IODP borehole observatories to monitor slow slip at the offshore Hikurangi subduction zone

The offshore northern Hikurangi subduction margin, New Zealand, is the site of some of the world's best-documented shallow slow slip events. Seafloor geodetic studies have suggested that SSEs there occur to within at least 2 km of the seafloor, and it is possible that they propagate all the way to the trench. IODP Expedition 375 recently completed installation of two subseafloor observatories at the offshore Hikurangi margin, aimed at monitoring slow slip event processes in the very near field. One of the observatories intersects a major active fault near the deformation front of the subduction zone. The fault zone observatory includes 3 levels of formation pressure monitoring (for volumetric strain), high-resolution temperature monitoring, and geochemical sampling and fluid flow monitoring within the fault zone. A second, simpler observatory was installed directly above the area of large slow slip on the offshore Hikurangi subduction zone, which involves two levels of formation pressure monitoring and high-resolution temperature monitoring. We will describe the installation of these observatories, and the types of instrumentation that was installed. We will discuss the scientific aims of the observatories, and the ability of the observatory configuration to resolve many outstanding questions about slow slip event processes. We will also address placement of the observatory monitoring intervals in the context of the coring and Logging-While-Drilling data that was acquired in advance of the observatory installation on Expeditions 372 and 375.