Spatial Variation of In-Situ Stress Orientation Along the Hikurangi
Subduction Margin: Insights from Borehole Image Logging
Abstract
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.