Updip Migration of Slow Slip Revealed Through Seafloor Geodesy during 2019 East Coast Slow Slip at the Hikurangi Margin, New Zealand

Abstract:

Slow slip event (SSE) distribution along shallow, offshore subduction interfaces has long been elusive due to lack of resolution from onshore geodetic instruments. To address this, we deployed seafloor geodetic instruments off the east coast of North Island, New Zealand from October 2018 to November 2019, during which a large SSE was recorded in April-May 2019. Absolute pressure gauges (APGs) near Gisborne recorded 1-2 cm of uplift coincident with the displacement of onshore Global Navigation Satellite System (GNSS) sites. In Hawke Bay, APGs reveal seafloor pressure decreases corresponding to uplift from 0.5 cm to 5 cm, delayed by a few weeks relative to the largest onshore GNSS displacements, suggesting slow slip beneath this region migrated updip towards the trench over a few weeks. Near trench GPS-acoustic measurements (GPS-A) indicate possible cm-level southeast displacement of the seafloor, providing further evidence of near trench deformation during the 2019 SSE. We use the time-dependent inversion code TDEFNODE and numerical Green’s functions accounting for heterogeneous elastic properties (constrained by the NZ-wide seismic velocity model) to invert GNSS, APG and GPS-A timeseries for slip on the Hikurangi interface during the 2019 SSE. Our preliminary model suggests >10 cm of slip on the plate interface at 6-12 km depth, with the SSE beginning beneath the offshore Gisborne area in late-March, migrating southward under Hawke Bay over a four week period, and then updip towards the central Hikurangi trench in mid-May. Slip appears to reach near the trench (to 3-6 km depth) beneath both the offshore Gisborne and Hawke Bay areas. Seismicity during the event wraps around the downdip edge of the SSE with clusters of earthquakes occurring near Mahia Peninsula. This 2019 SSE distribution coincides with the source area of prehistoric subduction megathrust earthquakes beneath Hawke Bay inferred from previous paleoseismic investigations. This has important seismic and tsunami hazard implications as regions that host aseismic slow slip may also rupture seismically. Seafloor geodesy was vital in resolving the offshore slip distribution and observation of updip SSE migration, adding to the argument that seafloor observations are essential to revealing the distribution of deformation at offshore subduction zones.