Abstract:

The seismic activity in Dongtai, Jiangsu, suddenly intensified from November to December 2022. The largest earthquake observed during this period was a magnitude M_S3.0 event on 25 December, which was felt reported by many nearby residents and caused a certain degree of social impact. The Dongtai area is situated in the central part of Jiangsu Province, within the Dongtai depression in structure, which is a secondary tectonic unit in the southern part of the Subei-South Yellow Sea Basin. Multiple fault zones developed in the region. The prominent known fault zones near the epicenter include the Taizhou fault, the Chenjiabao-Xiaohai Fault, and the Benchahe Fault. Among them, the closest to the epicentral area is the Taizhou fault. Additionally, the Subei Basin has a long history of industrial activity. Its geological conditions are complex, and the resources are extremely scattered and fragmented. The scale of underground resource extraction is predominantly small to medium-sized and has entered the middle to high exploration level. Historically, Dongtai has experienced weak seismic activity with only six earthquakes of M_S≥3 within 50 kilometers of the epicenter since 1970. The sudden increase in seismic activity prompts investigation into its cause. Analyzing the structural features of the Dongtai earthquake sequence can enhance understanding of seismic activity and seismogenic mechanisms in the region.

Previous studies on regional velocity structure have primarily focused on large scales, such as the Tan-Lu fault zone, with no specific research dedicated to the Dongtai earthquake sequence. In this study, we collected earthquake arrival time data recorded by the China Earthquake Networks Center from 2008 to 2022. Employing the double-difference tomography method, we conducted a joint inversion to investigate the velocity structure and earthquake locations in the Subei Basin. The resulting outcomes include the three-dimensional P-wave velocity structure of the area and the relocation results of 22 events within the seismic sequence. Furthermore, utilizing clear P-wave initial motion data from station waveform records, we inverted the focal mechanism solutions of the earthquake sequence using a modified grid search method. By integrating these inversion results with data on fault distribution and local industrial activity, we discussed the earthquake-triggering mechanism and possible seismogenic structures.

The results indicate that: 1)Following relocation, the seismic sequence exhibits a zonal distribution pattern. The earthquakes are predominantly situated north of the Tai-Zhou fault in a nearly north-south orientation, spanning approximately 15 kilometers in total length, with a predominant depth range of 11 to 16 kilometers. Notably, there is no apparent correlation between the earthquakes and the surrounding known fault structures. 2)The focal mechanism solution parameters for the largest earthquake in the sequence, M_S3.0, suggest a strike-slip seismogenic structure with a minor normal component. The direction of the stress axis aligns closely with the current tectonic stress field of the study area. Based on the focal mechanism solution and the distribution of the sequence, it is inferred that a dextral strike-slip hidden structure trending in a NNE-SSW direction may exist beneath the sequence. 3)The velocity structure of the epicenter area exhibits significant heterogeneity. The middle crust displays relatively high velocity, while the lower crust shows relatively low velocity. Notably, a spindle-shaped high-velocity anomaly with a P-wave velocity of 6.25km/s is observed at a depth of approximately 15km. The earthquakes primarily cluster southeast of this anomaly. 4)By examining the relationship between the spatial locations of earthquakes and their occurrence times, it is observed that the epicenters exhibit a seismogenic process extending far from the edge of the anomalous body. This suggests the outward release of accumulated elastic energy within the high-velocity anomaly, indicating a potential relationship between earthquake occurrences and the velocity anomaly. 5)Through on-site investigations of the epicentral area, data regarding local industrial activities have been collected. It was observed that three new wells and multiple industrial operation points have been established in the seismic area. Remarkably, 73% of earthquakes in the seismic sequence occurred within a 4.6km radius of well H1, with the largest earthquake in magnitude located approximately 1km from the well. A notable correspondence is observed between the Wulie-Shiyan-Qindong extraction points, the seismic sequence, and the deep high-velocity anomaly. Additionally, the operational timeframe of newly developed wells in the region closely aligns with the timing of earthquakes. However, the dominant depth of seismicity does not correspond with the drilling depth. A preliminary inference suggests that the occurrence of the earthquake sequence may be linked to the deep heterogeneous velocity structure, while industrial production activities near the epicenters may induce alterations in the regional stress state, leading to stress destabilization and subsequent energy release.

Key words: Dongtai earthquake sequence, double-difference tomography, earthquake relocation, P-wave velocity structure, Subei Basin

摘要：

2022年江苏东台接连发生数十次小地震, 其中, 震级最大的 M_S3.0 地震导致震中附近震感明显。 为探究该地区近期地震相对活跃的原因, 文中收集了2008—2022年的地震走时资料, 利用双差层析成像方法对苏北盆地的速度结构和地震位置进行了联合反演, 获得了震中区P波三维速度结构和地震精确定位结果, 并结合断裂分布、 震源机制解和当地工业活动等资料, 对地震序列可能的发震机制和发震构造进行了探讨。 结果显示: 1)重定位后, 地震序列总体呈近SN向带状展布, 发震优势深度在11~16km, 与已探明的断裂带缺乏明显联系。 结合震源机制结果推测, 序列下方可能存在NNE-SSW向隐伏构造。 2)震中区下方速度分布具有明显的非均匀性特征, 15km深度处存在纺锤状P波高速异常体, 地震序列的震源位置表现出从该异常体向远处扩散的发震特点。 3)震中区附近现存多处工业活动, 其中五烈—时堰—溱东作业点、 地震序列和深部高速异常三者的位置存在一定的关联性, 且作业时间和发震时间相近。 由此推断, 此次地震序列的发生与深部非均匀速度结构有关, 震中附近的工业活动可能改变了区域应力状态, 导致了能量的释放。

关键词: 东台地震序列, 双差层析成像, 地震精定位, P波速度结构, 苏北盆地