First look at seafloor geodetical pressure data acquired during a large 2024 Slow-Slip-Event at the Hikurangi Margin offshore New Zealand

Over the last two decades, the importance of Slow Slip Events (SSEs) in the deformation and seismic cycle of subduction zones has become more widely recognized. Knowledge of the evolution and slip distribution during SSEs can provide key insights into processes that influence SSE occurrence and their relationship to seismic slip. However, the offshore nature of many SSEs makes them difficult to observe with onshore geodetic methods alone, necessitating the deployment of seafloor-geodetic instruments to detect seabed deformation.

The Hikurangi subduction zone offshore New Zealand is characterized by frequent, large SSEs, and previous experiments have shown that such events regularly produce up to a few centimetres of seabed uplift that is detectable using seafloor pressure data. We are presenting a first look on ocean bottom pressure data recovered from the most recent GONDOR deployment across the northern Hikurangi subduction zone. The 2022-2025 GONDOR project is the largest seafloor geodetic experiment to date at Hikurangi, with over 50 seafloor instruments deployed in a dense array with several kilometres spacing, of which 39 were fitted with Absolute Pressure Gauges (APG) to detect vertical displacement of the seabed. 13 of these instruments have self-calibrating A-0-A sensors, enabling mitigation of instrument drift from the pressure record. The deployment is collocated with an IODP CORK observatory, allowing for future ground truthing of the pressure data and includes two arrays of Direct-Path-Acoustic sensors, giving horizontal deformation information of SSEs. During the 2024/2025 period, at least three large SSEs have occurred beneath the array, one beneath the centre and one each beneath the northern and southern subarrays.

I will present preliminary results from our analysis of seafloor pressure data, using depth-matched reference sites to mitigate oceanographic noise. I will also outline a processing workflow for pressure data analysis, including improved drift-removal techniques that do not depend on prolonged periods of oceanographic calm and are robust to sensor vertical displacement during the initial deployment phase when drift is most rapid. Further, I will explore the advantages of interspersing A-0-A instruments with conventional APGs for more robust drift mitigation to enable resolution of SSE vertical displacement during the early deployment phase.