Differences in crustal stress direction in the southern section of the Huayingshan fault zone in Sichuan Basin: Insights from in situ borehole image logging

Abstract

Objective The Huayingshan fault zone, the largest fault zone within Sichuan Basin, exhibits notable differences in geological structures on both sides. Historically, earthquakes with a magnitude of ≤5 have occurred frequently along this fault zone, which remains relatively active to this day, disturbing the crustal stress field of the basin. Clarifying the crustal stress state of the Huayingshan fault zone can enhance our understanding of its active deformation and its tectonic and seismic activities and serve as a reference for subsequent research regarding this fault zone. Currently, geostress studies in this area rely mainly on analyzing seismic data, whereas the investigation of borehole data remains relatively scarce. Methods We collected and collated imaging data regarding borehole collapse and borehole-induced tensile fractures from eight borehole logging sites in the southern segment of the Huayingshan fault zone; the geostress directions of these eight boreholes were determined by analyzing these data. Subsequently, a comprehensive analysis of the geostress characteristics was performed by combining the data regarding China’s modern stress field and the earthquake focal mechanism solutions in the southeastern margin of Sichuan Basin. Results The maximum horizontal principal stress in four boreholes located in the southern region of the southern segment of the Huayingshan fault zone was oriented in the NWW-SEE direction, which aligns with the regional stress field direction in Sichuan Basin; only one borehole in the southern region exhibited a maximum horizontal principal stress in the NEE—SWW direction, representing a counterclockwise deviation relative to the regional stress field of Sichuan Basin. Meanwhile, the maximum horizontal principal stress in all three boreholes in the central region of the southern segment of the Huayingshan fault zone was also oriented in the NEE—SWW direction, representing a counterclockwise deviation relative to the regional stress field of Sichuan Basin. Conclusion Through the comprehensive analysis of the above results and the tectonic features and basement properties of the study area, the following conclusions are drawn: (1) The deviations in stress direction in the southern segment of the fault zone are primarily caused by the combined effects of changes in the basement properties of the region, tectonic stress near the boreholes, and the regional stress field. (2) The geostress characteristics obtained in the present study, along with the focal mechanism solutions of earthquakes in the southern segment of the Huayingshan fault zone, indicate that the stress direction in the southeastern margin of Sichuan Basin is relatively divergent, resulting from the combined effects of complex local structures and the regional stress field in this region. Significance The present study aims to supplement the geostress data regarding the southern segment of the Huayingshan fault zone based on borehole data, analyze the current stress field state of the rock mass, and determine the direction of the maximum horizontal principal stress. These findings will provide supporting geostress data for subsequent research on the frequent seismic activities in this region.

Keywords

• borehole image logs
• in situ stress field
• huayingshan fault zone
• sichuan basin