# A New Look at the Stress State Across the Bohai Strait, China
## 1 Introduction  
In-situ stress, a crucial aspect of geomechanics research, directly drives crustal deformation and fault activity [1, 2] and can also induce the failure of rock masses in engineering [3, 4]. Thus, estimation of in-situ stress field is essential for understanding the mechanisms of earthquakes [5, 6], solving geomechanical issues in energy exploration [7, 8], assessing the geological safety of engineering projects [9], and predicting [10, 11] and mitigating geohazards [12]. The Bohai Strait is located in the northeastern section of North China. It is partitioned by the Zhangjiakou-Bohai Fault Zone (ZBFZ) and the Tan-Lu Fault Zone (TLFZ), the two major seismogenic faults in North China. Therefore, the Bohai Strait and adjacent regions suffer from intense fault activity and earthquakes [13, 14]. For example, large earthquakes, such as the Tancheng earthquake in 1668 [15], the Haicheng earthquake in 1975 [16], and the Tangshan earthquake in 1976 [17], all occurred in this region andcaused great casualties. Given the dense population and cities, developed economy, seismic risk, and the potential cross-sea corridor, a thorough understanding of the stress state across the Bohai Strait has been an attractive topic for decades [18,19]. The stress field in the Bohai Strait and adjacent regions has been preliminarily studied by several scholars. For example, [PERSON] [20] and [PERSON] et al. [21] pointed out that the Bohai Strait is under an NE-NEE trending regional compression stress field by integrating stress data around the Bohai Sea. [PERSON] et al. [22] estimated the stress state in the Pingdu area, Shandong Province, based on in situ stress data obtained from a deep borehole. Their estimation revealed that the maximum horizontal stress is oriented to E-W, and the stress level is low, far from the typical limit inducing fault activity in the upper crust. [PERSON] et al. [23] further analyzed the stress state in the Gaizhou area in the Liaodong Peninsula. They found that vertical stress plays a dominant role in the current stress field, and the maximum principal stress is oriented to NE-NEE, which is conducive to the occurrence of right-lateral strike-slip faulting on the Jinzhou Fault. [PERSON] et al. [24] analyzed the stress state in the nearshore areas of Penglai and the sea areas near Changdao. They noted that the maximum principal stress orientation ranges from NEE to E-W, and the horizontal stress increases with depth at greater gradient coefficients than vertical stress. Therefore, the faults intersect the principal stress orientation at a high angle and are in a contemporary stable state. [PERSON] et al. [25] estimated the stress state of the Shandong-Bohai-Liaoning section of the TLFZ using measured stress data around the Bohai Sea. They noted that the stress is oriented in N68\({}^{\circ}\) E in the Bohai section, and the stress accumulation level in the Bohai section is lower than that of the Shandong section but higher than that of the Liaoning section. The results also indicate that the stress state exhibits consistency with the fault kinematics. [PERSON] et al. [26] identified a reverse stress regime in the southern Bohai Sea, with a strike-slip regime in the northern area, though the \(S_{\rm H}\) orientations in the two regions are all NE-trending. These studies provide valuable insights into the stress state around the Bohai Strait but mainly focus on stress state either at a single borehole scale or regional scale; therefore, these studies do not thoroughly characterize the stress state across the Bohai Strait, especially the variation of stress state. It is argued that the seismicity in the Bohai Strait is dominantly controlled by the tectonic interactions between the TLFZ and ZBFZ [27,28]. Moreover, the intersection of faults striking in different directions and the fault activity may lead to notable variations in stress state [2,8,29], which in turn affects the assessment of fault reactivation potential.