Safety review report for IODP proposal 781A-Full : Unlocking the secrets of slow slip by drilling at the northern Hikurangi subduction margin, New Zealand

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Henrys, S.A.; Barnes, P.; Saffer, D.; Mountjoy, J.; Wallace, L.M.; Underwood, M.; Torres, M. 2013 Safety review report for IODP proposal 781A-Full : Unlocking the secrets of slow slip by drilling at the northern Hikurangi subduction margin, New Zealand. Lower Hutt, N.Z.: GNS Science. GNS Science report 2013/09 126 p.

Abstract: This document constitutes the proponents’ report to the Environmental Protection and Safety Panel (EPSP) for IODP proposal 781A-Full. 781A-Full is the riserless component of a recently submitted Multi-phase Drilling Project (MDP; 781-MDP) proposal that outlines a plan for IODP drilling to discern the mechanisms behind subduction zone slow slip events (SSEs) by drilling at the northern Hikurangi subduction margin, New Zealand. Sampling of the upper plate and subducting section via shallow drilling (400-1200 m), and installation of borehole observatories are key components of this project. In 781A-Full, we proposed seven riserless boreholes (plus one alternate) in a transect across the northern Hikurangi subduction margin, intended to collect samples, geophysical logs, make downhole measurements, and install observatories. These riserless boreholes are designed to address three fundamental scientific objectives: (1) characterize the state and composition of the incoming plate and shallow plate boundary fault near the trench, which comprise the protolith and initial conditions for fault zone rock at greater depth in the SSE slip zone; (2) characterize material properties, thermal regime, and stress conditions in the upper plate above the SSE source region; and (3) install borehole observatory instruments to monitor a transect of holes above the SSE source, to measure temporal variations in deformation, fluid flow, and seismicity. Northern Hikurangi subduction margin SSEs recur on average every two years, and thus provide an excellent setting to monitor changes in deformation rate and associated chemical and physical properties surrounding the SSE source area throughout a slow slip cycle. Sampling material from the sedimentary section and oceanic basement of the subducting plate, and from the primary active thrust in the outer wedge near the trench will reveal the rock properties, composition, and lithological and structural character of the material that will be transported downdip to the known SSE source region. If we find that SSEs propagate all the way to the trench, the shallow fault zone target may even lie within the SSE rupture area, providing riserless access to the SSE source. Together, data from these riserless boreholes and observatories will test a suite of hypotheses about the fundamental mechanics and behavior of slow slip events, and their relationship to great earthquakes along the subduction interface. In this report, we conduct a full characterization for each site from all available data, and evaluate the potential hazards that each of the proposed drill sites could face to assist with the EPSP evaluation of this proposed project. (auth)