthquakes, the distributions of foreshocks and aftershocks, the facts of tidal modulation, the rupture directivity of major earthquakes, fault slip-tendency analysis, and the results of previous studies on similar seismic activities in northwestern Yunnan, we preliminarily suggest that the Yangbi earthquake sequence is significantly affected by the action of deep fluids. The first foreshock activity climax at 18:00 on 18 May, in a tensile fault step (should be permeable channel) area of the NW-trending fault, likely initiated along NE-trending faults (greater slip-tendency) by deep overpressure fluid, and then migrated to the northwest. The second climax of foreshock activity that began at night on the 19 May was concentrated near the hypocenter of the main shock. The triggering of these foreshocks and the action of deep fluid jointly promoted the activity of the NW-trending fault (smaller slip-tendency), but the main shock was mainly promoted by the action of deep fluid.

     

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