Earthquake early warning (EEW) works by detecting the first arrivals from a rupture, estimating its size and location in seconds, and broadcasting an alert before the stronger shaking arrives. For a Cascadia megathrust rupture, the instruments best positioned to give the earliest warning are on the seafloor, above the locked fault itself.
Why offshore matters
The USGS ShakeAlert system already operates along the U.S. West Coast using onshore seismic networks. For onshore or near-shore events, this works well. For a great Cascadia rupture initiating offshore, however, the first P-wave arrivals at onshore stations are delayed by the travel time from the hypocenter to the coast, typically 15 to 30 seconds. Seafloor sensors shorten that delay dramatically.
How COSZO contributes
Three COSZO subsystems contribute directly to EEW:
- Strong-motion accelerometers near the expected rupture surface detect initial ground motion without clipping.
- Short-period seismometers catch low-magnitude precursors and enable rapid magnitude estimation.
- Seafloor pressure sensors operating in the tsunami band provide independent tsunami-source information within minutes.
Integration with ShakeAlert
Integration of COSZO data into ShakeAlert is an explicit project goal, coordinated between the COSZO team, the Pacific Northwest Seismic Network, and the USGS. Technical integration is scheduled for the third project year, following full instrument commissioning.
Earlier work
COSZO builds on extensive community study, including the 2019 Feasibility Study that first laid out the quantitative case for seafloor EEW off Cascadia.