THREE-DIMENSIONAL WAVE VELOCITY STRUCTURE AND SEISMOGENIC STRUCTURE FOR THE JIANGDU EARTHQUAKE SWARM IN JIANGSU

doi:10.3969/j.issn.0253-4967.2025.05.20240114

Abstract

Abstract:

Since April 23, 2023, multiple earthquakes have occurred in Jiangdu, Jiangsu Province. On April 27, 2023, at 09:39 local time, an M3.1 event struck the region, followed by a series of seismic activities that constituted the Jiangdu earthquake swarm(hereafter referred to as “Jiangdu swarm 1”). Seismicity gradually diminished by June 22, 2023. On May 28, 2024, renewed seismic activity was observed in the same area, forming another swarm(hereafter “Jiangdu swarm 2”). On July 8, 2024, at 16:07 local time, an M3.6 earthquake occurred, after which activity again subsided by July 16, 2024. To investigate the velocity structure, seismotectonic setting, and possible relationship between these two swarms, both sequences were analyzed collectively(hereafter referred to as the Jiangdu earthquake swarm).

In this study, PhaseNet, a deep learning-based phase detection method, was employed to detect earthquakes in the epicentral area. The HypoDD algorithm was then used for precise relocation, producing a high-resolution catalog of the Jiangdu swarm. Additionally, seismic reports from January 2009 to July 2024 covering Jiangsu and adjacent provinces were compiled. Using the TomoDD double-difference tomography method, we inverted the three-dimensional velocity structure of V_P, V_S, and Poisson's ratio in the epicentral area. To constrain the seismogenic fault properties, focal mechanism solutions for seven M_L≥3.0 earthquakes were obtained with the HASH algorithm. Integrating precise locations, 3D velocity structures, and focal mechanisms, we identified the seismogenic faults of the Jiangdu swarm and analyzed its seismotectonic environment.

The results show that earthquakes in the Jiangdu swarms exhibit two predominant alignments, trending NNW and NNE, with focal depths concentrated between 7~16km. For Jiangdu swarm 1, focal mechanisms of four earthquakes indicate a NW-striking plane I, consistent with the NNW alignment of the relocated sequence. This plane is interpreted as the causative fault, which is a left-lateral strike-slip structure with minor normal faulting. For Jiangdu swarm 2, focal mechanisms of three earthquakes reveal a NE-striking plane I, consistent with the NNE alignment, and interpreted as a left-lateral strike-slip fault with a minor reverse component. Overall, the seven focal mechanism solutions show good agreement with the relocation results, indicating predominantly sinistral strike-slip motion.

Near the epicentral area, remarkable velocity contrasts are observed, with the Chenjiapu-Xiaohai Fault exerting a significant segmentation effect. The NW side of the fault is marked by low velocity and low Poisson's ratio anomalies, while the SE side displays increasing high-velocity anomalies with depth. Strong stratification of velocity and Poisson's ratio is also evident. The Jiangdu swarm is situated in a low-V_P, low-V_S, and low-Poisson's ratio anomaly zone, where the drop in P-wave velocity is more pronounced than in S-wave velocity, suggesting no involvement of fluids during the sequence. The low Poisson's ratio and narrow fault zone indicate that brittle fracture of rock strata was the dominant mechanism.

The Jiangdu source region is rich in shale oil and gas. The abundant shale gas is adsorbed in the pores and fractures of the rock formation. Long-term extraction and hydraulic fracturing enlarge rock fractures, reducing the effective elastic modulus and lowering seismic wave velocities. Based on precise relocation, seismogenic fault geometry, and crustal velocity structures, we infer that the two seismogenic faults of the Jiangdu swarm are likely subsidiary branches of the Chenjiapu-Xiaohai Fault. One is a concealed left-lateral strike-slip fault trending SSE-NNW, and the other is a concealed left-lateral strike-slip with thrust component trending SSW-NNE. The brittle failure of the hard rock strata directly triggered the Jiangdu swarms, representing two concentrated episodes of stress release. These findings provide new insights into the seismogenic environment and mechanisms of earthquake swarms in the Jiangdu region.

