Calibrated Absolute Seafloor Pressure Measurements for Geodesy in Cascadia

Abstract

The boundary between the overriding and subducting plates is locked along some portions of the Cascadia subduction zone. The extent and location of locking affects the potential size and frequency of great earthquakes in the region. Because much of the boundary is offshore, measurements on land are incapable of completely defining a locked zone in the up‐dip region. Deformation models indicate that a record of seafloor height changes on the accretionary prism can reveal the extent of locking. To detect such changes, we have initiated a series of calibrated pressure measurements using an absolute self‐calibrating pressure recorder. A piston‐gauge calibrator under careful metrological considerations produces an absolutely known reference pressure to correct seafloor pressure observations to an absolute value. We report an accuracy of about 25 ppm of the water depth, or 0.02 kPa (0.2 cm equivalent) at 100 m to 0.8 kPa (8 cm equivalent) at 3,000 m. These campaign survey‐style absolute pressure measurements on seven offshore benchmarks in a line extending 100 km westward from Newport, Oregon from 2014 to 2017 establish a long‐term, sensor‐independent time series that can, over decades, reveal the extent of vertical deformation and thus the extent of plate locking and place initial limits on rates of subsidence or uplift. Continued surveys spanning years could serve as calibration values for co‐located or nearby continuous pressure records and provide useful information on possible crustal deformation rates, while epoch measurements spanning decades would provide further limits and additional insights on deformation.

Shallow Water Seafloor Geodesy With Wave Glider‐Based GNSS‐Acoustic Surveying of a Single Transponder

“…Earth's surface water forms a natural electromagnetic barrier for direct use of space‐based geodetic techniques to measure seafloor motion, resulting in a large portion of the solid Earth surface not being geodetically monitored. Community efforts in developing seafloor geodetic methods, including GNSS‐acoustic ranging (GNSS‐A) (e.g., Spiess, 1985; Spiess et al., 1998; Gagnon et al., 2005; Chadwell & Spiess, 2008; Chadwell & Sweeney, 2010; Chen et al., 2020; Watanabe et al., 2020), direct or indirect‐path acoustic ranging (e.g., Blum et al., 2010; McGuire & Collins, 2013; Petersen et al., 2019), ocean bottom pressure measurements (e.g., Sasagawa & Zumberge, 2013; Wilcock et al., 2021; Sasagawa et al., 2023; Cook et al., 2023), spar‐buoys (e.g., De Martino et al., 2014; Xie et al., 2019), borehole pressure measurements (e.g., Davis et al., 1992), and cabled strainmeters (Zumberge et al., 2018), have improved geodetic coverage of the poorly monitored seafloor. Among them, GNSS‐A has been proven suitable for long‐term and long‐distance seafloor displacement monitoring due to its ability to provide vector measurements in a global reference frame, to be free of instrument drift, and to be flexibly deployable in a range of water depths.…”