Figure 5. Map of SHmax at the East Coast of North Island determined from borehole Breakouts in (orange arrows) and IODP borehole image analysis from McNamara et al., 2021 (blue arrows). Green shaded region represents the East Coast cumulative slow slip (Wallace et al., 2012a). Active faults (normal faults: yellow; strike slip: red; reverse fault: black) traces from Litchfield et al. (2014) and Langridge et al, (2016). The bold black line shows the Hikurangi Through. Black arrow indicate the relative convergence vector between the Pacific and Australian Plates from DeMets et al., (1994) and grey arrows show shortening rates at the Hikurangi Trough from Wallace et al. (2012). Abbreviations: NIDFB, North Island Dextral Fault Belt; TVZ, Taupo Volcanic Zone.
Geodetic measurements over the last 25 years have been used to determine New Zealand’s contemporary surface strain field (Dimitrova et al., 2016; Haines &Wallace, 2020). The rotation of borehole-derived SHmax orientations along the HSM in the upper plate is remarkably consistent with the variation in coupling behavior on the HSM subduction interface (Figure 1b) and the along-strike rotation observed in the maximum contraction directions determined from geodetic measurements along the HSM forearc (Dimitrova et al., 2016; Haines & Wallace, 2020; Figure 6a). Maximum contraction directions determined from campaign GPS data reveal that the central HSM has a dominant E-W maximum contraction direction, whereas the southern HSM shows a NW-SE contraction direction (Figure 6a; Haines & Wallace, 2020), broadly compatible with the borehole-derived SHmax orientations reported here. The along-strike variations in maximum contraction directions from GPS are suggested to be related to along-strike changes in interseismic coupling on the HSM plate interface (Wallace et al., 2004; Dimitrova et al., 2016). In the northern and central HSM, the subduction interface is largely creeping and experiences shallow (<15 km), episodic slow slip events on the offshore. At the southern HSM the plate interface is strongly interseismically coupled to ~30 km depth, and is currently accumulating elastic strain in the surrounding crust that will eventually be relieved in future large earthquakes (Wallace, 2020; Figure 1b). The strong agreement between the geodetic maximum contraction directions and the observed stress field from the borehole observations indicate that SHmax orientations and their variability at HSM are strongly influenced by along-strike variations in elastic strain accumulation in the overriding plate due to changes in interseismic coupling on the subduction interface.
In the central HSM (where the plate boundary is largely creeping) borehole-derived SHmax orientations and maximum contraction directions are reasonably consistent and sub-parallel to far-field relative Pacific-Australian plate motion. In contrast, at the southern HSM borehole-derived SHmax orientations and maximum contraction directions are roughly perpendicular to far-field relative Pacific-Australian plate motion, but are parallel to the convergence direction. This observation may be due to the fact that the subducting plate drags the forearc wedge in the direction of subduction and increases elastic compression strain and stress on the plate interface and within the forearc itself aligning elastic strain accumulation and SHmax stress orientations into that orientation. These SHmax orientation observations are at odds with those of Townend and Zoback (2006) showing that stress orientations in central Japan (from earthquake focal mechanisms) do not agree with maximum contraction directions associated with cyclical subduction zone locking. This difference in the conclusions between our study and Townend and Zoback (2006) may be related to the fact that we utilize shallow (<3 km) stress orientations, which may be more susceptible to temporal changes in elastic strain resulting from interseismic coupling rather than stress orientations from earthquake focal mechanisms sampling greater depths. This observation at the HSM may be reflecting longer-term accumulation of stress (related to long-term tectonic processes) independent of stage within the earthquake cycle.