Key words: Jiangdu earthquake swarm, deep-learning, accurate earthquake location, TomoDD, velocity structure, seismogenic structure

摘要：

江苏省江都地区地处下扬子板块的苏中盆地, 是在中古生界复杂构造基础上演变形成的中新生代陆相沉积盆地, 区内发育大量演化的断裂、隆起及坳陷构造。文中基于深度学习的PhaseNet检测技术, 对2023年1月1日-2024年7月31日江都震群及附近地区进行地震重检测和精定位, 并通过TomoDD双差层析成像方法反演震源区的三维速度结构。基于地震精定位结果和反演的三维速度结构特征, 结合震源区M_L3.0以上地震的震源机制解, 对江都震群的孕震环境和发震机制进行了深入讨论。研究结果表明: 1)江都震群发震断裂为2条未知的左旋走滑隐伏断裂。其中, 江都震群1序列的发震断裂沿NNW走向, 断层面倾向为NE向; 江都震群2序列的发震断裂沿NNE向, 断层面略向NW倾。2条发震断裂近直立, 破裂均未延伸至地表。2)震源区附近波速变化剧烈, 陈家堡-小海断裂对震源区存在明显的分割作用, 断裂的北西侧存在明显的低波速、低泊松比异常。随着深度的增加, 断裂南东侧的高速异常逐渐增强。3)江都震群序列整体位于V_P、 V_S和泊松比低值异常体内, 且P波速度比S波速度下降更快, 整个发震过程没有地下流体的渗入与参与。基于发震断裂的性质及震源区速度结构特征, 分析认为江都震群序列的2次发震断裂或为陈家堡-小海断裂的2条分支断裂, 一条呈SSE-NNW向展布的左旋走滑隐伏构造, 一条呈SSW-NNE向展布的左旋走滑兼逆冲隐伏构造。江都震群是震源区区域应力的2次集中释放过程, 其直接成因是由震源区坚硬岩层发生脆性破裂。

关键词: 江都震群, 深度学习, 精定位, TomoDD, 速度结构, 构造特征

LI Ting-ting, MIAO Fa-jun, SUN Ye-jun, FAN Wen-hua, GONG Jie, GU Qin-ping, DU Hang, SUN Xiao-hang, ZHANG Cen, LI Zi-ye. THREE-DIMENSIONAL WAVE VELOCITY STRUCTURE AND SEISMOGENIC STRUCTURE FOR THE JIANGDU EARTHQUAKE SWARM IN JIANGSU[J]. SEISMOLOGY AND GEOLOGY, 2025, 47(5): 1343-1363.

李婷婷, 缪发军, 孙业君, 范文华, 宫杰, 顾勤平, 杜航, 孙小航, 张岑, 李子烨. 江苏江都震群震源区速度结构及构造特征[J]. 地震地质, 2025, 47(5): 1343-1363.

Figures/Tables 13

Fig. 1 Seismic structure map of Suzhong Basin(a), distribution of faults and historical earthquakes in the epicenter region of Jiangdu earthquake(b).

Fig. 2 Phase time distance curves of P-wave and S-wave.

Fig. 3 The hypocentres, stations and rays in the horizontal plane for VP and VS tomography.

Table1 Initial 1-D velocity model

序号	深度/km	V_P/km·s^-1	V_S/km·s^-1
1	0	3.792	2.239
2	5	5.121	3.025
3	10	6.062	3.581
4	15	6.212	3.669
5	20	6.451	3.810
6	25	6.500	3.759
7	30	6.824	3.972
8	34	7.400	4.299
9	40	8.100	4.6900

Fig. 4 L-curves of the smoothing and damping factors.

Fig. 5 Histograms of arrival time residuals for P-phase and S-phase.

Fig. 6 P-wave and S-wave checkerboard resolution at depths of 0km、 5km、 10km、 15km and 20km.

Fig. 7 Comparison of magnitude-frequency in different catalogs.

Fig. 8 M-T plot and epicenter distribution of earthquakes from manually and automatically detected catalogs.

Fig. 9 Tomography results of P-wave and S-Wave velocity and poisson's ratio at different depth.

Fig. 10 Depth profiles of seismic precise positioning distribution and focal mechanism solutions of selected earthquakes with magnitude ML≥3.0.

Table2 Focal mechanism solutions of selected earthquakes with magnitude ML≥3.0

序号	节面Ⅰ			节面Ⅱ			发震时刻	震级/M_L
序号	走向 /(°)	倾角 /(°)	滑动角 /(°)	走向 /(°)	倾角 /(°)	滑动角 /(°)	发震时刻	震级/M_L
1	305	75	-10	33	78	-136	2023-04-27, 09:39:59	3.8
2	312	71	-23	42	75	-125	2023-05-10, 23:20:43	3.5
3	298	68	-26	205	80	-145	2023-06-19, 02:15:24	3.4
4	293	75	-11	199	77	-138	2023-07-10, 14:17:43	3.4
5	234	73	39	323	72	135	2024-07-06, 21:31:33	3.4
6	226	76	37	318	63	126	2024-07-08, 16:07:19	4.1
7	246	69	29	321	64	140	2024-07-11, 05:22:18	3.6

Fig. 11 Poisson's ratio profile and P-wave and S-wave velocity near the source area of the Jiangdu earthquake swarm.

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