Seismological characterisation of slow slip regions at the northern Hikurangi margin, Aotearoa New Zealand
File(s)
Author(s)
Woodward, Amy
Type
Thesis or dissertation
Abstract
Slow slip events (SSEs) accommodate strain at plate boundaries without ground shaking. Understanding SSEs is important for characterising hazards, but the mechanisms for their occurrence is debated. The Hikurangi margin hosts a diverse range of SSEs along a margin that varies significantly in subduction zone geometry, lithology, rheology, and hydrology. The northern Hikurangi margin experiences shallow (20 km depth) SSEs. Most studies have focused on the offshore region at depths 15 km. The onshore, deeper part of the slow slip region is comparatively poorly constrained. I use a temporary, high-density onshore seismograph network to provide seismological constraints on this deeper part of the slow slip patch, and to test whether it is continuous with the better-characterised offshore shallow SSE source region. First, I create catalogues of 3,071 earthquakes and 86 focal mechanisms and investigate two previously unconstrained∼Mw6 SSEs. Earthquake clusters occur during each SSE, consistent with offshore observations of∼Mw7 SSEs. Second, I use local earthquake tomography to investigate how variations in wavespeeds relate to properties of the slow slip source regions. High VP/VS (∼1.9) in the subducting slab and at the plate interface is in close agreement with existing plate interface models, and is likely due to the composition of the slab and the presence of fluids. I suggest that linear bands of seismicity, normal-faulting focal mechanisms, and high VP are associated with subducted seamounts. I interpret these seamounts as relatively buoyant plate interface topography that results in localised uplift and normal faulting, enhanced fluid pathways from slab mantle to the plate interface, and the supply of fluids from the slab to surface hot springs, cold seeps, and mud volcanoes. I argue that fluid availability, modulated by subducted topography and plate interface roughness, is likely a key control on the occurrence and downdip limit of shallow SSEs.
Version
Open Access
Date Issued
2026-05-21
Date Awarded
2026-08-01
Copyright Statement
Attribution-NonCommercial 4.0 International Licence (CC BY-NC)
License URL
Advisor
Bastow, Ian
Bell, Rebecca
Publisher Department
Department of Earth Science & Engineering
Publisher Institution
Imperial College London
Qualification Level
Doctoral
Qualification Name
Doctor of Philosophy (PhD)