Quantifying tectonic stress magnitudes is crucial in understanding crustal deformation processes, fault geomechanics, and variable plate interface slip behaviors in subduction zones. The Hikurangi Subduction Margin (HSM), New Zealand is characterized by along-strike variation in interface slip behavior, which may be linked to tectonic stress variations within the overriding plate. This study constrains in-situ stress magnitudes of the shallow (<3km) overriding plate of the HSM to better understand its tectonics and how they relate to larger scale subduction dynamics. Results reveal σ3: Sv ratios of 0.6-1 at depths above 650-700 m TVD and 0.92-1 below this depth interval along the HSM and SHmax: Sv ratios of 0.95-1.81 in the central HSM, and 0.95-3.12 in the southern HSM. These stress ratios suggest a prevalent thrust to strike-slip (σ1=SHmax) faulting regime across the central and southern HSM. In the central HSM, the presence of NNE-NE striking reverse faults co-existing with a modern σ1 aligned ENE-WSW (SHmax) suggests that overtime the stress state here evolved from a contractional to a strike-slip state, where the compressional direction changes from perpendicular (NW-SE) to subparallel (ENE-WSW) to the Hikurangi margin. This temporal change in stress state may be explained by forearc rotation, likely combined with development of upper plate overpressures. In the southern HSM, the modern WNW-ESE/ NW-SE σ1 (SHmax) and pre-existing NNE-NE striking reverse faults indicate that stress state remains contractional and subparallel (NW-SE) to the Hikurangi margin overtime. This may reflect the interseismic locked nature of the plate interface.

Quantifying the orientation and magnitude of tectonic stresses is essential to better understand active crustal deformation and faulting in the Hikurangi Subduction Margin (HSM), North Island, New Zealand. In this study, We estimate the horizontal stress magnitudes (Shmin and SHmax ) utilizing leak-off test (LOTs) data, borehole breakout widths measured from borehole image logs, and rock unconfined compressive strengths (UCS) derived from empirical relationships using P-wave velocity wireline logs. Stress field results are used to infer the tectonic regime experienced in the region where three boreholes, Makareao-1, Kauhauroa-5, and Tuhara-1A, are drilled. Relative stress magnitudes in Makareao-1 at 260-900 m TVDss (True vertical depth from sea level) suggest thrust or strike-slip tectonics (SHmax≥ Shmin= Sv). Moving east to Kauhauroa-5, the stress results report a gradual transition from shallow normal/strike-slip tectonics (Sv > Shmin) to thrust or strike-slip tectonics (SHmax> Sv≥ Shmin) at depth. Further east again, at borehole Tuhara-1A, stress results suggest normal/strike-slip tectonics (Sv≥SHmax> Shmin) from 555-2264 m TVDss. The tectonic regimes in individual boreholes are consistent with fault interpretations of seismic reflection profiles from this region. These three boreholes are located within the hangingwall of active, NE-SW striking thrust faults and from borehole breakout azimuths we find a mean SHmax orientation of 065° ± 17° (NE-SW) for the deeper parts of these boreholes. The SHmax orientation is broadly compatible with maximum contraction directions determined from campaign GPS and sub-parallel to far-field relative Pacific-Australian plate motion. This, combined with our stress magnitude observations in Makareao- 1 and Kauhauroa-5 suggests these NE-SW striking faults predominantly experience strike and/or oblique slip despite appearing in seismic profiles as thrust faults. We suggest that these faults originated as thrust faults during older stages of subduction along this margin, which over time have become reactivated in a more strike-slip manner.

We constrain the orientation of the horizontal stress field from borehole image data in a transect across the Hikurangi Subduction Margin. This region experiences NW-SE convergence leading to recurrent slow slip events. The direction of the horizontal maximum stress is E-W at an active thrust fault near the subduction margin trench. This trend changes to NNW-SSE in the Tuaheni Basin in the offshore accretionary wedge, and to NE-SW in the onshore forearc. Multiple, tectonic and geological processes, either individually or in concert, may explain this variability. A general offshore-onshore stress rotation may reflect a change from dominantly compressional tectonics at the deformation front, to a strike-slip and/or extensional stress regime closer to the Taupo Volcanic Zone. In addition, the offshore stress may be affected by topography and/or stress rotation around subducting seamounts, and/or temporal stress changes during the slow slip cycle.

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 variation in the stress state and the mechanical strength of faults and their hanging walls, or, alternatively, these variations in seismic behavior may generate variation in the stress state in space and time. In this study, we analyze borehole image and oriented four-arm caliper logs acquired from thirteen boreholes along the HSM to present the first comprehensive stress orientation dataset within the HSM upper plate. Our results reveal a NE-SW SHmax orientation (parallel to the Hikurangi margin) within the central HSM (Hawke’s Bay region) which rotates to a WNW- ESE SHmax orientation (roughly perpendicular to the Hikurangi margin) in the southern HSM. This rotation of SHmax orientation spatially correlates 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 southern HSM. Observed borehole SHmax orientations are largely parallel to maximum contraction directions derived from geodetic surface deformation measurements, suggesting that modern stress orientations may reflect contemporary elastic strain accumulation processes related to subduction megathrust locking.