2021年5月21日漾濞MS6.4地震的发震断层及其破裂特征: 地震序列的重定位分析结果

据中国地震台网测定,2021年5月21日21时48分在云南省大理州漾濞县发生M_S6.4地震,及时查明此次地震的发震构造及震源破裂特征,可为认识该区孕震条件和判别未来强震危险性提供关键依据。采用双差定位方法对漾濞地震序列进行重新定位,得到3863次地震事件的精确震源位置。结果显示:漾濞地震序列整体呈北西—南东向分布,长约25 km;整体走向135°;M_S6.4主震震中位置为25.688°N,99.877°E;震源深度约9.6 km。综合地震序列深度剖面和震源机制解结果可知,发震断层应为北西走向、整体向西南方向陡倾的右旋走滑断层,倾角具有自北西向南东逐渐变缓的趋势。进一步分析地震序列的时空演化过程发现,该地震具有典型的"前震-主震-余震型"地震序列活动特点,其破裂过程主要包括3个阶段。破裂成核阶段:首先在发震断层10~12 km深度处相对脆弱部位产生小尺度破裂,之后失稳加速破裂,发生M_S5.6地震;主震破裂阶段:在构造应力场持续加载和周围小尺度破裂的共同影响下,促使浅部较高强度断层闭锁区破裂,形成M_S6.4主震;尾端拉张破裂阶段:主震破裂向东南扩展过程中,在东南端形成与之呈马尾状斜交的、具有正断性质的次级破裂,并产生M_S5.2余震。而且此次地震还在源区北东侧触发了北北东向的左旋走滑破裂。综合分析认为,漾濞地震是兰坪-思茅地块内部北西向草坪断裂在近南北向区域应力挤压作用下发生右旋走滑运动的结果,具有明显的新生断裂特征。近年来兰坪-思茅地块内部一系列中强地震的发生表明,青藏高原物质向东南持续挤出的过程中,遇到该地块的阻挡,正在导致地块内部早期断层贯通形成新的活动断裂。因此,川滇地块西南边界带上或相邻地块内部老断层的复活和新生断裂的产生是区域中强地震危险性分析评价中值得关注的重要课题,同时建议需重视未来该区中强地震进一步向东南和向北的迁移或扩展的可能性。 

Seismogenic fault and it's rupture characteristics of the 21 May, 2021 Yangbi MS 6.4 earthquake: Analysis results from the relocation of the earthquake sequence

According to China Earthquake Network Center (CENC), the M_S 6.4 Yangbi earthquake struck northwestern Yunnan Province on 21 May, 2021 at 21:48(Beijing time). Figuring out the seismogenic fault and source rupture characteristics in time can provide a key basis for understanding the dynamic conditions in this region and estimating the risk of strong earthquakes in the future. We employed the double-difference relocation algorithm to relocate the Yangbi earthquake sequence, and obtained precise locations of 3, 863 earthquakes. In general, the result revealed a narrow 25-km-long, linear southeast seismicity trend concentrated in the 2~14 km depth range, and the orientation is 135°. The M_S 6.4 mainshock located at (25.688°N, 99.877°E) after relocation, and the focal depth is 9.6 km. Based on the results of precise locations and focal mechanism solutions, the seismogenic fault might be a NW dextral strike-slip fault with southwest dip, and the dip angle tends to gradually decrease from NW to SE. The Yangbi earthquake sequence belongs to the "foreshock-mainshock-aftershock" type, revealed by the temporal and spatial evolution process of the earthquake sequence, and the fracture process mainly includes three stages:fracture nucleation stage, mainshock rupture stage, and tension rupture stage. In the first stage, small-scale fractures occurred at the relatively weak part of the seismogenic fault at the depth between 10~12 km, after two-days'' nucleation, the fault entered into an unstably accelerated rupture state, resulting in the M_S 5.6 foreshock. Under the joint influence of continuous loading of tectonic stress and surrounding small-scale fractures, the higher strength blocking area in the shallow part of the fault ruptured, and the M_S 6.4 mainshock occurred. The tension rupture mainly occurred at the southeast end of the seimogenic fault. A horsetail splay with normal fault features was formed at the southeast end of the aftershock sequence, started by the largest aftershock of M_S 5.2. In addition, the mainshock triggered small-scale fractures on a NEN sinistral strike-slip fault near the source area. The comprehensive study shows that the seismogenic fault of the Yangbi earthquake is not the well-known Weixi-Qiaohou fault, but the Caoping fault in the Lanping-Simao block. The Yangbi M_S 6.4 mainshock is the result of the dextral strike-slip motion of the Caoping fault, which has been revived under the NWN-SES regional principal compressive stress, and the fault has obviously new fracture characteristics. This study indicates that the continuous southeastward extrusion of material from the Tibetan Plateau is leading to the reconnection and reactivation of the old faults in the junction zone between the eastern Lanping-Simao block and the Lijiang-Dali fault system, resulting in relatively frequent moderate-to-strong earthquakes in this area. Therefore, the reactivation of old faults and the generation of new faults in the southwestern boundary zone of Sichuan-Yunnan block are worthy to pay attention on the risk estimation and evaluation of regional moderate-to-strong earthquakes. We suggest that more attention should be paid to the possibility of further southern or northern migration (or expansion) of moderate-to-strong earthquakes.