The northern Hikurangi margin three-dimensional plate interface in New Zealand remains rough 100 km from the trench

Abstract

At the northern Hikurangi margin (North Island, New Zealand), shallow slow slip events (SSEs) frequently accommodate subduction-interface plate motion from landward of the trench to <20 km depth. SSEs may be spatially related to geometrical interface heterogeneity, though kilometer-scale plate-interface roughness imaged by active-source seismic methods is only constrained offshore at <12 km depth. Onshore constraints are comparatively lacking, but we mapped the Hikurangi margin plate interface using receiver functions from data collected by a dense 22 × 10 km array of 49 broadband seismometers. The plate interface manifests as a positive-amplitude conversion (velocity increase with depth) dipping west from 10 to 17 km depth. This interface corroborates relocated earthquake hypocenters, seismic velocity models, and downdip extrapolation of depth-converted two-dimensional active-source lines. Our mapped plate interface has kilometer-amplitude roughness we interpret as oceanic volcanics or seamounts, and is 1–4 km shallower than the regional-scale plate-interface model used in geodetic inversions. Slip during SSEs may thus have different magnitudes and/or distributions than previously thought. We show interface roughness also leads to shear-strength variability, where slip may nucleate in locally weak areas and propagate across areas of low shear-strength gradient. Heterogeneous shear strength throughout the depth range of the northern Hikurangi margin may govern the nature of plate deformation, including the localization of both slow slip and hazardous earthquakes.

Geophysical transect reveals seismic P-wave velocity structure of the northern Hikurangi margin, New Zealand

“…These were interpreted as regions of high fluid content, with faults acting as conduits for fluid migration from the mantle into the Pacific Plate (Mochizuki et al, 2021), with the Pacific mantle experiencing a low degree (<10%) of serpentinization (Grevemeyer et al, 2018;Gase et al, 2021) 2.2 Plate interface The plate interface, as inferred by Williams et al (2013) using MCS observations, earthquake hypocenters, and regional tomography models, displays a northwest dipping interface and variations in dip with a wavelength of 10s of km, increasing from a dip of ~7° near the trench to ~20° near the intersection with the Australian Moho. Finer scale imaging of the plate interface beneath the eastern Raukumara Peninsula from receiver functions showed a plate roughness on the scale of 1s of km, interpreted as volcanic sediments and/or seamounts, which leads to a variability of shear-strength along the plate interface (Leah et al, 2022). Marine multichannel seismic data from Gase et al (2021) imaged a rough subducting plate, with volcaniclastic sediments producing strong reflectivity and contributing to geometric roughness near the decollement.…”

Permeability and Elastic Properties of Rocks From the Northern Hikurangi Margin: Implications for Slow‐Slip Events

“…The relatively large seismic moment released by shallow SSEs, comparable to that of earthquakes (Passarelli et al., 2021), proves the importance to understand SSEs and how they influence the seismogenic character of a convergent margin. Frictional properties and stress heterogeneities along the plate interface might favor SSEs (Barnes et al., 2020; Bell et al., 2010; Boulton et al., 2019; Im et al., 2020; Leah et al., 2022; Rabinowitz et al., 2018; Shreedharan et al., 2023). Moreover, subducting oceanic crust and sediments release large volumes of fluids whose pressure can exceed hydrostatic conditions when confined within low permeability rocks, lowering the effective stress on the shallow megathrust or splay faults and creating conditions conducive to SSEs (Ellis et al., 2015; Kitajima & Saffer, 2012; Tsuji et al., 2008; Warren‐Smith et al., 2019).…”

“…The northern Hikurangi margin (HM) in New Zealand, is a subduction zone with a shallow forearc and plate interface, where sediment accretion, compaction, and deformation have been modulated for millions of years by underthrusting seamounts (Gase et al., 2021; Sun et al., 2020). Subducting seamounts may cause stress heterogeneities (Bangs et al., 2023; Leah et al., 2022; Sun et al., 2020) and fluid pressure transients (Shaddox & Schwartz, 2019) that can lead to SSEs, several of which have been characterized in great detail by onshore geodetic and offshore absolute pressure gauge (APG) data (Yohler et al., 2019). Offshore Gisborne SSEs occur every 1–2 years and can last several weeks, during which time 5–30 cm of slip may be accommodated (L. Wallace, 2020).…”

Permeability and elastic properties of rocks from the northern Hikurangi margin: Implications for slow-slip events

“…S1). Subducting topography (e.g., seamounts) may cause stress heterogeneities (Leah et al, 2022;Sun et al, 2020) and fluid pressure transients (Shaddox & Schwartz, 2019) that can lead to SSEs, several of which have been characterized in great detail by onshore geodetic and offshore absolute pressure gauge (APG) data (Yohler et al, 2019). Offshore Gisborne SSEs occur every 1-2 years and can last for several weeks, during which 5 to 30 cm of slip may be accommodated (Wallace, 2020).…”