Investigation of Hikurangi subduction zone slow slip events using onshore and offshore geodetic data

Slow slip events (SSEs) involve episodic transient fault slip of millimetres to tens of centimetres over days to years. They have been detected at subduction zones worldwide. SSEs play a major role in accommodating the plate motion budget in SSE source regions and are capable of interacting with other modes of slip, such as fast seismic earthquakes, making understanding SSEs important to assessing seismic hazard. Frequent SSEs occur at both the deep (25–50 km depth) and shallow (<15 km depth) portions of the Hikurangi subduction zone, where the Pacific plate subducts westward beneath the Australian plate at North Island New Zealand.

In this research we use onshore and offshore geodetic data to detect deep and shallow Hikurangi subduction SSEs. The data are three-component positional time series data from New Zealand's Global Navigation Satellite System (GNSS) network, displacements from temporary onshore (campaign) and offshore pressure sensor deployments and Interferometric Synthetic Aperture Radar (InSAR) acquisitions. Time-dependent inversions of these geodetic data using the elastic block model code TDEFNODE allow the spatial distribution and temporal evolution of SSEs to be resolved. Determining the SSE spatiotemporal behaviour contributes to our understanding of the role of SSEs in accommodating plate motion at the Hikurangi subduction zone, investigating the relationship between SSEs and earthquakes, and improving our ability to forecast future large earthquakes and tsunamis.

We document an intriguing sequence of deep SSEs beneath the Kāpiti and Manawatū regions in 2013 and 2014–2016. These SSEs occurred at close proximity and overlapping in time with three Mw >6 local earthquakes: 21 July 2013 Mw 6.6 Cook Strait, 16 August 2013 Mw 6.6 Lake Grassmere, and 20 January 2014 Mw 6.3 Eketāhuna earthquakes. Multiple SSE, coseismic, and postseismic slip sources are resolved simultaneously to reliably characterise the sequence. We find that the deep 2013 Kāpiti and 2014/2015 Manawatū SSEs involved >300 mm of peak slip at 40 km depth (over ~370 days) and >400 mm at 40 km depth (over ~720 days) respectively, reaching a combined equivalent moment release to an Mw 7.3 earthquake. It was the longest (and one of the largest) SSEs observed in New Zealand to-date. Over the past ~20 years, Kāpiti and Manawatū SSEs have released 60–100% of slip deficit at the 25–50 km SSE source region. However, heterogeneous models of the deep SSEs are required to more accurately assess this, as we find homogeneous half-space assumptions could result in a 20–40% over-estimation of the seismic potency of deep SSEs (30–150 mm over-estimation of SSE slip in our study).

We also investigate the spatiotemporal evolution of shallow, offshore Hikurangi SSEs from February–July 2019 using onshore and offshore geodetic data. This is the first study to investigate <12 km depth SSEs beneath Hawke Bay using seafloor geodetic data (revealing 1.1–2.7 cm uplift of the seafloor), and the second experiment to capture a large Gisborne SSE with seafloor geodesy (1.0–3.3 cm uplift). Through the use of self-calibrating seafloor pressure sensors, we demonstrate that signals due to ocean variability can influence the inferred centimetre-level seafloor vertical displacement estimates by 0–2.6 cm, which could result in misinterpretation of offshore tectonic activity if not corrected for. Drawing on this finding, we make recommendations to optimise future experiments aiming to detect SSE-related vertical displacement of the seafloor using seafloor pressure arrays. Along with onshore geodetic data, the seafloor geodetic data are then inverted to estimate the spatiotemporal evolution of the shallow SSEs in 2019.

Peaks of 150–200 mm and >200 mm total slip are resolved offshore Gisborne at 6–9 km depth and Hawke Bay at 9–12 km depth respectively. The improved offshore resolution in 2019 leads to the detection of longer-duration (>1 month) SSE slip on the <9 km depth portions of the subduction interface offshore Gisborne. The seafloor pressure data also suggest trenchward migration of the SSE occurred beneath Hawke Bay, from 9–12 km depth to <9 km depth over 1–3 weeks in late May/early June 2019, which was not detectable by onshore GNSS stations. Over the past 20 years, shallow SSEs appear to have released 70–100% of the slip deficit at 9–12 km depth immediately offshore Gisborne but not at the 12–25 km portion of the subduction interface beneath onshore Gisborne (30–80% released), or at <12 km depth subduction interface beneath Hawke Bay (as much as 60% of slip deficit released). Further seafloor observations are required to detect and characterise the SSEs in the offshore area and more accurately estimate the role of SSEs at the <9 km depth subduction interface.

The frequent occurrence of Hikurangi SSEs, potential interaction with seismicity, and possible spatial overlap with past major megathrust ruptures highlights how critical it is that the SSEs are reliably and accurately monitored and modelled. Our characterisation of SSEs using onshore and offshore geodetic data develops the current understanding of the spatial extent of SSE phenomena on offshore subduction zones and of the role of SSEs in accommodating the plate motion budget at the Hikurangi subduction zone. Knowledge of SSE behaviour is important to eventually understanding the relationship between SSEs and seismic slip, ultimately enabling SSEs to be incorporated into operational earthquake forecasting and seismic hazard assessments.