The Cascadia subduction zone stretches roughly 1,000 kilometers from northern California to British Columbia, capable of generating magnitude 9+ megathrust earthquakes and far-field tsunamis. Our ability to understand and warn against these events has, until now, been constrained by the fact that the locked portion of the fault lies almost entirely offshore.
The Cascadia Hazard
The last great Cascadia earthquake occurred on January 26, 1700. Paleoseismic, geodetic, and historical records now place the recurrence interval in the 200 to 800 year range, and the plate is accumulating strain at a rate that implies a large event is a matter of when, not whether. Coastal communities from California to British Columbia would be directly affected, and a tsunami would reach shore in 15 to 30 minutes.
Decades of onshore GPS and seismometer networks have transformed our picture of the onshore deformation field. But the locked region of the megathrust, the part that stores and will eventually release most of the strain, sits beneath the continental shelf and abyssal plain. Observing it requires instruments on the seafloor.
An Observation Gap
Offshore geophysical observations have historically been episodic. Ocean-bottom seismometer (OBS) deployments are valuable but typically operate for a year or two at a time, limiting their ability to capture long-term deformation transients or the full spectrum of pre-seismic behavior. Permanent, cabled, real-time seafloor instrumentation is what the community has needed, and what the Cascadia Offshore Subduction Zone Observatory provides.
What we still do not know
- The precise updip and downdip limits of the locked zone.
- Whether slow slip events extend offshore and how often they occur.
- The timing and magnitude of potential precursory signals.
- How intraslab seismicity relates to megathrust loading.
Building on the RCA
The OOI Regional Cabled Array, commissioned in 2015, provides something rare in ocean science: a permanent power and communications backbone on the seafloor, with real-time data telemetry. The RCA was not originally designed as a geophysical observatory. It was built for coastal oceanography. But the cable runs directly across the Cascadia margin, and the nodes it feeds are fortuitously positioned in the region of greatest geophysical interest.
COSZO augments the RCA with a geophysical sensor suite: strong-motion accelerometers, broadband seismometers, absolute and differential pressure gauges, acoustic hydrophones, and current meters. Each contributes a different piece of the puzzle.
Why COSZO, Why Now
With the RCA infrastructure in place, the engineering risk of adding geophysical instruments is greatly reduced. What remains is a science opportunity of unusual magnitude: decades of continuous, open-access, real-time observation across the locked region of a major subduction zone. Few facilities anywhere in the world will match it.
For a complete statement of the science drivers, see our Scientific Objectives.