Abstract:

On June 1, 2022, a M_S6.1 earthquake occurred in Lushan, Sichuan Province, western China, which is approximately 10km from the Lushan M_S7.0 event on April 20, 2013. To understand if the earthquake has the same seismogenic structure as the Lushan M_S7.0, we relocated the event in the Lushan area using the multi-stage locating method based on the seismic phase arrival data of the Sichuan Seismic Network from April 20, 2013, to July 1, 2022. A total of 6992 M_L≥1.0 earthquakes were acquired, with a relative locating error of 0.5km and 0.7km in the horizontal and vertical directions, respectively, with a travel time residual(RMS)of 0.18s. The results show that the M_S6.1 event is located at 102.943°E, 30.382°N with an initial-rupture focal depth of 15.6km, lying on the NW side of the 2013 Lushan M_S7.0 event. The sub-surface rupture length of the long and short axis is 10 and 8km, measured from the dense aftershock area in NE-SW and NW-SE directions, respectively. The NE-SW profile in the Lushan area shows that the depth of Lushan M_S7.0 earthquake in 2013 was about 15km, similar to that of Lushan M_S6.1 and M_S4.5 on June 1, 2022. The M_S6.1 earthquake sequence, located at the NE end of the long axis, shows no evidence to break through the rupture termination point of the Lushan M_S7.0 earthquake and enters the Dayi seismic gap, which is bounded by the 2008 Wenchuan M_S8.0 and 2013 Lushan M_S7.0 aftershock regions. The short-axis profile shows that the M_S6.1 earthquake sequence occurred on a new back-thrust fault in the pre-existing seismogenic structure of the 2013 Lushan M_S7.0. The new structure dips SE and ruptures in a slight arc protruding into the NW, parallel to the northern segment of the seismogenic structure of the 2013 Lushan M_S7.0 earthquake with a horizontal distance of about 5km. The new and old structures connect at the detachment base to the main segment of the 2013 Lushan M_S7.0 earthquake.

We also inverted the focal mechanism of the Lushan M_S6.1 earthquake using the CAP(Cut and Paste)method. The result indicates that the centroid depth of the M_W5.7 main event is 14km which is very close to the initial-ruptured depth of 15km calculated by the phase arrival times. The best double couple parameters are 221°/40°/105° for nodal plane Ⅰ and 22°/52°/78° for nodal plane Ⅱ. The parameters are in order of the strike, dip, and rake angles. Combined with the realization of the NE-striking, SE-dipping seismogenic structure characteristics determined by the accurate locating of the earthquake sequence, it can be quickly confirmed that the nodal plane Ⅱ is the fault plane.

Based on the accurate locating results, focal mechanism solutions, and geodynamic background of the focal area, it is inferred that the seismogenic structure of the Lushan M_S6.1 earthquake is induced by the thrust dislocation of a NE-SW trending and SE inclining thrust fault in the southern section of Longmenshan fault zone. Finally, we discussed the relationship between M_S7.0 and M_S6.1 in the Lushan area. The two could be considered a unique sequence: the mainshock and the maximum aftershock, respectively, regarding spatial relationship and tectonic correlation. However, the time interval of these two earthquakes significantly overextends the statistical relationship between the principal earthquake and the maximum aftershock. Furthermore, considering the effects of the Coulomb stress change produced by the earthquakes repeated at the end of the Dayi gap, Lushan earthquake further enhanced the stress level in the Dayi seismic gap located in its northern segment.

Key words: Lushan earthquake, earthquake location, focal mechanism, seismogenic structure

摘要：

2022年6月1日在四川芦山发生了 M_S6.1 地震。为厘清这次地震的发震构造与2013年4月20日芦山 M_S7.0 地震是否相同, 文中基于2013年4月20日-2022年7月1日四川地震台网记录到的芦山地区的震相数据, 采用多阶段地震定位方法进行了精确定位, 最终获得了6 992次M_L≥1.0地震事件的精确定位结果, 定位误差为: 水平向0.5km, 垂直向0.7km, 走时残差0.18s。主震的精确位置为(30.382°N, 102.943°E), 震源深度为15.6km。定位结果显示, 2022年芦山 M_S6.1 地震序列整体位于2013年芦山 M_S7.0 地震序列北东端的西南侧, 余震密集区NE-SW向长约10km, NW-SE向长约8km。整个芦山地震序列震中分布区域的长轴呈NE向, 沿其走向的震源深度剖面显示, 2022年6月1日芦山 M_S6.1 主震和 M_S4.5 最大余震的震源深度约为15km, 与2013年芦山 M_S7.0 主震的震源深度相当。 M_S6.1 地震序列位于长轴的北东端, 并未突破 M_S7.0 地震的破裂终止点而进入大邑地震空段。沿芦山地震序列震中分布区域短轴的震源深度剖面显示, M_S6.1 地震序列发生在一条新的反冲断层上, 该破裂面略呈弧形向NW突出, 位于既有的芦山 M_S7.0 发震构造的北西侧, 倾向SE, 与2013年芦山 M_S7.0 北段的反冲断层近平行, 水平间距约5km, 在底部连接至2013年芦山 M_S7.0 地震的主断面。同时, 文中采用CAP(Cut and Paste)方法反演得到此次芦山 M_S6.1 地震的震源机制解为逆冲型, 最佳双力偶解分别为: 节面I, 走向221°、 倾角40°、 滑动角105°; 节面Ⅱ, 走向22°、 倾角52°、 滑动角78°; 矩震级 M_W5.7, 矩心深度为14km, 与初始破裂深度一致, 结合精确定位余震分布及断裂走向结果判定节面Ⅱ为其发震断层面。根据精确定位结果、 震源机制解及震源区构造特征推测, 此次芦山 M_S6.1 地震为龙门山断裂带南段的一条走向NE-SW、 倾向SE的反冲断裂发生逆冲错动所致。文中也讨论了2次芦山 M_S7.0 和 M_S6.1 地震的关系, 认为从空间关系和构造相关来看, 可以将二者看作同一序列, 但这2次事件的发震时间间隔显然突破了统计意义上的主震-最大余震关系。此外, 此次地震的发生进一步增强了位于其北段的大邑地震空段的应力水平。

关键词: 芦山地震, 精确定位, 震源机制解, 发震构造