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The more-complicated observatory at Site U1518 contains multi- to date. Wellhead sensors in both observatories will provide data on
level pressure sensing above, within, and below the fault zone, as changes in pressure at the seafloor, which will help to resolve vertical
well as a string of autonomous miniature temperature loggers. deformation of the seafloor during SSEs (e.g., Wallace et al., 2016).
OsmoSamplers and an OsmoFlowmeter (see Jannasch et al., 2004;
Solomon et al., 2009) will capture time series of fluid-flow rates and IODP Expeditions 372 and 375 are pioneering because they are
fluid chemistry within the fault zone. The observatory at Site U1519 the first to target slow-slip event processes. Ultimately, the team of
involves a simpler design, with two levels of formation-pressure sixty scientists from thirteen countries who participated in these
sensing and a distributed string of temperature loggers. Pressure expeditions will integrate their coring, logging, and observatory
sensing in the formation provides a sensitive indicator of volumetric data with detailed post-expedition lab studies and modeling to test a
strain during SSEs (e.g., Araki et al., 2017); those results will greatly broad suite of hypotheses concerning the fundamentals of slow-slip
improve the spatiotemporal resolution of offshore Hikurangi SSEs events, and their relationship to great megathrust earthquakes. The
and enable detection of SSEs much smaller than those documented central theme addresses several important goals of GeoPRISMS. ■

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Spring 2018 Issue No. 40 GeoPRISMS Newsletter • 19