HOBITSS: Hikurangi Ocean Bottom Investigation of Tremor and Slow Slip

Due to the shallow dip of the subducting plate, the Hikurangi site offers a unique opportunity to document the small signals associated with slow slip events (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.

Our 2014-2015 deployment of a network of pressure gauges and seismometers on the Hikurangi portion of the subduction zone off North Island New Zealand was designed to record a slow-slip event (SSE) that was expected to occur on the plate boundary fault in the 2014-2015 timeframe, and was successful.  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 helps illustrate the evolution of forces and associated hazards in this region. Twenty US seafloor instruments, including 10 from the US Ocean Bottom Seismic Instrument Program (OBSIP), were combined with a similar number of Japanese instruments for 13 months. These data are being evaluated together with data from onshore geodetic and seismic stations in this international collaboration. Results helped inform planning for seafloor drilling by IODP in 2018 and subsequent in situ measurements.