Collaborative Research: Constraints on Interseismic Deformation Offshore Oregon from Calibrated Continuous Pressure Records

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The Cascadia Subduction zone lies off the coast of the Pacific Northwest and is of both scientific and public interest because it is the site of catastrophic earthquakes similar to those that have occurred over the last decade in Japan, Chile, and Sumatra. On land, networks of instruments are available to measure the build-up of strain across the Cascadia subduction zone fault. Similar observations are required offshore to understand the dynamics of the subduction zone better, and to place stronger constraints on the earthquakes and tsunamis the fault produces every few hundred years. This project uses calibrated seafloor pressure sensors to make accurate measurements of the elevation of the seafloor at six sites off the coast of central Oregon. The project will initiate a decadal study that will measure changes in seafloor elevation that result from the build-up, and potentially the release, of strain across the subduction zone fault. The study will constrain the width of the locked portion of the fault that will break in the next large subduction zone earthquake. The project will involve two graduate students, and include at least four undergraduates on each research expedition.High-quality offshore vertical geodetic measurements will be used to provide a baseline suitable for resolving secular strain caused by locking across the Cascadia subduction zone, and for detecting slow slip events. Several previous studies have shown that seafloor pressure measurements can detect slow-slip events, but the drift of seafloor pressure sensors has limited their application to measuring secular strain. The University of Washington (UW) and Scripps Institution of Oceanography (SIO) have collaborated to build and successfully test a campaign version of the SIO seafloor calibrated pressure recorder, the Absolute Seafloor Calibrated Pressure Recorder (ASCPR). This instrument measures absolute pressures to an accuracy equivalent to ~1 cm of water depth. Repeat measurements with the ASCPR on stable seafloor benchmarks equipped with a continuously recording bottom pressure gauge will be used to remove the drift from pressure time series and enable measurements of secular strain. The UW and SIO have deployed a trench-perpendicular profile of 6 benchmarks and bottom pressure recorders off central Oregon. This project supports repeat campaign-style absolute pressure measurements over two years on each benchmark to fully characterize the repeatability of ASCPR measurements and initiate a decadal study required to measure secular strain. By utilizing stable, undisturbed benchmarks to make accurate absolute pressure measurements, this effort could transform our capabilities for making long-term geodetic observations offshore. Complementary geodetic and physical oceanographic modeling studies are being used to further understand the expected geodetic signals and develop improved methods to separate geodetic and oceanographic components of pressure.