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.