An Absolute Self-Calibrating Pressure Recorder for Campaign-Style Detection of Vertical Seafloor Deformation in the Cascadia Subduction Zone

Seawater pressure can be used to detect vertical seafloor deformation because small changes in seafloor height produce measurable pressure changes. Vertical deformation rates in subduction zones due to secular strain are expected to be less than 1 cm/year, signals that are difficult to measure with pressure gauges because of gauge drift. The Self-Calibrating Pressure Recorder (SCPR) was designed to circumvent the problem of gauge drift by employing a deadweight calibrator, which periodically provides a reference pressure that is used to correct for drift in a continuously recorded pressure record. Alternatively, the SCPR can be used to make campaign-style determinations of true seafloor pressure to support long-term deformation measurements and provide an exact reference for nearby pressure gauges. This Absolute Self-Calibrating Pressure Recorder (ASCPR) requires a metrological assessment of measurement parameters to ensure that its absolute accuracy is sufficient to resolve secular deformation. While on a concrete seafloor benchmark, alternating calibration and seawater observations are made every 10-20 minutes for several hours. The difference between the known reference pressure and the seafloor pressure is observed, which allows the calculation of the true, absolute seafloor pressure. In 2014 and 2015, seven concrete benchmarks were placed on the seafloor in the Cascadia subduction zone off central Oregon along a profile that extends from 20 km to 105 km offshore. We surveyed two benchmarks in 2014, 2015, and 2016, a third one in 2015 and 2016, and four more in 2016. Current measurement repeatability varies from 2 to 5 cm, but several corrections still need to be incorporated. The expected resolution is 1 cm.