Collaborative Research: Hikurangi Ocean Bottom Investigation of Tremor and Slow Slip (HOBITSS)

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Deployment of a network of pressure gauges and seismometers on the Hikurangi portion of the subduction zone off North Island New Zealand is designed to record a slow-slip event (SSE) expected to occur on the plate boundary fault in the 2014-2015 timeframe. SSE occur every ~18 months in this region, so documenting the deformation associated with this type of event and comparing that couple-week activity with ongoing microseismicity should illustrate the evolution of forces and associated hazards in this region. Twenty US seafloor instruments, including 10 from OBSIP, will be combined with a similar number of Japanese instruments for ~12 months. These data will be evaluated together with data from onshore geodetic and seismic stations in this international collaboration. Results will inform planning for possible future seafloor drilling by IODP and subsequent in-situ measurements.Due to the shallow dip of the subducting plate, the Hikurangi site offers a unique opportunity to document the small signals associated with SSE, for which motion is too minor for human perception. Insights into this newly-recognized mode of plate interaction are expected to be applicable to other convergent margins. How far 'up-dip' the slip extends, whether all the way to the seafloor near the subduction trench or not, is a key unknown in current estimates of earthquake shaking and tsunami hazard. The extent of slow slip can indicate how much stress on the plate boundary fault is relieved versus building up toward an eventual megathrust earthquake.