STUDY ON THE SEISMOGENIC STRUCTURE OF THE 2022 GUJIAO ML4.1 EARTHQUAKE IN SHANXI PROVINCE BASED ON FOCAL MECHANISM AND SEISMIC LOCATION

doi:10.3969/j.issn.0253-4967.2024.02.010

Abstract

Abstract:

Understanding the mechanism of earthquake sequence in the mining area is important for the time-dependent hazard assessment. An earthquake of M_L4.1 occurred in Gujiao, Taiyuan, Shanxi on February 20th, 2022, which caused strong ground motion in Gujiao and surrounding counties. The epicenter of this earthquake is located in the area of Lvliang uplift, where historical earthquakes are relatively rare. In addition, the coal resources are well developed in the earthquake source area which has attracted much attention from society and local governments.

To investigate the mechanism and the seismogenic fault of Gujiao M_L4.1 earthquake, we first apply the double-difference location method to retrieve highly accurate hypocenter locations. The results show that the earthquakes mainly occur at a depth range of 3~5km, and display a dominant distribution direction nearly EW-trending, which differs significantly from the NE-trending fault distribution pattern in this region. We further collect the broad-band seismic waveforms from the regional network of Shanxi province to perform focal mechanism inversion. The inversion results show that the Gujiao earthquake is a left-slip seismic event with a moment magnitude of M_W3.96. The optimal double-couple solution is characterized by a strike of 90°, dip of 80°, and a rake angle of -21° for fault plane Ⅰ, while for the fault plane Ⅱ, the strike is 184°, dip is 69°, and rake angle is -169°. The best centroid depth is estimated to be at 3km. This earthquake shows an extremely shallow focal depth. Moreover, By using cluster analysis method, we obtained the central solution for the seismogenic fault plane of the GuJiao earthquake, with a fault strike of 91°and a dip angle of 70°. The focal solutions show that the earthquake exhibit a strike-slip type, and the orientations of earthquake sequence coincide well with the focal mechanisms.

In addition, to discuss the effect of Gujiao M_L4.1 earthquake on regional stress, we calculate the stress drop of this seismic sequence. The results show that the stress drop is significantly smaller than that of the regional earthquakes, exhibiting at least one order of magnitude lower than that of the background earthquakes in the same region. This phenomenon reflects that the stress level in the focal area of the GuJiao earthquake is not high, suggesting that the background stress enhancement in the focal area is not obvious.

Based on regional geological structure, we found that the known faults in the region are all high-angle normal faults, and the strike of these faults are inconsistent with the focal mechanism solution of Gujiao earthquake sequence, which suggests that the existing faults are not the seismogenic fault. Taking the regional mining activities into account, we speculated that mining may cause strong disturbance to the stress field, and lead to stress redistribution within the rock mass. Such coal mining activity may generate a high stress disturbance on the hidden fault plane, and then the fault become the carrier of stress transfer. So we conclude that the seismogenic mechanism of the Gujiao-seismic sequence may be related to coal mining activities near the focal area, which leads to local stress changes, thus resulting in the activation of preexisting hidden faults and triggering the occurrence of the Gujiao earthquake.

Key words: Gujiao earthquake, focal mechanism solution, hidden fault, gCAP method, seismic stress drop

摘要：

2022年2月20日山西太原古交市发生了M_L4.1地震, 古交及周边县市震感强烈。此次地震的震中位于历史地震相对稀少、煤矿资源丰富的吕梁隆起区, 其发震断裂尚不明确。为了更好地认识这次地震的发震构造, 文中利用山西省测震台网的近震波形资料反演了地震的震源机制解, 并获得最佳矩心深度。反演结果显示, 古交地震是一次左旋走滑型地震事件, 矩震级为 M_W3.96, 最佳双力偶解为: 节面Ⅰ, 走向90°、倾角86°、滑动角-19°; 节面Ⅱ, 走向182°、倾角70°、滑动角-175°; 最佳矩心深度为3km, 属于极浅源地震。地震序列重定位结果显示震中的优势展布方向近EW, 与区域内断层基本为NE走向的分布格局差异显著。利用聚类分析方法得到古交地震的发震断层面中心解, 结果显示断层的走向为91°, 倾角为70°。另外, 古交地震序列的应力降显著偏小, 低于同区域背景地震至少1个数量级。结合实地调研的矿区生产情况和区域地质构造, 认为古交地震序列的发震机制可能与震源区附近的煤田开采活动有关, 开采活动致使局部应力改变, 从而导致先存隐伏断层活化, 进而触发此次地震。

