← all scenarios

Cascadia megathrust earthquake and tsunami

The Cascadia subduction zone off the US and Canadian Pacific Northwest last ruptured on 26 January 1700, an earthquake of about magnitude 9 documented by Japanese tsunami records. It will rupture again. Coastal communities would have minutes, not hours, between the shaking and the tsunami.

The tier rates the quality of the evidence, not how likely the event is.

How it could unfold

1. 1A rupture of hundreds of kilometres of the plate boundary, magnitude about 9● strong evidence
2. 2Minutes of violent shaking across the region● strong evidence
3. 3A tsunami reaches the coast within minutes to under an hour● strong evidence
4. 4Roads, ports and power lines fail; outside help takes days to arrive◐ moderate evidence

Historical anchors

• Cascadia, 1700.
• Tōhoku, Japan, 11 March 2011 (magnitude 9.0-9.1): a tsunami killed about 18,000 people.

What experts and models say

Expert estimates are personal judgements, not measurements, and experts disagree. This site never adds its own probabilities.

What you can do

• Know the tsunami evacuation routes and walk them. Move to high ground on strong shaking: do not wait for a siren.
• Keep a grab-and-go bag and 2 weeks of supplies at home, as local authorities advise.

Open the preparedness checklist for this scenario →

Related scenarios

• A large earthquake near Istanbul (Marmara)

References

1. USGS Fact Sheet 2025-3050: Earthquake probabilities and hazards in the U.S. Pacific Northwest. link ↗