New slow slip events along the Hikurangi margin detected using wavelet analysis

Along the Hikurangi margin, where the Pacific plate subducts beneath the Australian plate, slow slip events (SSEs) have been detected at both shallow and deep depths. With the aim of improving the SSE catalog along the Hikurangi margin, we use a wavelet-based method developed by Ducellier et al. (2022) to detect SSEs recorded by the GPS sites operated by the GeoNet network. We apply wavelet decomposition to the east component of the GPS stations along Hikurangi. To do so, we consider two transects, transect 1 and transect 2 that target the shallow and deep SSE regions, respectively.
We take equally spaced points along the transects and group stations within a 50-km radius of a given point. Then we apply wavelet decomposition to each station within the radius. We then stack each detail over all stations within the radius and sum the stacked details at each level of decomposition. We find that SSEs are best distinguishable in levels 5, 6 and 7 for shallow SSEs and levels 7, 8 and 9 for deep SSEs. We then define a displacement threshold of one standard deviation of the stacked details. To define an SSE, we consider the stacked details below the displacement threshold that are followed by stacked details above the displacement threshold, following Ducellier et al. 2022. Considering the stacked details along transect 1, which targets shallow SSEs, we find 54 SSE detections from 2005 to 2023. Of those, 20 have been reported in previous work, which leaves 34 potential new SSE detections. The stacked details along transect 2, which runs close to the deep SSE region, indicate 15 SSE detections over the same period; six of which were previously reported and nine potential new detections. We then geodetically model the new SSE detections using the software TDEFNODE (McCaffrey 2009) to study their spatial and temporal distribution along the margin.