Mobile Pressure Calibrator for the Development of Submarine Geodetic Monitoring Systems

Monitoring long-term vertical seafloor displacements at the centimeter scale using a pressure gauge network in the seismogenic zone is key to understanding the real-time strength of interplate coupling. The mobile pressure calibrator (MPC) was developed for calibrating a seafloor pressure gauge network with a resolution of less than 1 hPa/year, equivalent to about 1 cm/year. Inherent drift of the seafloor pressure gauges is estimated by comparing the raw observations between the seafloor pressure gauges and the MPC and subsequently subtracting the long-term pressure data. In October 2018, three in situ pressure measurements were conducted to evaluate the measurement uncertainty of the MPC. The calibrator was placed adjacent to a seafloor pressure gauge, which is installed at a depth of about 1,750 m in the Nankai Trough offshore Japan, and measured the pressure differences between the calibrator and the seafloor pressure gauge. The drift component of the pressure gauge used inside the MPC was estimated and corrected based on calibrations performed in the laboratory before and after the in situ seafloor pressure measurements. The results indicated an uncertainty of 0.04 hPa for the in situ measurements and therefore the possibility of detecting small, long-term seafloor displacements. We continue to compare the MPC and seafloor pressure gauge deviations over intervals of several months to years and estimate the drift of the seafloor pressure gauge. Plain Language Summary In the Nankai Trough, Japan, large earthquakes have repeatedly occurred. The Dense Ocean floor Network system for Earthquakes and Tsunamis (DONET) consisting of 51 observatories, each equipped with a pressure gauge to study vertical deformation and tsunami signals, was installed spanning the region. Crustal deformation is a key parameter for estimating plate coupling strength in subduction zones. The DONET pressure gauge network could be useful for measuring long-term vertical deformation; however, pressure gauges inherently drift at rates up to 10 hPa/year, about 10 cm/year, which likely exceed seafloor displacement rates. It is necessary to correct gauge drift to reliably detect small seafloor displacements. The mobile pressure calibrator (MPC) was developed to correct drift within the DONET pressure gauge network and enable seafloor displacement measurements on the order of 1 cm. In October 2018, we conducted three pressure measurements at a DONET pressure gauge to evaluate the MPC accuracy. We determined an uncertainty of 0.04 hPa, 0.04 cm equivalent, which should enable seafloor vertical displacement measurements better than 1 cm. Continued DONET pressure gauge MPC calibrations could enable high-density and real-time vertical displacement measurements above the seismogenic zone and contribute to understanding plate coupling strength and earthquake and tsunami hazards.