CHARACTERISTICS AND SEISMIC STRUCTURE ANALYSIS OF THE MS5.9 AND MS4.7 EARTHQUAKE SEQUENCES IN ALUKEERQIN BANNER, INNER MONGOLIA

doi:10.3969/j.issn.0253-4967.2024.06.006

Abstract

Abstract:

The Alukeerqin Banner region in Chifeng city, Inner Mongolia, experienced two notable earthquakes, with magnitudes of M_S5.9 and M_S4.7, in 2003 and 2021, respectively. These were the most significant seismic events in the area in recent years, however, neither resulted in surface rupture. The distribution of aftershocks also did not align with known fault lines, and both the characteristics and mechanisms of these seismic activities remain unclear. To address this, we utilized data from the Inner Mongolia Seismic Monitoring Network to reposition the M_S5.9 and M_S4.7 earthquake sequences in Alukeerqin Banner.

The results indicate that the M_S5.9 and M_S4.7 earthquake sequences occurred on the western and eastern sides of the Shuiquanzigou Tianshankou fault, respectively, aligning in a northwest-southeast(NW-SE)direction. The main shocks are approximately 45km apart, with focal depths of 2-12km and 8-22km for aftershocks. The main earthquakes are situated in the southeastern portion of the aftershock sequences, which also trend NW-SE. Depth-profile analysis of the aftershock zones shows relatively simple structures, with clusters oriented in a NW trend and inclined southwest(SW). The 2003 M_S5.9 earthquake sequence exhibits a fault plane with a dip angle of approximately 60°, while the 2021 M_S4.7 earthquake sequence has a nearly vertical fault plane.

Using the CAP method and P-wave first-motion polarity analysis, we derived focal mechanism solutions and source depths for earthquakes of M_L≥1.5 in the region. The focal mechanism solution indicates that the M_S4.7 main shock primarily involved left-lateral strike-slip motion at a source depth of 19.9km, which closely matches the initial rupture depth of 21km obtained from relocation. Other significant earthquakes in the series similarly exhibit left-lateral strike-slip characteristics, with most developing along a NW-SE strike plane, consistent with the seismogenic fault characteristics identified in the relocated series. Previous research also shows that the main shock of the M_S5.9 earthquake sequence involved left-lateral strike-slip motion, with the B-node plane orientation(NW direction)aligning with the distribution of fine-located events and the long axis of the macroscopic intensity isoseismal line.

The temporal-spatial distribution and focal mechanism analyses of the M_S5.9 and M_S4.7 earthquake sequences suggest that their primary faults are consistent in strike and mechanical properties with the Shuiquanzi-Tianshankou Fault, trending NW but located at different positions. This confirms that the seismogenic structure of the M_S5.9 earthquake is likely a left-lateral strike-slip secondary fault on the western side of the Shuiquanzi-Tianshankou Fault, trending SW. The seismogenic structure of the M_S4.7 earthquake may be a concealed fault nearby with similar characteristics to the Shuiquanzi-Tianshankou Fault.

We also analyzed 71 earthquakes of magnitude 2 and above in the southeastern segment of the Daxing'anling uplift since 2012, based on observation data from the China Earthquake Networks Center. Using comprehensive focal mechanism inversion, we determined the regional P-axis distribution, finding that the primary compressive stress direction in the southeastern Daxing'anling uplift is predominantly NW and nearly east-west(EW). In the vicinity of Alukeerqin Banner, the P-axis orientation is mainly EW, reflecting a relatively simple stress field. The focal mechanisms of the 2003 M_S5.9 and 2021 M_S4.7 earthquakes are consistent with this regional stress field, suggesting that these earthquakes were likely caused by faulting influenced by the nearly EW-oriented regional principal compressive stress.

Key words: Alukeerqin Banner earthquake, earthquake sequence relocation, focal mechanism, seismogenic structure

摘要：

内蒙古赤峰市阿鲁科尔沁旗在2003年与2021年先后发生 M_S5.9 地震与 M_S4.7 地震。这2次地震是该地区近年来震级最大的地震事件, 均未造成地表破裂, 余震分布与已知断层的相关性不强, 地震活动特征和发震机制仍不甚明确。针对该问题, 文中使用内蒙古地震监测台网观测数据, 对阿鲁科尔沁旗2次地震序列进行重新定位。结果显示, M_S5.9 和 M_S4.7 地震序列的余震震源深度分别为2~12km和8~22km间, 主震位于余震序列的南东段, 余震带走向为NW-SE。余震带深度剖面分析显示2次地震序列结构相对简单, 均呈一个NW走向、倾向SW的地震集群。进一步采用CAP方法与P波初动极性方法, 得到了震源区M_L≥1.5地震的震源机制解和震源矩心深度。震源机制解结果显示, M_S4.7 主震的发震断裂为左旋走滑性质, 震源矩心深度为19.9km, 与重定位得到的震源初始破裂深度21km较为接近, 余震与主震的性质整体相同, 均发育NW-SE走向的断层面。前期研究结果显示, M_S5.9 地震序列的主震也为左旋走滑型。文中对阿鲁科尔沁旗 M_S5.9 与 M_S4.7 地震序列的时空分布特征与震源机制解的分析表明, 阿鲁科尔沁旗2次地震序列的主要发震断层的产状及力学性质应与NW走向的水泉子沟-天山口断裂较为一致, 但展布位置不同, 证实 M_S5.9 地震的发震构造应是位于该断裂西侧、倾向SW的一条左旋走滑次级断裂, M_S4.7 地震的发震构造可能是震中附近与水泉子沟-天山口断裂性质相近的一条隐伏断裂。2次地震序列均是发震构造受到近EW向主压应力作用导致的。

