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Hikurangi Ocean Bottom Investigation of

Tremor and Slow Slip (HOBITSS) -

Revealing the environment of shallow slow slip

Susan Schwartz (UC Santa Cruz), Anne Sheehan (University Colorado, Boulder),
Rachel Abercrombie (Boston University)

n the last fifteen years, it has become evident that slow slip events (SSEs) are a common and important
part of the subduction process. They produce millimeters to centimeters of surface displacement over
Idays to years that can be measured by geodesy and are often accompanied by seismic tremor and
earthquake swarms. Slow slip and tremor have been observed in subduction zones in Cascadia, Japan,
Mexico, Alaska, Ecuador, northern Peru, Costa Rica and New Zealand.
The 2014-15 HOBITSS deployment of 24 ocean bottom pressure sensors and fifteen ocean bottom
seismometers (OBSs) at the northern Hikurangi margin, New Zealand captured a M7.0 SSE. The
vertical deformation data collected were used to image one of the best-resolved slow slip distributions
to date, and indicated slip very close, if not all the way to the trench (Wallace et al., 2016). The Fall 2016
GeoPRISMS Newsletter reported on this experiment and how for the first time, ocean bottom pressure
Rough seas off the shores recorders successfully mapped a SSE displacement field (Wallace et al., 2016). The HOBITSS results were
of New Zealand during the instrumental in demonstrating that Absolute Pressure Gauges are a valuable tool for seafloor geodesy.
HOBITSS deployment in 2014. Seismologists from UC Santa Cruz, University of Colorado Boulder and Boston University are now using
Retrieved from the Fall 2016
issue of the GeoPRISMS the seismic data collected during the same experiment to evaluate the spatiotemporal relationship between
Newsletter. Photo credit: seismicity (both earthquakes and tremor) and the slow slip event and the role that seismic structure plays
Justin Ball in controlling slip behavior. One of our primary goals is to determine if slow and fast interplate slip modes
spatially overlap or are segregated.

An initial catalog of local earthquakes was constructed and relocated in a New Zealand-derived velocity
model to produce a catalog of 2,619 earthquakes ranging in magnitude between 0.5 and 4.7. Locations
indicate that Hikurangi seismicity is concentrated in two NE-SW bands, one offshore beneath the Hikurangi
trough and outer forearc wedge, and one onshore beneath the eastern Raukumara Peninsula, with a gap
in seismicity between the two beneath the inner forearc wedge. We do not find an increase in seismicity
during the 2014 slow slip event, though seismicity is slightly higher in the month following the SSE. The
majority of earthquakes are within the subducting slab rather than at the plate interface. The few events that
locate close to the plate interface were assumed to be thrust events and used as templates in a waveform
matching technique to identify similarly located earthquake swarms within the entire dataset.

20 • GeoPRISMS Newsletter Issue No. 40 Spring 2018