Detection of Shallow Slow Slip events on the Northern Hikurangi Margin using Ocean Bottom Pressure Recorders

The Pacific Plate subducts westward beneath the eastern North Island of New Zealand along the Hikurangi Trough at 3 to 6 cm per year. Slow slip events (SSE) occur at the northern Hikurangi margin, offshore Gisborne, New Zealand, every 18 to 24 months (Wallace et al., 2010). Recently, the SSEs have been observed by offshore ocean bottom pressure recorders (OBPR) as well as on the onshore c-GPS network (www.geonet.org.nz). OBPR data spanning from May 2014 to June 2015 show that a SSE which occurred in September-October 2014 possibly extended updip to the trench. The best-fitting slip model for that SSE reveals the major slip (10 to 20 cm slip) was focused between 7 and 4 km depth (Wallace et al., 2016). Here we report on the OBPR data from June 2015 to June 2016, covering a time period where onshore-c-GPS stations observed at least one SSE in early June 2016, just before the offshore OBPR instruments were recovered. In addition, a small SSE may also have been observed in August and October 2015. In order to precisely evaluate the crustal deformation using OBPR, we need to correct other components in the OBPR. Ocean tides were primarily estimated using BAYTAP-G (Tamura et al. 1991). Non-tial components were calculated and removed using a numerical ocean model forced by air pressure and wind on the sea surface (Inazu et al. 2012). Finally, we take the difference between the corrected OBPR at two observation points. Comparing the OBPR data to onshore c-GPS data, we detect the vertical deformation due to the SSE. We also identify whether other possible SSEs occur near the trench, which were not observed by onshore c-GPS sites. Our rolling network of OBPR provides a feasible method of detecting shallow SSE events and recovery of the frictional conditions on the plate interface near the trench.