Unlocking plate motions of the Cascadia subduction zone with seafloor geodesy

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Seeking understanding of the fundamental constraints on plate motions, rates of convergence, and shallow strain accumulation across one of the United States’ most hazardous fault zones.

USGS scientists from the Earthquake Hazards and Coastal Marine Hazards and Resources Programs have joined forces with academic colleagues (Scripps Institution of Oceanography, University of Washington, University of Hawai‘i/University of Stuttgart) in a pioneering effort to instrument the seafloor in the Pacific Northwest. The goal is to understand where and how much strain is accumulating along shallow portions of the Cascadia subduction zone that have produced large earthquakes and tsunamis in the past. From March 12-17, two new seafloor geodetic sites, each consisting of three seafloor benchmarks, will be placed on the seafloor to measure small (centimeter-scale) tectonic plate motions at each site. The benchmarks will be deployed from R/V Sikuliaq. Plans are underway to retrieve data from these sites in summer 2021 using the USGS waveglider. Over the next decade, repeat measurements at these and other seafloor geodetic sites in Cascadia will provide fundamental constraints on plate motions, rates of convergence, and shallow strain accumulation across one of the United States’ most hazardous fault zones.

Photo of the stern of a ship has a piston crane with pulleys across top, and instruments with floats sit on the ship deck.

Six orange seafloor transponders rest on the deck of reasearch vessel Sikuliaq, which is owned by the National Science Foundation and operated by the University of Alaska Fairbanks. The transponders will be placed on the seafloor in the Cascadia subduction zone offshore Oregon and Washington. As they relay their positions to surface vessels or wavegliders, they will serve as benchmarks recording the relative motions of the tectonic plates on which they sit.

(Public domain.)

An apparatus with various fins and equipment sits on the deck of a ship in a harbor.

Waveglider used to communicate with a geodesy station’s seafloor transponders.

(Credit: Rob Wyland, USGS Pacific Coastal and Marine Science Center. Public domain.)

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