基于构造应力场研究2025年西藏定日6.8级地震的发震构造

doi:10.3969/j.issn.0253-4967.2025.01.004

摘要/Abstract

摘要：

双力偶模型给出的震源机制解的2个节面具有等效性, 需要借助于其他资料或方法识别其中的发震断层面, 基于应力场识别震源机制解2个节面中的发震断层面即为方法之一。基于GCMT的震源机制解, 文中将上述方法应用于2025年1月7日西藏定日6.8级地震发震构造的研究, 获得以下结论: 1)震源区最大主压应力轴近直立, 最大主张应力轴近水平且方向近EW, 为正断层型应力场; 2)2025年1月7日西藏定日6.8级地震的发震断层为震源机制解中走向S、倾向W的节面, 为一次正断层型地震, 推断其发震断层为登么错断裂; 3)该地区历史地震发震断层面的识别结果显示西藏定日地区历史地震的发震断层走向S, 倾向W, 倾角范围为37°~48°, 断层类型为正断层。文中结果为该次地震发震构造的研究提供了地震学证据, 可为相关发震构造研究提供参考。

关键词: 西藏定日地震, 应力场, 断层不稳定系数, 震源机制解节面, 发震断层

Abstract:

On January 7, 2025, at 09:05 Beijing Time, an M_S6.8 earthquake struck Dingri County in Shigatse City, Xizang, as reported by the China Earthquake Networks Center. The earthquake occurred at 28.50°N, 87.45°E with a hypocentral depth of 10km, resulting in significant casualties and economic losses. In the immediate aftermath, major earthquake research institutions and seismologists, both domestic and international, promptly released the focal mechanism solution, providing crucial data for understanding the earthquake's origin and its seismogenic structure. However, the two nodal planes of the focal mechanism, derived from a double-couple source model, are equivalent, necessitating additional data or methodologies to distinguish the actual seismogenic fault plane. The parameters of the seismogenic fault are fundamental for the accurate calculation of ground motion maps, and they provide key information for seismic hazard assessment and post-earthquake rapid response guidance. Therefore, it is imperative to identify the seismogenic fault plane for the given focal mechanism solution.

This study employs the tectonic stress field in the source region of the Dingri earthquake to calculate the instability coefficients of the two nodal planes, selecting the most unstable plane as the actual seismogenic fault. This method is based on the tectonic stress field to identify the seismogenic fault plane in the two nodal planes of the focal mechanism solution. The approach is applied to identify the seismogenic fault plane of the Dingri earthquake and nearby historical seismic events.

Using the Global Centroid Moment Tensor(GCMT)focal mechanism solution, the study inverts the shallow tectonic stress field in the source region. The results reveal the maximum principal compressive stress axis is nearly vertical, and the maximum principal tensile stress axis is nearly horizontal with a strike orientation of E-W, which is a normal faulting stress regime. The stress field result is consistent with the normal faulting characteristics of the regions main fault structures.

The seismogenic fault for the Dingri 6.8 earthquake is the one-striking southward and dipping westward nodal plane of the focal mechanism solution, determined to be a normal fault. Thus, we can infer that the seismogenic fault is the Dengmocuo Fault. In addition, the identification of the seismogenic fault for the historical earthquakes in the Dingri area shows that the fault is characterized by a southward strike and westward dip, with dip angles ranging from 37° to 48°, and the fault type is normal faulting.

Identifying the seismogenic fault plane in the nodal planes of the focal mechanism solution based on the tectonic stress field, this study accurately identifies the seismogenic faults associated with the Dingri earthquake and surrounding historical events. It contributes seismological evidence for understanding the seismogenic structure of the region. It offers valuable insights for future research on seismogenic structures, particularly the determination of seismogenic faults of small and medium-magnitude earthquakes.

Key words: Dingri earthquake, Xizang, stress field, fault instability, the nodal plane of focal mechanism, seismogenic fault

盛书中, 王倩茹, 李振月, 李红星, 张小娟, 葛坤朋, 宫猛. 基于构造应力场研究2025年西藏定日6.8级地震的发震构造[J]. 地震地质, 2025, 47(1): 49-63.

