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

 


Slow slip events (SSE) are patches of displacement across a fault zone that occur on time scales that very long compared to the typical earthquake. Very large (several cm of displacement), shallow (<10km depth) SSEs are observed to occur roughly every 18-24 months on the Hikurangi subduction zone near Gisborne, New Zealand. To study these events, the HOBITSS experiment lead by Laura Wallace (UTIG), with Anne Sheehan (U.CO), Susan Schwartz (UCSC), and Spahr Webb working in collaboration with scientists from Japan (Yoshihiro Ito, and Kimihiro Mochizuki, and Hiroshi Ichihara) deployed 15 Ocean Bottom Seismometers (OBS), 24 absolute pressure gauges (APG) and 3 ocean bottom electromagnetic (OBEM) from the R/V Tangaroa in May 2014. The instruments will be recovered from the R/V Revelle in July, 2015. 


The APG measure the pressure at the seafloor and should record up to several cm’s of vertical uplift during SSEs beneath the array. This is the first time pressure gauges have been deployed and the first offshore array to study SSEs. (Detection with the APGs will require successful processing to suppress noise from oceanographic signals).  Two SSE events have occurred during the deployment near the array as of Feb., 2015. Each involved several cm of horizontal displacement (as observed by GPS stations on land) over about a two week interval.

HOBITTS array. LDEO OBS with APG (pink), LDEO APG (brown), Texas APG (yellow), ERI OBS (blue), ERI APG (purple), Sendai APG (green), Tohuku (red triangle).