Collaborative Research: Unlocking the secrets of slow slip by drilling at the northern Hikurangi subduction margin, New Zealand: CORK observatory development and installation

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The Hikurangi subduction zone offshore the eastern North Island of New Zealand has characteristics that provide a globally unique window into a fundamental aspect of how stress builds up and is relieved as an oceanic plate thrusts (or "subducts") beneath continental crust. Recently, seismologists have recognized that subduction megathrust faults undergo slip in episodic "slow slip events". Prior to the discovery of slow slip events in the last decade, our paradigms of fault slip were focused on seismic slip in major earthquakes. Since the discovery of slow slip, most studies have focused on deep (20-40 km depth) slow slip events, but we now realize that shallow slow slip events (5-15 km depth) exist that are much more accessible to near-source investigations. These shallow slow fault slip events are centered beneath the ocean so onshore instruments have not been able to measure how much surface deformation occurs. This study takes advantage of planned International Ocean Discovery Program drilling to install sensors in two boreholes that will measure properties indicative of change in the shallow crust that the episodic fault activity causes. Senior researchers in borehole instrumentation will mentor mid-early career scientists, passing along extensive knowledge of such complex implementation and research. Japanese, New Zealand, and German collaborators will participate with the US investigators in this international study, and local communities will benefit from scientist outreach associated with cruises off New Zealand. This study is being undertaken as a ?community proposal?, whereby all data will be immediately publicly available. The shallowest, well-documented slow slip events on Earth occur at the northern Hikurangi margin, offshore Gisborne, New Zealand, at 5-15 km depth. They recur approximately every 1-2 years, with slip on the plate boundary interface of up to 15-20 cm. The unusually close proximity of the Gisborne slow slip events to the seafloor, the large slow slip event magnitudes (equivalent to an earthquake magnitude Mw ~6.3-6.8), and short recurrence intervals makes this the ideal setting for evaluating the near-source hydrological, geochemical, and thermal response of the offshore subduction margin to large, frequent, well-documented deformation transients. The borehole observatory data will clarify the role that fluid pressures, fluid flow, and temperature play in the occurrence of slow slip events, as well as the influence of slow slip event deformation on fluid and geochemical cycling within the forearc. This project will establish the borehole facility and set-up open data access. Analysis of the first two years of data, recovered by ROV, will evaluate the deformation, hydrological, and geochemical responses near the source of slow slip, as well as document the thermal regime of shallow slow slip. One of the borehole installations will target the region surrounding the shallow portion of the megathrust fault , while sensors in a second borehole will target the over-riding plate ~5 km above the slow slip event source.