Meeting Documents

Observations of Vertical Secular Deformation in the Offshore Hikurangi Subduction Zone

Abstract

In October 2022, we deployed a seafloor pressure geodetic network on the Hikurangi Subduction Zone, offshore the Gisborne region of New Zealand’s north island. The area regularly hosts offshore slow slip, but this ~1 year deployment coincided with a period of quiescence that provides the opportunity to investigate the secular strain signal on the Hikurangi margin from ongoing convergence. The bottom pressure data are sensitive to vertical deformation of the seafloor and the highly precise quartz-resonator sensors used in these instruments are capable of millimeter-level resolution. However, the presence of sensor drift–an artificial exponential-linear signal–complicates the observation of long-period, low-amplitude signals. In this experiment, we deployed instruments capable of “A-0-A” calibration, in which the sensors can measure their own drift by calibrating against the stable internal pressure of the instrument housing. We show that the drift for these sensors is as large as 20 hPa/yr (~cm/yr) and correcting for this effect yields pressure data with trends at the hPa/yr level. The corrected data compare well with predicted bottom pressures from the GLORYS and ECCO oceanographic models. By comparing the trends of the observations and models, we attempt to discriminate between the solid-Earth and oceanographic components of the net pressure change over the duration of the deployment.