关键词: 古交地震, 震源机制解, 隐伏断层, gCAP方法, 地震应力降

DONG Chun-li, ZHANG Guang-wei, LI Xin-wei, WANG Yue-jie, DING Da-ye, GONG Zhuo-hong. STUDY ON THE SEISMOGENIC STRUCTURE OF THE 2022 GUJIAO M_L4.1 EARTHQUAKE IN SHANXI PROVINCE BASED ON FOCAL MECHANISM AND SEISMIC LOCATION[J]. SEISMOLOGY AND GEOLOGY, 2024, 46(2): 414-432.

董春丽, 张广伟, 李欣蔚, 王跃杰, 丁大业, 宫卓宏. 基于震源机制和地震定位研究2022年山西古交M_L4.1地震的发震构造[J]. 地震地质, 2024, 46(2): 414-432.

Figures/Tables 13

Fig. 1 Geological structure of the research area and location of earthquake stations in this study.

Fig. 2 Distribution of regional and local faults in Gujiao area(modified after Hydrogeology survey report of Gujiao Ordovician Limestone).

Table1 Velocity model used for earthquake relocation

上界面深度/km	0	4	14	23	40	80
P波速度/km·s^-1	4.80	5.90	6.30	6.75	7.90	8.10

Fig. 3 Comparison of the Gujiao earthquake sequence before(a)and after(b)precise localization.

Fig. 4 Comparison between synthetics(grey)and observed(black)seismograms of Feb.20, 2022 Gujiao ML4.1 event.

Fig. 5 Distribution of seismic mechanisms at different depths with the gCAP method.

Table2 The Gujiao ML4.1 earthquake and six ML≥2.0 aftershocks on 20 February 2022

序号	日期	发震时刻	北纬/(°)	东经/(°)	震级/M_L	深度/km
主震①	2022-02-20	08:27:27.4	37.954	112.108	4.1	4
余震②	2022-02-21	08:49:11.3	37.942	112.114	2.2	5
余震③	2022-02-22	04:31:01.7	37.953	112.115	2.9	3
余震④	2022-02-22	18:36:07.0	37.952	112.102	2.9	5
余震⑤	2022-02-22	22:28:53.5	37.950	112.106	2.5	5
余震⑥	2022-02-26	21:42:20.3	37.951	112.109	2.6	5
余震⑦	2022-03-21	01:30:01.9	37.952	112.118	2.8	4

Table3 Parameters of the ML≥2.0 source mechanism solution in the Gujiao earthquake sequence

使用方法	节面Ⅰ/(°)			节面Ⅱ/(°)			P轴/(°)		T轴/(°)		B轴/(°)		矩震级 /M_W	矩心深度 /km
使用方法	走向	倾角	滑动角	走向	倾角	滑动角	走向	倾角	走向	倾角	走向	倾角	矩震级 /M_W	矩心深度 /km
gCAP	90	86	-19	182	70	-175	45	18	138	10	256	69	4.0	3.2
Snoke①	94	52	-12	192	80	-141	60	34	317	19	203	50
Snoke②	92	66	51	334	45	144	208	12	315	52	109	35
Snoke③	267	81	-65	16	27	-160	204	48	337	31	83	25
Snoke④	92	63	-14	188	78	-152	53	28	317	10	210	60
Snoke⑤	92	66	33	348	61	152	219	3	312	40	125	50
Snoke⑥	94	52	-12	192	80	-141	60	34	317	19	203	50
Snoke⑦	89	74	-12	183	79	-163	47	20	315	4	216	70

Fig. 6 The epicenter location distribution and source mechanism solution of the seven ML≥2.0 earthquakes in the Gujiao sequence by using the Snoke method.

Fig. 7 Comparison of the Gujiao earthquake sequence with the regional seismic stress drop.

Fig. 8 Spatial distribution of mainshock and aftershock activity within one week.

Fig. 9 The results of clustering of the nodal planes in the Gujiao earthquake sequence.

Fig. 10 Distribution of fault and mining block in central earthquake area.

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