E. Behboudi1,2, D.D. McNamara3, I.
Lokmer1,2, L. Wallace4,5, and D.
Saffer 5
1 Irish Centre for Applied Geosciences (iCRAG),
University College Dublin, Republic of Ireland.
2 School of Earth Sciences, University College Dublin,
Republic of Ireland.
3 Department of Earth, Ocean and Ecological Sciences,
University of Liverpool, UK.
4 GNS Science, New Zealand.
5 University of Texas Institute for Geophysics, USA.
Corresponding author: Effat Behboudi
(effat.behboudi@ucdconnect.ie)
Key Points:
- Maximum horizontal stress orientation varies along strike in the
Hikurangi Subduction Margin upper plate
- Stress orientations reflect contemporary elastic strain accumulation
processes related to subduction megathrust locking
- Stress orientations in the southern HSM are oblique to focal-mechanism
stress suggesting the subduction interface is mechanically weak
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.
Plain Language Summery
Movement along faults at tectonic plate boundaries can cause changes in
the orientations of the forces, known as stress, that make them move.
Such changes may help us explain how deformation at the surface occurs
when these faults move, the way fluid moves along these faults, and why
different types of earthquakes occur on these faults. The Hikurangi
Subduction Margin, New Zealand’s largest and most hazardous plate
boundary fault, shows a variety of deformation and earthquake types that
occur in the over-riding plate which may be linked to stress
orientation. In this study, we found that variability in the stress
orientations within the upper plate of Hikurangi Subduction Margin
matches areas where we see different earthquake types occurring, and
observed patterns of surface deformation. We suggest that stress
orientations reflect the accumulation and release of strain caused by
subduction at the HSM.
1 Introduction
In-situ stress measurements can provide important insights into stress
states at global and localized scales, the geomechanical state of
earthquake-hosting faults, shear traction on faults, and processes of
stress accumulation and release on plate boundary faults. Such
measurements also assist with understanding how crustal stresses relate
to strain observed geodetically and geologically (e.g., Zoback et al.,
1987; Magee & Zoback, 1993; Townend & Zoback, 2006; Byrne et al.,
2009; Chang et al., 2010; Lin et al., 2013, 2016; Brodsky et al., 2017).
Earthquake occurrence and many earthquake rupture characteristics are
partly dependent on the shear to normal stress ratio, which is a
function of the relative magnitude of in-situ principal stresses, the
orientation of the fault plane with respect to the orientation of the
principal stress orientations, pore pressure, and fault plane friction
coefficients (Jaeger et al., 2007; Schellart & Rawlinson, 2013;
Vavryčuk, 2015). Additionally, earthquakes can redistribute stress and
change both shear and normal stress on adjacent fault planes and
surrounding rocks either statically (a shift in the stress state from
before to after the earthquake) or dynamically (oscillating stress
changes that occur with the passage of seismic waves) (Stein, 1999;
Seeber & Armbruster, 2000; Hardebeck, 2004; Ma et al., 2005; Lin et
al., 2007, 2016; Hardebeck & Okada, 2018).
The Hikurangi Subduction Margin (HSM), on the east coast of the North
Island of New Zealand (Figure 1a), experiences strong along-strike
variations in megathrust slip behaviour, ranging from deep interseismic
locking (and stress accumulation) beneath the southern North Island, to
episodic slow slip events (SSEs) and creep at the northern and central
HSM (Figure 1b). Creep and shallow (<15 km depth) SSEs lasting
for 2-3 weeks recur every 18-24 months offshore of the northern and
central HSM (Wallace & Beavan 2010; Wallace, 2020; Figure 1b). Deep
(>25 km), long-term (>1 year) slow slip events
occur approximately every ~5 years at the southern HSM
(Wallace & Beavan 2010), just down-dip of a portion of the plate
interface that is strongly locked and accumulating stress likely to be
released in a future great earthquake (Mw > 8.0). Despite
the recognized importance of in-situ stress states along active
subduction zones in understanding strain accumulation and release, few
studies have been undertaken to directly estimate stress magnitudes in
these settings (Chang et al., 2010; Huffman & Saffer, 2016; Lin et al.,
2010, 2013, 2016; Malinverno et al., 2016; Brodsky et al., 2017;
McNamara et al., 2021).