关键词: 阿鲁科尔沁旗地震, 地震序列精定位, 震源机制, 发震构造

WANG Xin, ZHANG Ke, WANG Yue. CHARACTERISTICS AND SEISMIC STRUCTURE ANALYSIS OF THE M_S5.9 AND M_S4.7 EARTHQUAKE SEQUENCES IN ALUKEERQIN BANNER, INNER MONGOLIA[J]. SEISMOLOGY AND GEOLOGY, 2024, 46(6): 1314-1331.

王鑫, 张珂, 王玥. 内蒙古阿鲁科尔沁旗M_S5.9与M_S4.7地震序列特征及其发震构造分析[J]. 地震地质, 2024, 46(6): 1314-1331.

Figures/Tables 13

Fig. 1 Seismo-tectonic background of the Alukeerqinqii MS5.9 and MS4.7 earthquakes(a), epicenter location of the earthquake sequence(ML≥0.0)(b), and seismic stations of the research area(c).

Fig. 2 The plot of amplitude-time of aftershocks in the MS5.9 (a) and MS4.7 (b) earthquake sequences in Alukeerqinqi, Inner Mongolia.

Table1 Crustal velocity model in the north region of Inner Mongolia

层号	顶层深度/km	v_P/km·s^-1
1	0	6.10
2	23.00	6.72
3	39.00	8.05

Fig. 3 Location residuals and travel time residuals by double-difference method.

Fig. 4 Epicenter location distribution(a), variation characteristics of the source depth along the latitude and longitude directions(b, c), and the distribution of the source depth after relocation(d).

Fig. 5 Depth distribution of aftershock sequence on different sections after relocation.

Fig. 6 Isoseismal distribution of earthquake intensity (a) and focal mechanism(b) of 2003 Alukeerqinqi MS5.9 earthquake.

Table2 Focal mechanism solutions of the 2003 MS5.9 Alukeerqinqi main earthquake

序号	节面Ⅰ			节面Ⅱ			P轴		T轴		震源球图解	资料来源
序号	走向 /(°)	倾角 /(°)	滑动角 /(°)	走向 /(°)	倾角 /(°)	滑动角 /(°)	方位 /(°)	仰角 /(°)	方位 /(°)	仰角 /(°)	震源球图解	资料来源
1	209	73	19	305	72	163	78	2	167	25		内蒙古地震局测震台网
2	202	56	174	296	85	34	64	20	165	27		USGS^①
3	46	NW	65.4	319.5	NE	70	267.5	17	7.5	52.2		内蒙古地震局地震分析预报中心
4	70	NW	80	172	NE	40	16	41	131	25		中国地震局地球物理研究所

Fig. 7 Variations of inversion error of focal mechanism solution with depth for the Alukeerqinqi MS4.7 earthquake by using the CAP method.

Fig. 8 Waveform fitting curves under the optimal inversion depth of 19.9km of the Alukeerqinqi MS4.7 earthquake by using the CAP method.

Table3 Focal mechanism solutions of typical ML≥1.5 events of the 2021 MS4.7 Alukeerqinqi earthquake sequence

序号	发震时刻		震中位置		震级 /M_L	深度 /km	节面Ⅰ			节面Ⅱ			震源球图解
序号	发震时刻		东经 /(°)	北纬 /(°)	震级 /M_L	深度 /km	走向 /(°)	倾角 /(°)	滑动角 /(°)	走向 /(°)	倾角 /(°)	滑动角 /(°)	震源球图解
1	2021-10-15	5:10:03	120.298	44.183	5.1	37	220	90	160	310	80	10
2	2021-10-15	5:24:48	120.292	44.187	2.9	21	227	62	169	322	80	29
3	2021-10-15	5:33:57	120.286	44.182	3.5	21	58	75	167	152	77	16
4	2021-10-15	10:48:14	120.254	44.213	2.9	14	219	70	178	310	88	10
5	2021-10-15	15:47:57	120.265	44.192	3.3	20	219	70	176	310	87	20
6	2021-10-16	5:05:11	120.281	44.197	2.4	22	270	90	170	360	80	0
7	2021-10-16	9:29:40	120.25	44.205	2.8	15	270	90	170	360	80	0
8	2021-10-20	8:55:29	120.275	44.188	2.8	19	69	85	150	162	61	6
9	2021-10-23	7:47:11	120.268	44.19	2.5	20	179	79	-126	41	62	-67
10	2021-11-13	9:25:03	120.284	44.187	3.2	21	169	36	-126	31	62	67
11	2021-12-20	8:36:57	120.286	44.185	2.3	21	220	70	-176	129	87	-20

Fig. 9 The simplified geological map of the study area.

Fig. 10 The distribution of the azimuth of P axis and the contradictory ratio.

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