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

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