SHENG Shu-zhong, WANG Qian-ru, LI Zhen-yue, LI Hong-xing, ZHANG Xiao-juan, GE Kun-peng, GONG Meng. INVESTIGATION OF THE SEISMOGENIC STRUCTURE OF THE 2025 DINGRI M_S6.8 EARTHQUAKE IN XIZANG BASED ON THE TECTONIC STRESS FIELD PERSPECTIVE[J]. SEISMOLOGY AND GEOLOGY, 2025, 47(1): 49-63.

图/表 7

图1 研究区地质构造背景图图中五角星示意定日地震的震中位置, 蓝色方框示意本研究的研究区, 登么错断层资料参考地质所四川局西藏局联合调查团队结果 ① , 其余断层资料参考文献(李雨森等, 2025)

Fig. 1 Tectonic background of the study area.

表1 震源机制解参数及其节面上的不稳定系数计算结果表

Table1 The parameters of the earthquake focal mechanisms and the fault instability of nodal planes

序号	日期	东经 /(°)	北纬 /(°)	震级		深度 /km	节面Ⅰ/(°)			节面Ⅱ/(°)			不稳定系数I
序号	日期	东经 /(°)	北纬 /(°)	M_W	M_S	深度 /km	走向	倾角	滑动角	走向	倾角	滑动角	节面Ⅰ	节面Ⅱ
1	1993-03-20	87.25	29.2	6.2	6.4	22	161	46	-121	22	52	-62	0.993	0.752
2	1993-03-20	87.4	29.0	5.1	5.2	33	357	75	-86	160	16	-106	0.991	0.715
3	2016-05-22	87.62	28.31	4.8	4.4	7	189	41	-66	338	53	-109	0.991	0.838
4	2016-05-22	87.60	28.36	5.3	5.4	9.4	203	45	-46	329	59	-125	0.970	0.864
5	2016-05-22	87.59	28.41	5.2	5.1	4.4	175	37	-83	346	53	-95	0.990	0.864
6	2020-03-20	87.42	28.63	5.7	5.9	10	184	47	-77	345	45	-103	0.990	0.726
7	2021-02-02	87.50	28.55	5.1	4.9	10.8	159	43	-96	347	48	-85	0.995	0.786
8	2021-02-04	87.52	28.64	4.9	4.8	10	168	41	-112	16	52	-72	0.999	0.794
9	2021-11-05	87.44	28.73	4.8	4.4	10	173	38	-96	1	52	-85	0.994	0.853
10	2025-01-07	87.45	28.50	7.1	6.8	10	173	48	-92	356	42	-88	0.982	0.676

图2 震源机制解空间分布及断层面识别结果图图中序号与表1中的地震序号对应; 震源机制解上的红色箭头示意识别出的发震断层走向

Fig. 2 Spatial distribution of focal mechanisms and the identification result of their seismogenic faults.

图3 a 应力场反演结果; b R值分布图图中不同颜色圆示意主应力轴的95%置信区间

Fig. 3 Stress field inversion results(a)and distribution of R-value(b).

表2 震源区应力场反演结果表

Table2 The stress field inversion results of the source region

最优主压应力轴σ₁		最优中间应力轴σ₂		最优主张应力轴σ₃		R值(不确定度)	摩擦系数μ
走向/(°)	倾伏角/(°)	走向/(°)	倾伏角/(°)	走向/(°)	倾伏角/(°)	R值(不确定度)	摩擦系数μ
272	76	175	2	84	14	0.15(±0.23)	0.40

图4 断层平均不稳定系数与摩擦系数取值关系图

Fig. 4 Relationship between average instability coefficient of fault and friction coefficient.

图5 不同倾角时断层面和辅助面的不稳定系数图

Fig. 5 The fault instability of fault planes and auxiliary planes with different dip angles.

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