Using Seafloor Geodesy to Detect Vertical Deformation at the Hikurangi Subduction Zone: Insights From Self‐Calibrating Pressure Sensors and Ocean General Circulation Models

• Published in: Journal of geophysical research. Solid earth Vol. 127; no. 12
• Main Authors: Woods, K., Webb, S. C., Wallace, L. M., Ito, Y., Collins, C., Palmer, N., Hino, R., Savage, M. K., Saffer, D. M., Davis, E. E., Barker, D. H. N.
• Format: Journal Article
• Language: English
• Published: 01.12.2022
• ISSN: 2169-9313; 2169-9356
• DOI: 10.1029/2022JB023989

Seafloor pressure sensor data is emerging as a promising approach to resolve vertical displacement of the seafloor in the offshore reaches of subduction zones, particularly in response to slow slip events (SSEs), although such signals are challenging to resolve due to sensor drift and oceanographic signals. Constraining offshore SSE slip distribution is of key importance to understanding earthquake and tsunami hazards posed by subduction zones. We processed seafloor pressure data from January to October 2019 acquired at the Hikurangi subduction zone, offshore New Zealand, to estimate vertical displacement associated with a large SSE that occurred beneath the seafloor array. The experiment included three self‐calibrating sensors designed to remove sensor drift, which, together with ocean general circulation models, were essential to the identification and correction of long‐period ocean variability remaining in the data after applying traditional processing techniques. We estimate that long‐period oceanographic signals that were not synchronous between pressure sensors and reference sites influenced our inferred displacements by 0.3–2.6 cm, suggesting that regionally deployed reference sites alone may not provide sufficient ocean noise correction. After incorporating long‐period ocean variability corrections into the processing, we calculate 1.0–3.3 cm of uplift during the SSE offshore Gisborne at northern Hikurangi, and 1.1–2.7 cm of uplift offshore the Hawke's Bay area at central Hikurangi. Some Hawke Bay displacements detected by pressure sensors near the trench were delayed by 6 weeks compared to the timing of slip onset detected by onshore Global Navigation Satellite System sites, suggesting updip migration of the SSE. Plain Language Summary We use pressure sensors to estimate centimeter‐level vertical motion of the seafloor due to New Zealand slow slip event activity in 2019. These tectonic events have been observed at subduction zones worldwide. They involve a similar amount of slip as fast earthquakes, but the slip occurs over longer periods of time (days to years). Seafloor pressure sensors are important to detecting and determining the location of offshore slow slip events, knowledge that contributes to our understanding of subduction zone processes, and to assessing the risk of future large earthquakes and tsunamis. Variations in ocean circulation can cause high noise levels in the pressure data compared to the tectonic signals of interest, which makes it challenging to calculate seafloor uplift or subsidence. Here we show that the deployment of a self‐calibrating type of pressure sensor, along with knowledge from global models of ocean circulation, can significantly reduce the data noise levels. We estimate up to 3.3 cm of seafloor uplift occurred during a large slow slip event between March and June 2019 offshore New Zealand. Key Points Seafloor pressure data detected vertical seafloor deformation during 2019 slow slip events at the Hikurangi subduction zone Ocean Global Circulation Models can help with the removal of long‐period (>3 month) oceanographic signals in seafloor pressure records Self‐calibrated pressure sensors and depth‐matched reference sites optimize resolution of seafloor vertical deformation