The COSZO facility is designed around five interlocking scientific thrusts. Each leverages a different subset of the sensor suite and each addresses fundamental, unresolved questions about how subduction zone faults work.
1. Constrain the updip and downdip limits of the locked zone
Strong-motion and broadband seismometer coverage across the continental shelf will improve our ability to locate small earthquakes along the megathrust, refining where the fault is seismically active and where it is locked. Complementary seafloor pressure gauges track vertical deformation at the sensor sites with sub-centimeter sensitivity.
2. Detect and characterize offshore slow slip
Onshore geodesy has revealed a rich catalog of episodic tremor and slow slip in the Cascadia forearc. Whether analogous slow slip occurs offshore, and whether it migrates along strike, is a central open question. COSZO pressure sensors operating at tsunami-band frequencies, together with the seismic array, will provide the first sustained offshore geodetic look at this phenomenon.
3. Characterize the full seismicity catalog beneath the shelf
Intraslab and crustal seismicity beneath the Cascadia margin remain under-sampled. A dense seafloor seismic network closes the detection gap and enables magnitude-of-completeness improvements of roughly one unit compared to onshore-only networks.
4. Quantify near-source ground motion
Strong-motion accelerometers on the seafloor give the first direct measurements of ground shaking near the expected rupture surface of a future great earthquake. These records are essential for calibrating ground-motion prediction equations used by engineers and emergency planners.
Offshore earthquake early warning
In addition to the science objectives, COSZO will provide a test bed for offshore earthquake early warning.
Because COSZO instruments sit offshore of population centers, they can detect rupture initiation seconds to tens of seconds before onshore networks. Integrating COSZO data streams into the USGS ShakeAlert system is an explicit project goal and is the focus of our Early Warning work.