2022年芦山MS6.1地震序列的精确定位及发震构造

doi:10.3969/j.issn.0253-4967.2023.04.011

摘要/Abstract

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

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

傅莺, 胡斌, 赵敏, 龙锋. 2022年芦山M_S6.1地震序列的精确定位及发震构造[J]. 地震地质, 2023, 45(4): 987-1005.

FU Ying, HU Bin, ZHAO Min, LONG Feng. PRECISE LOCATION AND SEISMOGENIC STRUCTURE OF THE 2022 LUSHAN M_S6.1 EARTHQUAKE[J]. SEISMOLOGY AND GEOLOGY, 2023, 45(4): 987-1005.

图/表 9

图 1 a 区域大地构造简图; b 研究区初始震中分布图 a F1玛曲-荷叶断裂; F2龙日坝断裂; F3阿坝断裂; F4岷江断裂; F5雪山梁子断裂; F6虎牙断裂; F7抚边河断裂; F8米亚罗断裂; F9青川断裂; F10大邑隐伏断裂; F11双石-大川断裂; F12盐井-五龙断裂; F13耿达-陇东断裂; F14山前隐伏断裂; F15灌县-江油断裂; F16汶川-茂汶断裂; F17北川-映秀断裂; F18蒲江-新津断裂; F19龙泉山断裂带; F20锦屏山断裂; F21大凉山断裂; F22汉源-甘洛断裂; F23峨嵋-烟峰断裂; F24马边断裂带; F25长山镇断裂; F26华蓥山断裂带。 b F1茂汶-汶川断裂; F2北川-映秀断裂; F3双石-大川断裂; F4新开店断裂

Fig. 1 Schematic map of regional tectonics(a)and location of initial epicenter of earthquakes in study area(b).

图 2 芦山 MS6.1 地震序列的震中和地震台站分布图蓝色三角形为固定台和历史流动台, 绿色三角形为此次2022年芦山 MS6.1 地震后布设的流动台

Fig. 2 Distribution of seismic stations in determination of the epicenter of MS6.1 Lushan earthquake sequence.

表 1 芦山地区一维速度模型

Table 1 One dimensional P wave velocity model of Lushan region

层号	1	2	3	4	5	6	7	8	9	10
顶层深度/km	0.0	4.0	8.0	14.0	16.0	18.0	20.0	25.0	28.0	43.0
P波速度/km·s^-1	5.47	5.87	6.04	6.11	6.20	6.38	6.47	6.74	6.82	7.31

图 3 芦山震区精定位震中分布图 F1茂汶-汶川断裂; F2北川-映秀断裂; F3双石-大川断裂; F4新开店断裂

Fig. 3 Epicenter distribution map of precise seismic location in Lushan earthquake area.

图 4 沿2013年芦山 MS7.0 地震序列长轴走向的震源深度剖面AA' a 2013-2021年; b 2022年。红色直线表示2013年芦山 MS7.0 地震的破裂终止点

Fig. 4 The focal depth profile AA' along the long axis of strike of the 2013 Lushan MS7.0 earthquake sequence.

图 5 沿BB'(a)、 CC'(b)、 DD'(c)和EE'(d)剖面的震源深度投影图图b给出了断层宽度和厚度的测量方式

Fig. 5 Plots of the focal depths at BB'(a)、 CC'(b)、 DD'(c)and EE'(d)profile.

图 6 2022年6月1日芦山 MS6.1 地震的震源机制解反演残差随深度的分布图

Fig. 6 Variation of fitting error with depth in the focal mechanism inversion for the MS6.1 Lushan earthquake on June 1, 2022.

图 7 2022年6月1日芦山 MS6.1 地震震源机制和理论波形与观测波形拟合图红色实线为理论地震图; 黑色实线为观测地震图; 台站名上方的数字为震中距(km); 台站名下方的数字为方位角(°); 波形下方第1排数字为理论地震图相对观测地震图的移动时间(s); 波形下方第2排数字为理论地震图与观测地震图的相关系数(%)

Fig. 7 Focal mechanism solution and comparison between observed and synthetic waveforms of the MS6.1 Lushan earthquake on June 1, 2022.

表2 2022年6月1日芦山 MS6.1 地震震源机制解

Table2 Focal mechanism solution of Lushan MS6.1 earthquake

发震时间	震中位置		深度/km	节面Ⅰ/(°)			节面Ⅱ/(°)			P轴 /(°)		T轴 /(°)		B轴/(°)		震级		资料来源
发震时间	东经/(°)	北纬/(°)	深度/km	走向	倾角	滑动角	走向	倾角	滑动角	方位角	仰角	方位角	仰角	方位角	仰角	M_W	M	资料来源
2022-06-01-17:00:08.24	102.943	30.382	14	221	40	105	22	52	78	120	6	242	79	29	10	5.67	6.1	本文
2022-06-01-17:00:08.00	102.958	30.395	19.5	224	53	105	20	40	71	304	7	185	76	35	12	5.83		USGS^①
2022-06-01-17:00:11.80	103.06	30.34	21.7	216	44	101	21	47	80	118	1	213	81	28	9	5.8	5.8	GCMT^②
2022-06-01-17:00:08.37	102.91	30.36	16	211	31	93	28	58	88	119	13	292	76	29	1	5.8		GFZ^③
2022-06-01-17:00:08.50	103.12	30.51	17.5	218	52	94	31	38	85	305	7	149	82	35	3	5.8		CPPT^④
2022-06-01-17:00:08.00	102.94	30.37	17	211	39	98	22	52	84	116	6	259	82	25	5	6		中国地震局地球物理研究所^⑤

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