# A New Look at the Stress State Across the Bohai Strait, China
## 5 Discussion  
### Relationship Between the Stress State and Tectonic Feature  
In-situ stress mainly originates from tectonic stress [64], gravity [65], thermal stress [66], etc. It is influenced by topography [67; 68], faulting [69; 70; 71], and lithological strength variations [72; 73; 74]. It is noted that stress states across different scales are governed by distinct conditions [2]. For the four boreholes in this study, the variation in topography is small. In detail, PD and XL show elevation differences of \(<\)20 m; the CD and GZ exhibit max. evaluation variations of \(<\)50 m. Previous studies [68; 75] found that the influence of topographic relief is primarily controlled by elevation differences; therefore, its effect  
Figure 8: Recent tectonic stress map in the Bohai and adjacent area (modified after [62]). The blue circles represent the results resolved from the four boreholes in this study. on the four boreholes in this study can be neglected. Additionally, core samples are mainly granitoid rocks and show no significant lithological variations along the entire depth. Combined with existing studies [56], the influence of lithology is also negligible. Thus, variations in stress states are likely associated with the tectonic framework, fault activity, etc., in the Bohai Strait region. We discuss the relation between the stress state and the tectonic framework of the Bohai Strait region. Previous studies reveal that the Bohai segment of the TLFZ currently exhibits dextral strike-slip motion, accompanied by a thrust component [25]. The Holocene activity intensity in the central Bohai segment is greater than that in the Liaodong segment to the north and the Laizhou Bay segment to the south. The ZBFZ exhibited dextral strike-slip activity from the Mesozoic to the Early Paleogene and has transitioned into sinistral strike-slip since the Late Paleogene [76] or the Neogene [77, 78]. The current stress field in North China is predominantly characterized by NEE compressive stress, which is induced by the lateral subduction of the Pacific Plate beneath the Eurasian Plate in conjunction with NW-directed compression from the Philippine Plate. Under such an NEE-oriented stress field, the NWW-trending ZBFZ is more prone to strike-slip motion, whereas the NNE-trending TLFZ exhibits a weaker compressive pattern, due to the relatively weak regional compressive stress, as proven by [PERSON] et al. [17] and [PERSON] et al. [79]. Moreover, the stress is likely to accumulate in the intersection region of the TLFZ and ZBFZ, particularly in the southeastern and southwestern quadrants of the intersection zone. Among the four boreholes, the GZ and PD boreholes are located far from the fault intersection zone, resulting in minimal deviations in stress magnitude and stress regime from the regional stress field. In contrast, the XL and CD boreholes, situated in the southeastern quadrant of the fault intersection zone, exhibit higher stress magnitudes. The stress regimes in the XL and CD sites, which are predominantly reverse faulting regimes, contrast with the strike-slip and normal faulting stress regimes that dominate in North China. Furthermore, we estimated the maximum horizontal shear stress, i.e., \((\tau=S_{1}-S_{3})/2\) at 500 m depth across the Bohai Strait, based on the measured stress data. The results indicate that a progressive decrease in the maximum horizontal shear stress is observed with increasing proximity to the ZBFZ (Figure 9). The shear stresses decrease from 8.57 MPa in the CD site (i.e., closest to the ZBFZ) to 1.17 MPa in the GZ site (i.e., farthest from the ZBFZ), showing an 86.3% reduction. This is contrary to the findings in the San Andreas fault and the Longmenshan fault belt [80]. We speculate that such uniqueness is related to the intersection of the TLFZ and ZBFZ, since previous studies have focused on the single major fault zones, but the Bohai Strait area is influenced by two interacting faults.