Slow slip events are part of a spectrum of aseismic processes that relieve tectonic stress on faults. Their spatial distribution in subduction zones has been linked to perturbations in fluid pressure within the megathrust shear zone and subducting oceanic crust. However, physical observations of temporal fluid pressure fluctuations through slow slip cycles remain elusive. Here, we use earthquake focal mechanisms recorded on an ocean-bottom seismic network to show that crustal stresses and fluid pressures within subducting oceanic crust evolve before and during slow slip events. Specifically, we observe that the retrieved stress ratio, which describes the relative magnitudes of the principal compressive stresses, systematically decreases before slow slip events in New Zealand’s northern Hikurangi subduction zone, and subsequently increases during the evolution of each slow slip event. We propose that these changes represent the accumulation and release of fluid pressure within overpressured subducting oceanic crust, the episodicity of which may influence the timing of slow slip event occurrence on subduction megathrusts. This work contributes an improved understanding of the physical driving forces underlying slow subduction earthquakes, and a potential means by which to monitor stress and fluid pressure accumulation in such regions.
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Onshore GeoNet seismic waveforms were obtained from https://www.geonet.org.nz/data/types/seismic_waveforms. Offshore HOBITSS raw seismic and geodetic data are archived at the Incorporated Research Institutions for Seismology Data Management Center (https://doi.org/10.7914/SN/YH_2014).
Code used for the geodetic slip inversions (TDEFNODE) is available from http://www.web.pdx.edu/~mccaf/defnode/manual/tdefnode.html. MATLAB code (MSATSI) used for temporal stress inversions is available from https://www.induced.pl/software/msatsi.
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This study was funded by a Royal Society of New Zealand Marsden Fund grant (15-GNS-026) and an MBIE Endeavour Fund grant to GNS Science. The HOBITSS OBS network was funded by NSF grants OCE-1334654, 1333311, 1332875 and 1333025 (to L.W., S.W., S.S. and A.S.). K.M. was funded by ERI JURP 2013-B-09. E.W.-S. thanks C. Boulton and C. Williams for discussions that helped to enhance the manuscript. The authors thank P. Vannucchi and V. Cruz-Atienza for comments that helped to improve the manuscript.
Supplementary Table 1 and Figs. 1–9