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

摘要：

2021年5月21日21时, 云南大理白族自治州漾濞县发生 M_S6.4 地震。该地震的震中位于川滇块体的西南边界, 是该区40多年来震级最大的一次地震。地震未产生地表破裂, 余震也未沿震区附近已知的断裂分布。研究者针对这次地震的发震构造已有若干研究结果, 但采用不同数据、 方法和思考角度对这些结果进行验证并同时补充新认识是必要的。文中利用云南地震台网观测资料分析了漾濞地震序列的时空分布特征, 进行重新定位, 并通过CAP(Cut and Paste)方法获取序列中较大地震的震源机制解与矩心深度。结果表明, 漾濞地震的余震震源深度主要集中在4~13km, 余震带总体呈NW-SE走向, 空间分段性明显: 主震震中北西侧余震稀少且分布相对集中, 东南侧余震密集且余震带宽度变大; 前震序列发生在主震震中的东南侧, 与余震密集段的位置基本重叠, 反映主震震中北西侧的稀疏余震应属于触发型, 而主震破裂可能属于由震中向SE扩展的单侧破裂型。余震带的深度横剖面显示主震破裂具有明显的分段性, 序列北西段的结构较为简单, 显示出一个地震丛集, 而南东段则相对复杂, 很可能由2条倾向SW的高倾角断层组成。漾濞地震序列中29个M_S≥3.0地震的矩心深度主要分布于3~13km, 震源机制解以右旋走滑型为主, 大多存在一个NW-SE走向的高倾角节面, 且具有一定的正断倾滑分量。主震是以右旋走滑为主的破裂所引发的, 矩心深度为5.2km, 与重定位所得的震源初始破裂深度8.9km较为接近, 反映该地震发生在上地壳, 也反映震区的地震活动深度偏浅。文中对2021年5月云南漾濞序列的时-空分布特征与震源机制解的分析, 反映此次地震的发震断层产状及力学性质与NW走向的维西-乔后-巍山断裂较为一致, 但位置不同, 证实这次地震的发震构造是该断裂南段西侧一条SW陡倾的右旋走滑次级断裂。

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