E. Behboudi^1,2, D.D. McNamara³, I. Lokmer^1,2, L. Wallace^4,5, 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:

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 S_Hmax orientation (parallel to the Hikurangi margin) within the central HSM (Hawke’s Bay region) which rotates to a WNW- ESE S_Hmax orientation (roughly perpendicular to the Hikurangi margin) in the southern HSM. This rotation of S_Hmax 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 S_Hmax 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).