Spatial Variation of Shallow Stress Orientation Along the Hikurangi Subduction Margin: Insights From In-Situ Borehole Image Logging

Knowledge of the contemporary in-situ stress orientations in the Earth's crust can improve our understanding of active crustal deformation, geodynamic processes, and seismicity in tectonically active regions such as the Hikurangi Subduction Margin (HSM), New Zealand. The HSM subduction interface is characterized by varying slip behavior along strike, which may be a manifestation of combined variations in both stress state and the mechanical properties of faults and their hanging walls. Alternatively, these variations in subduction thrust slip behavior may drive heterogeneity in the stress state in space and time. In this study, we analyze borehole image and oriented four-arm caliper logs acquired from 13 boreholes along the HSM to present a comprehensive stress orientation data set from borehole data within the shallow (<3 km) upper plate of the subduction thrust. Our results reveal a 065°/245° S_Hmax orientation within the central HSM (Hawke's Bay region) which rotates to 112°/292° and 140°/320° in the southern HSM. This rotation of S_Hmax orientation correlates spatially with along-strike variations in subduction interface slip behavior, characterized by creep and/or shallow episodic slip events in the central HSM and interseismic locking in the south. The borehole S_Hmax orientations suggest that contemporary stress orientations may be caused in part by along-strike variation in deformation style imposed by clockwise rotation of forearc. In the southern HSM, borehole-derived S_Hmax orientations are inconsistent with S_Hmax orientations derived from focal mechanism solutions in the subducting plate, implying some degree of mechanical decoupling between the shallow hanging wall and subducting slab.