Spatial Variation of Shallow Stress Orientation Along the Hikurangi Subduction Margin: Insights From In‐Situ 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 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° SHmax orientation within the central HSM (Hawke's Bay region) which rotates to 112°/292° and 140°/320° in the southern HSM. This rotation of SHmax 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 SHmax 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 SHmax orientations are inconsistent with SHmax orientations derived from focal mechanism solutions in the subducting plate, implying some degree of mechanical decoupling between the shallow hanging wall and subducting slab.

Shallow Tectonic Stress Magnitudes at the Hikurangi Subduction Margin, New Zealand

“…The observed dominant thrust to strike-slip faulting regime is consistent with observed contractional tectonics in the HSM developed by the subduction of the Hikurangi Plateau beneath the North Island (Barnes et al, 1998;Nicol & Beavan, 2003), and the strikeslip faulting generated by forearc rotation of the East Coast (Beanland & Haines, 1998;Litchfield et al, 2014;Nicol et al, 2007;Wallace et al, 2004). Behboudi et al (2022) report a dominant ENE-WSW shallow crust S Hmax orientation within the central HSM, and WNW-ESE or NW-SE S Hmax orientations for the southern HSM (Figure 1b). Considering σ 1 =S Hmax along the HSM, observed S Hmax orientations suggest the contemporary maximum compressional stress switches from subparallel (ENE-WSW) to the Hikurangi margin in the north and central HSM, to roughly perpendicular (WNW-ESE or NW-SE) to the Hikurangi margin in the southern HSM.…”

“…Clockwise rotation of the forearc, which accommodates the margin-parallel component of oblique Pacific-Australian plate motion, drives strike-slip and/or normal faulting within the onshore portion of the northern and central HSM, and transpressional faulting in the southern HSM (Figure 5, Fagereng & Ellis, 2009;Nicol et al, 2007;Wallace et al, 2004;Wallace, Fagereng, & Ellis, 2012). Behboudi et al (2022) suggest that this forearc rotation is likely responsible for generating strike-slip stress state with ENE-WSW S Hmax = σ 1 in the central HSM, and contemporary contractional stress state with WNW-ESE/ NW-SE S Hmax = σ 1 in the southern HSM. However, our stress magnitude results of σ 3 : S v = 0.92-1 and σ 1 = S Hmax leave a possibility for both strike-slip and contractional stress states to occur across both the central and southern HSM due to poorly constrained UCS values used in this study, a limitation of the study that could be restricted by laboratory rock strength testing of both onshore and offshore HSM lithologies.…”

“…Reiter (2021) investigated the impact of physical and elastic parameter contrasts on S Hmax orientation and proposed that contrasts in Young's modulus can introduce S Hmax rotations up to 78°, with larger stress rotations occurring within the softer lithologies. Behboudi et al (2022) proposed that basement uplift in the southern HSM may introduce lateral geomechanical heterogeneities and variations in rock and sediment physical m depth, 0). Some studies suggest that SSEs can release the amount of energy equivalent to a M w 6.5-8 earthquakes (Dixon et al, 2014;Wallace, Beavan, et al, 2012).…”

“…al., 2018;Lawrence, 2018;. Behboudi et al (2022) provides a comprehensive overview of the along-strike and depth related variability in HSM stress orientations. Borehole-derived S Hmax orientations rotate from ENE-WSW (065°/245° ± 10°) in the central HSM to WNW-ESE (112°/292° ± 20°) and NW-SE (140°/320° ± 22°) in the southern HSM (Figure 1b).…”

“…Deep stress orientations are defined by focal mechanism inversions (Townend et al, 2012), shear wave anisotropy (Illsley-Kemp et al, 2019), and gravitational stresses (Evanzia et al, 2017). Earthquake focal mechanism solutions (≤60 km depth) indicate a regional S Hmax orientation of 060°/240° ± 17° and 066°/246° ± 22° in the central and southern HSM, respectively (Figure 1b) (Behboudi et al, 2022;Townend et al, 2012).…”