关键词: 2021年漾濞M_S6.4地震, 地震序列精定位, 震源机制, 发震构造

Abstract:

An M_S6.4 earthquake occurred in Yangbi county, Dali Prefecture, Yunnan on May 21, 2021. It is the biggest earthquake in the region during past 40 years, and its epicenter is located in the southwest boundary of the Sichuan-Yunnan rhomboid block. The type of this earthquake is of a typical “fore-main-residual” type, and cause no surface rupture, its aftershock sequence was not distributed along any known fault in the vicinity. There have been several research results which are on the seismogenic structure of this earthquake that occurred in Yangbi county, but it is also necessary to use a different type and source of data, methods and perspectives thinking angles to verify these results and supply new understandings. In this paper, based on the Yangbi sequence(M_L≥2.0)digital waveform recording and its earthquake phase data recorded by Yunnan Seismic Network between May 18, 2021 and June 13, 2021, the Yangbi sequence is relocated by HypoDD double-difference method and the spatiotemporal Yangbi sequence is also analyzed. The focal mechanism solution and centroid depth of the larger earthquakes in the sequence is obtained by the Cut & Paste(CAP)method. The results indicate that the Yangbi earthquake is distributed along the NW-SE direction as a whole, and its extension length is about 34km. The foreshock sequence has an obvious spatiotemporal migration and has round-trip activity characteristics, while the aftershock sequence has irregular spatiotemporal migration characteristics. The depth range of the aftershocks is mainly between 4km and 13km, and there were a few aftershocks whose depth are below 4km, which is reflecting that this series of earthquakes occurred in the shallow layer of the upper crust, and the rupture of the main earthquake may not extend to the surface. The trend of the belt of the aftershock is generally from the direction NW to SE, which has the obvious spatial segmentation: the aftershocks, which are located in the northwest of the main earthquake epicenter, are rare and relatively concentrated, while the aftershocks, which are located in the southeast, are dense and the width of the aftershock zone becomes larger; The foreshock sequence occurred in the southeast side of the epicenter of the main earthquake, which basically overlapped with the location of the dense segment of aftershocks, indicating that the sparse aftershocks in the northwest side of the main earthquake should belong to the triggering type, while the main earthquake rupture may belong to the unilateral rupture type extending from the epicenter to the SE direction. Besides, its fracture length is about 37km and its downdip width is about 16km. The depth cross-section of the foreshock sequence indicates that the focal depth of the sequence earthquake is generally deep in the southwest and shallow in the northeast, and the fault rupture surface is inclined to SW, with a large dip angle. While the depth cross-section of the aftershock zone shows that the main earthquake rupture is obviously segmented: the NW segment of the sequence has a simple structure, which is there existed one earthquake cluster, while the SE segment is relatively complex, which is there probably composed of two high-dip faults with SW inclination. The centroid depth of the 29 M_S≥3.0 events in the Yangbi sequence, mainly range from 3km to 13km, and their focal mechanism solutions are mostly of right-handed strike-slip type with a nodal plane of high dip Angle in NW-SE direction, and possess a certain normal fault component. In the NW segment of the sequence, the focal properties are mainly dextral strike-slip, and a few earthquakes which have positive fault components shows that there is a NW trending earthquake cluster with a SW inclination. Although the SE segment is still dominated by strike-slip faults, there are more positive faults, of which are two NW trending faults with the SW inclination. This difference reflects that the SE segment is likely to bifurcate and develop into two faults. The main shock is a right-handed strike-slip rupture, the source parameters of fault plane Ⅰ are strike 139°, dip 78° and slip angle -¹64°, and the source parameters of fault plane Ⅱ are strike 45°, dip 74°, and slip angle -12°. The centroid depth of this main shock is 5.2km, which is close to the predominant focal depth of 8.9km obtained by repositioning, indicating that the earthquake occurred in the upper crust, and the depth of seismic activity in the earthquake area is shallow. According to the spatial and temporal distribution characteristics of relocated sequence, combined with the focal mechanism solutions of theYangbi series in Yunnan in May 2021, it is indicated that both the Yangbi earthquake sequence and the source fault plane Ⅰ of main shock are NW-SE trending, which is in good agreement with the middle section of the Weixi-Qiaohou-Weishan fault(the closest to the epicentre). In addition, the focal mechanism solution of the sequence earthquakes is consistent with the properties of the Weixi-Qiaohou-Weishan fault, both of which are right-lateral strike-slip type. We conclude that the seismogenic structure of the Yangbi earthquake may be correlated with the Weixi-Qiaohou-Weishan fault, but the epicentre distribution of the sequence earthquakes is different from that of the Weixi-Qiaohou-Weishan fault. It is confirmed that in this fault, the seismogenic structure of this earthquake is a right-lateral strike-slip secondary fault with a steep dip toward SW on the west side of the southern section. Besides, in this fault, there is another NW trending branch fault in the SE section. In addition, combined with the results of the existing regional tectonic stress field in the focal area, it is believed that the earthquake should be caused by a right-handed strike-slip activity in the focal area which is under the force of NNW-SSE direction.

Key words: 2021 Yangbi M_S6.4 earthquake, earthquake sequence relocation, focal mechanism, seismogenic structure