DISCUSSION ON SEISMOGENIC STRUCTURE OF THE JUNE 2022 MS6.1 EARTHQUAKE AND ITS RELATIONSHIP WITH THE APRIL 2013 MS7.0 EARTHQUAKE IN LUSHAN, SICHUAN PROVINCE

doi:10.3969/j.issn.0253-4967.2023.05.012

Abstract

Abstract:

In this paper, we relocated earthquakes occurred from April 2013 to July 2022 in Lushan seismic zone, inversed focal mechanism solution of the Lushan M_S6.1 earthquake on June 1, 2022 and discussed the seismogenic structure of the Lushan M_S6.1 earthquake and its relationship with the M_S7.0 earthquake in April 2013.

The results of the focal mechanism solution show that the Lushan M_S6.1 earthquake in 2022 is a thrust earthquake. The strike, dip and azimuth of nodal plane Ⅰ are 228°, 46° and 104° and for nodal plane Ⅱ are 28°, 46° and 76° respectively. The results of earthquake relocation show that the focal depth of the Lushan M_S6.1 earthquake sequence is shallow in the north and deep in the south, the fault length is about 10km. The focal depth is mainly concentrated between 10km to 19km. The fault dip is southeast with an angle of 60°. The initial rupture point of the main shock of the Lushan M_S6.1 earthquake is at a depth of 20km, located at the deepest part of the fault. The fault ruptured from deep to shallow. The Lushan M_S7.0 earthquake occurred on April 2013 strikes northeast and dips northwestward, but there exists a reverse fault in the aftershock sequence that has the same direction of strike but the opposite direction of dip. This reverse fault is consistent with the strike and dip of the M_S6.1 earthquake occurred in June 2022. It appears as two parallel faults in the profile. In addition to the reverse fault on the west side, the embryonic of another reverse fault seems to appear on the east side of the middle of earthquake sequence. These faults are about 10km away from the surface. The distribution of earthquakes in two northwest-oriented depth profiles shows that the dip angles of the main shock and the reverse fault of the M_S7.0 earthquake is different at different locations, and these faults are not simple straight planar sections. From one year after occurrence of the M_S7.0 earthquake to occurrence of the M_S6.1 earthquake, the seismic activity on the main fault decreased but the seismic activity on the reverse fault on the west side of the M_S7.0 earthquake sequence was more active during this period, most of the seismic activity occurred near the reverse fault that is parallel to the M_S6.1 earthquake fault.

By analyzing the seismogenic structure and seismic activity characteristics of the Lushan seismic zone, we concluded the Lushan M_S6.1 earthquake on June 1, 2022 is caused by a blind thrust fault with strike towards northeast and dip towards southeast, located 10km away from the surface. It has the opposite directions of strike and dip of the Longmenshan Fault. The epicenters of the Lushan M_S7.0 earthquake in April 2013 and the M_S6.1 earthquake in June 2022 are located near the surface exposure traces of the Shuangshi-Dachuan Fault and the Xiaoguanzi Fault, respectively. However, according to the analysis of the relocation aftershock depth in profile, the aftershock extension to the surface does not coincide with the surface exposure positions of the Shuangshi-Dachuan Fault and the Xiaoguanzi Fault. Therefore, the seismogenic faults of these two earthquakes are not the Shuangshi-Dachuan Fault and the Xiaoguanzi Fault, but two blind reverse faults. The Shuangshi-Dachuan Fault near the M_S6.1 earthquake sequence and the main shock fault of the 2013 M_S7.0 earthquake are thrust faults dipping northwest, while the Lushan M_S6.1 seismogenic fault has opposite direction of dip. The seismogenic fault of the Lushan M_S6.1 earthquake and the main thrust fault of the 2013 M_S7.0 earthquake, which strikes northeast and dips northwest with the reverse thrust fault of the hanging wall, which strikes northeast and dips southeast, together form a double layer Y-shaped structure. These faults are all blind thrust faults and belong to the Qianshan-Shanqian Fault system in the southern segment of the Longmenshan fault zone. The seismogenic structure in the Lushan seismic zone is a complex fault system composed of one main northeast strike fault with dipping northwest, and three faults dipping southeast.

From one year after occurrence of the Lushan M_S7.0 earthquake to the occurrence of the Lushan M_S6.1 earthquake, most of earthquakes in the Lushan seismic zone occurred near a reverse fault which is parallel to the Lushan M_S6.1 earthquake seismogenic fault. These earthquakes are located in the area where the coulomb stress change caused by the M_S7.0 earthquake acts as loading effect. The Lushan M_S6.1 earthquake sequence is mainly distributed in the area where the coulomb stress change plays an unloading role caused by the Lushan M_S7.0 earthquake. The research results showed that the coulomb rupture stress caused by the Lushan M_S7.0 earthquake on the seismic nodal plane of the M_S6.1 earthquake has a restraining effect on the M_S6.1 Lushan earthquake.

Key words: Lushan earthquake, precise earthquake relocation, focal mechanism, seismogenic structure

摘要：

文中对2013年4月—2022年7月发生在芦山震区的地震进行重定位, 并反演了芦山2022年6月1日 M_S6.1 地震的震源机制解。芦山 M_S6.1 地震序列的震源深度北浅南深, 断层长约10km, 自深向浅破裂。通过分析芦山震区的发震构造和地震活动特征, 认为震源断层为一条走向NE、倾向SE的盲逆冲断层, 其与2013年 M_S7.0 地震走向NE、倾向NW的主逆冲断层及其上盘走向NE、倾向SE的反向逆冲断层共同组成双层“Y”字形结构。进一步综合分析库仑应力变化, 认为2013年芦山 M_S7.0 地震对2022年芦山 M_S6.1 地震具有抑制作用。

关键词: 芦山地震, 地震精定位, 震源机制, 发震构造

CHEN Han-lin, WANG Qin-cai, ZHANG Jin-chuan, LIU Rui-feng. DISCUSSION ON SEISMOGENIC STRUCTURE OF THE JUNE 2022 M_S6.1 EARTHQUAKE AND ITS RELATIONSHIP WITH THE APRIL 2013 M_S7.0 EARTHQUAKE IN LUSHAN, SICHUAN PROVINCE[J]. SEISMOLOGY AND GEOLOGY, 2023, 45(5): 1233-1246.

陈翰林, 王勤彩, 张金川, 刘瑞丰. 四川芦山2022年6月 M_S6.1 地震的发震构造及其与2013年4月 M_S7.0 地震关系的探讨[J]. 地震地质, 2023, 45(5): 1233-1246.

Figures/Tables 7

Fig. 1 Location of stations, distribution of faults and epicenters in Lushan seismic zone and surrounding areas.

Fig. 2 Waveform fitting for inversion of focal mechanism solution of the MS6.1 earthquake on June 1, 2022.

Fig. 3 Profile of epicenter and depth distribution of magnitude MS6.1 earthquake sequence after precise location.

Fig. 4 Plot of magnitude-frequency of earthquakes in Lushan seismic zone.

Fig. 5 Magnitude and monthly frequency in Lushan seismic zone with time.

Fig. 6 Relocation results in Lushan seismic zone for earthquakes from April 2013 to July 2022.

Fig. 7 Time-space evolution of earthquakes in Lushan seismic zone.

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