Transient crustal deformation due to slow slip observed on ocean bottom pressure recorders in the Hikurangi margin

We describe two slow slip events observed using seafloor instruments at the northern Hikurangi margin, New Zealand. These transient deformation events were observed using ocean bottom pressure recorders (OBPR) as well as an onshore GPS network in 2013. The four OBPRs were deployed for over one year, from March 2013 to May 2014. Data from only two OBPRs are available as the other two instruments had trouble with their data loggers. One of the available OBPRs was installed on the incoming Pacific Plate near the trench. The other was installed approximately 40 km away from the trench on the Australian Plate, landward of the Hikurangi Trough, approximately 6 km above the plate interface. To remove tidal and oceanographic noise from the raw ocean bottom pressure data, we use the incoming plate site as a reference site to remove the oceanographic noise, which is largely common mode over such a small region. The first slow slip event was observed in July 2013, and produced up to 2 cm eastward displacement at onshore GPS sites over a period of 2-3 weeks. Crustal deformation observed by the OBPRs involved 16 mm of uplift during the slow slip event observed by the cGPS sites, followed by a subsequent subsidence over the following weeks. This suggests that the slow slip event initiated beneath the coastline and the rupture propagated seaward over ~3 weeks. We expect that large slip probably occurred near the trench in the later stages of the slow slip event. A second slow slip event was observed at onshore cGPS and the landward seafloor bottom pressure recorder in October 2013. The October SSE produced 36 mm total uplift at the OBPR. This suggests that in the offshore region, most of the slow slip in the October 2013 event was focused beneath the OBPR, and did not migrate trenchward significantly. Our results demonstrate the value of seafloor pressure observations to investigate slow slip events at offshore subduction margins.