Abstract: The Gaoyou-Baoying M_S4.9 earthquake on July 20, 2012 occurred in the Gaoyou Sag in the Subei Basin. This earthquake was a relatively rare medium-strength earthquake in the weak seismicity region of eastern China. Although studies on the seismogenic structure of this earthquake have been conducted previously, the seismogenic structure itself is still under debate and needs to be further studied. This paper uses the methods such as distribution of seismic intensity, precise positioning of earthquake sequence, focal mechanism, regional tectonic stress, seismic exploration, etc. to comprehensively study the seismogenic structure of this earthquake.
The characteristics of earthquake sequence show that the seismic structure is a high dip-angle fault spreading along the NNE direction, dipping ESE. The result of focal mechanism solutions shows that the strike of one of the two nodal planes is NNE, and the fault plane shows high dip angle. The earthquake is mainly characterized by strike-slip motion. Through the seismic exploration lines(GYL1, GYL2)laid at the epicenter area of the earthquake, a fault structure is identified, which strikes nearly NNE and dips near ESE. This fault is located between the Linze sag and the Liubao low uplift, coinciding with the distribution of the Liuling Fault, the boundary fault in the northwest of the Gaoyou Sag, so it can be judged that all the detected breakpoints belong to the Liuling Fault. The “Y-shaped” breakpoints detected by the two seismic exploration lines are characterized by high dip angle. There is a very obvious wave group disorder area at the distance of 6 500~9 000m on the GYL1 seismic exploration line. This area is about 2.5km in width displayed on the post-stack migration profile and shows an uplifting trend. The disordered uplifting of wave group is caused by intrusion of soft material into the structural breakage and weakness, squeezed by horizontal stress. The GYL2 post-stack migration profile shows obvious uplift appearing in the reflection wave group(T_g)on the top of the bedrock. This arc-shaped uplift also reflects the effect of strong compression of horizontal stress.
In order to further discuss the seismogenic structure of the Gaoyou-Baoying M_S4.9 earthquake, we used the focal mechanism data to invert the modern tectonic stress field in the Northern Jiangsu-South Yellow Sea Basin where the earthquake occurred. The maximum principal stress in this area is NE-SW, while the minimum principal stress is NW-SE; both of them are nearly horizontal, and the intermediate principal stress is nearly vertical. According to Zoback's rule for dividing the types of dislocation in the direction of the force axis, the distribution of principal stresses in the Northern Jiangsu-South Yellow Sea Basin is equivalent to a strike-slip dislocation.
To sum up, the stress characteristics reflected by the Liuling Fault are consistent with the horizontal forces on the P-axis and T-axis shown by the focal mechanism solution results, and also consistent with the horizontal state of the stress in the tectonic stress field in this region. The above characteristics indicate that the development of the Liuling Fault is affected and controlled by modern tectonic activities. At the same time, the characteristics of the strike and dip of the seismic fault reflected by the methods of seismic intensity investigation, precise earthquake positioning, focal mechanism solution and seismic exploration, etc. are consistent with each other. Therefore, the occurrence of this earthquake may be the result of continuous stress accumulation and sudden instability and rupture of the NNE-trending Liuling Fault under the long-term compression of the NE-direction principal stress.

Key words: Gaoyou-Baoying M_S4.9 earthquake, seismogenic structure, Gaoyou Sag, Liuling Fault, seismic exploration

摘要： 2012年7月20日高邮-宝应M_S4.9地震发生在苏北盆地高邮凹陷内, 此次地震是近年来东部弱震区内的一次较为显著的中等强度地震。 前人在一些研究中对此次地震的发震构造进行过讨论, 但已有结果仍存在诸多不确定性。 文中综合地震序列精确定位、 震源机制解、 地震烈度调查等研究结果, 结合在震区获得的反射地震勘探结果, 对高邮-宝应M_S4.9地震的发震构造进行了深入研究, 并对其发生机理和发震断裂的潜在危险性进行了探讨。 研究结果显示: 1)高邮-宝应M_S4.9地震的发震构造可能为柳菱断裂, 该断裂是一条NNE走向、 SEE倾向的高倾角右旋走滑断层, 为高邮凹陷的西北边界断裂; 2)本次地震是在NE向主应力长期挤压作用下, NNE走向的柳菱断裂不断积累应力、 突发失稳破裂的结果; 3)4.9级尚不能代表柳菱断层的最大发震能力, 仍需关注该断裂进一步活动的危险性。

关键词: 高邮-宝应M_S4.9地震, 发震构造, 高邮凹陷, 柳菱断裂, 地震勘探