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 (ASCPR). 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.