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.

This preprint has been deleted as it was posted before approval by USGS.

Measurements of ground tilt are a critical geodetic tool for monitoring active volcanoes because they provide multidimensional data that can resolve complex deformation signals. We are developing a Self-Calibrating Tilt Accelerometer (SCTA) for use in the marine environment and present results from two deployments: on land at the Scripps Institution of Oceanography Cecil and Ida Green Piñon Flat Observatory and on the seafloor at Axial Seamount on the Juan de Fuca Ridge. The SCTA utilizes a Quartz Sensor Solutions triaxial accelerometer on a gimbal system to periodically rotate the horizontal channels into the vertical to calibrate against the local g vector, achieving high precision and stability within 1 microradian. The SCTA tiltmeter has the added benefit of simultaneously measuring ground accelerations and recording seismic signals. We compare the SCTA performance at the center of the summit caldera at Axial Seamount against a co-located Jewell Instruments LILY tiltmeter on the OOI Cabled Array. The tilt measurements in one direction are consistent, but the data suggest that the deployment platform for the SCTA may be settling in the other direction. We are using data from the ensemble of 4 cabled pressure sensors and 5 tilt sensors at Axial, including the SCTA, to study its inflation behavior since its eruption in 2015. We have identified several significant, cm-scale deflation events of durations of tens of days. The tilt and relative elevations of instrument sites are asymmetric about their turning points, suggesting a more complex mechanism than a simple inflation reversal. We are conducting forward modeling of the deformation signals to determine if the geodetic signals are consistent with differential slip rates, normalized to the rate of inflation/deflation, on the caldera’s outwardly dipping ring faults between these periods. Another plausible mechanism that we plan to investigate is the lateral transport of magma from beneath the southern caldera to either the northern caldera or to a secondary reservoir, located 5 km to the east. These deflation events are potentially important for understanding the mechanisms of magma supply, storage, and transport at Axial Seamount, as well as for accurately forecasting future eruptions, which have been shown to be inflation-predictable.