Assessing occurrence patterns of shallow Hikurangi slow slip events using renewal processes

 

Frequent slow slip events (SSEs) occur along the shallow (< 15 km depth) Hikurangi margin, which marks the subduction of the Pacific plate beneath the Australian plate in the North Island of New Zealand. Geodetic observations suggest that these events exhibit varying recurrence intervals and slip patterns along the margin (e.g., Wallace et al. 2012, Wallace 2020). However, the recurrence patterns of these SSEs have not been well characterized. To address this knowledge gap, in this work we statistically model the occurrence of shallow Hikurangi SSEs in space and time. For that purpose, we first construct a catalog of these SSEs using the method developed by Ducellier et al. (2022), which uses wavelet analysis to identify SSEs in GPS time series. We complement the method with manual picking of GPS time series to determine the start and end times of SSEs at each GPS station. We identify 92 SSEs along the shallow Hikurangi margin between 2006 and 2024. To investigate the recurrence patterns of SSEs in the catalog, we fit a renewal process using Bayesian inference to obtain the posterior distribution of the parameters. These posterior estimates are then used to infer SSEs’ inter-arrival time and periodicity. Our results show that SSE inter-arrival time distribution vary along the margin, less frequent SSEs occur in the southern part of the margin (offshore Cape Turnagain) and more frequent events occur in the northern part (offshore Tolaga Bay and Gisborne areas). These results are consistent with previous observations (Wallace 2020). The periodicity of SSEs also changes along strike. SSEs in the northern and southern parts of the margin occur more regularly than those at the central part of the margin. We also compare the recurrence patterns of SSEs before and after the 2016 M_w7.8 Kaikoura earthquake, which ruptured New Zealand’s northeastern South Island and triggered widespread slow slip in the Hikurangi subduction zone. Our findings show that after the earthquake SSE occurrence is more periodic in some parts of the margin, while SSE mean length increases in the central part of the margin (offshore Gisborne). Our results highlight the patterns of SSE behavior along the Hikurangi margin and their sensitivity to external stress perturbations.