The Cascadia Subduction Zone Explained
By Quake Tracker Live Editorial · Published November 9, 2026
The Cascadia Subduction Zone is a roughly 1,000 km (about 600 miles) fault just off the Pacific coast, from northern Vancouver Island in British Columbia to Cape Mendocino in California. There the small Juan de Fuca Plate dives beneath North America. It is capable of a magnitude 9 earthquake and a major tsunami, and its last full rupture was on January 26, 1700. The USGS estimates a 10 to 15 percent chance of another in the next 50 years.
What is a subduction zone?
At a subduction zone, one tectonic plate slides beneath another. Cascadia's Juan de Fuca Plate moves toward North America at about 4 cm per year, according to the Pacific Northwest Seismic Network (PNSN). The two plates do not slide smoothly. The boundary between them, called the megathrust, can lock for centuries while the plates keep pushing. The upper plate is squeezed and bent. When the megathrust finally slips, the upper plate springs back, which shakes the ground and pushes up the seafloor, displacing a vast volume of water and creating a tsunami.
This is the same type of fault as those behind the 2004 Sumatra, 2010 Chile and 2011 Japan earthquakes. Subduction megathrusts are the only faults known to produce earthquakes larger than magnitude 8.5, which is why Cascadia gets so much attention. The San Andreas, by contrast, is a sideways-sliding fault, as we explain in the San Andreas Fault guide.
The 1700 earthquake: proof from two continents
There is no written record of a Cascadia earthquake, because the last one came before European settlement. The evidence is geologic and historical. Coastal marshes in Washington, Oregon and northern California contain buried soils that dropped suddenly below the tide line and were covered by tsunami sand. Ghost forests of dead cedars stand where land sank. Tree rings show these trees died between the growing seasons of 1699 and 1700.
The date came from Japan. Japanese records describe an "orphan tsunami" that hit the coast without an earthquake felt locally. Working backward from the travel time across the Pacific, scientists pinned the source earthquake to the night of January 26, 1700. The USGS estimates its magnitude at 8.7 to 9.2. It is the largest earthquake known in the contiguous United States.
How often do great Cascadia earthquakes happen?
Sediment cores offshore and coastal marsh records give a history of about 10,000 years, among the best of any subduction zone in the world. They show at least 19 great earthquakes of magnitude 8 or larger in that time, an average interval of about 500 years, according to PNSN, and the USGS national model uses about 500 years for full-margin ruptures. The PNSN gives a range of roughly 480 to 530 years for the average. Individual gaps are irregular: some intervals are only a couple of hundred years, others are close to a thousand.
The southern part of the fault appears to rupture more often than the northern part, and some earthquakes in the past broke only a section of the zone. That is why scientists distinguish between a full-margin rupture of about magnitude 9 and smaller, partial ones.
Logarithmic scale: each gridline is 10 times the previous one.
View the data
| Earthquake | Energy released (vs. Nisqually M6.8) |
|---|---|
| Cascadia, January 26, 1700 (upper estimate) | M9.2 · about 4,000× |
| Cascadia, 1700 (lower estimate) | M8.7 · about 710× |
| Nisqually, Washington (2001) | M6.8 · 1× |
The magnitude of the 1700 earthquake is an estimate of 8.7 to 9.2 (USGS). Energy scales by about 31.6 per magnitude unit.
What are the odds, and what do they mean?
The USGS fact sheet 2025-3050, "Earthquake Probabilities and Hazards in the U.S. Pacific Northwest" (September 2025), gives two numbers for a full-margin magnitude 9 earthquake in the next 50 years. A model that assumes earthquakes happen at random in time, with an average of about 500 years, gives about 10 percent. A model that takes into account that 1700 was the last one, and that earthquakes tend to recur after a long gap, raises it to about 15 percent.
- It is not a countdown. A 10 to 15 percent chance in 50 years means that it is more likely than not that it will not happen in that time frame, but that it is far from negligible.
- It is a full-margin number. Partial ruptures, deep earthquakes and shallow crustal earthquakes are separate hazards. For example, the same fact sheet puts the chance of a magnitude 6.5 or larger deep earthquake in the Puget Sound region at about 85 percent in 50 years, and a shallow crustal one at about 17 percent.
- The numbers can change. New geologic data and models update them. Check the USGS for the current version.
What would a great Cascadia earthquake be like?
The details depend on location, but the main features are well understood from scenario studies and from similar earthquakes elsewhere.
| Effect | What scientists expect | Source |
|---|---|---|
| Shaking | Strong shaking lasting about 3 to 5 minutes along the coast, much longer than a typical crustal earthquake | Oregon DOGAMI |
| Tsunami | Waves arriving at the coast roughly 10 to 30 minutes after shaking starts, depending on location | Oregon DOGAMI, NOAA |
| Coastal land | Sudden sinking of up to about 2 m (6 feet), which would enlarge floodplains | USGS-linked 2025 study |
| Aftershocks | Large aftershocks over months to years | See our aftershocks guide |
Arrival times and shaking duration vary by source and by place. Local tsunami arrival is counted from the start of the earthquake.
Because a local tsunami may arrive within minutes, officials in Oregon and Washington stress that the strong shaking is the warning. If you are on the coast and the shaking is long or strong, move to high ground or inland on foot right away, without waiting for an official alert. Our guide to what to do during an earthquake covers the basics.
Cascadia beyond the megathrust
The subduction zone is not the only source of earthquakes in the region. The subducting slab itself can break deep below Puget Sound, as in the 2001 Nisqually earthquake (magnitude 6.8), and shallow faults in the North American crust, like the Seattle Fault, add their own hazard. Smaller earthquakes happen all the time. To see them as they occur, check the live pages for Washington and Oregon, or the map on our homepage.
Getting ready
The most useful thing to do in advance is to make a plan: know your tsunami evacuation route if you live, work or vacation near the coast, keep supplies for at least two weeks if you can (emergency managers in the region recommend this because roads and bridges may be out), and secure heavy furniture. Our preparedness checklist lists the essentials. For the bigger picture of where Cascadia sits among other U.S. hazards, read U.S. earthquake zones.
Frequently asked questions
When was the last Cascadia earthquake?
On the night of January 26, 1700. Its date was determined from tree rings, coastal marsh deposits and Japanese records of an "orphan tsunami". The USGS estimates its magnitude at 8.7 to 9.2.
How often do Cascadia earthquakes happen?
Full-margin ruptures average roughly every 500 years over the past 10,000 years, with at least 19 great earthquakes in that time. The gaps are irregular, from a few centuries to around a thousand years.
What are the chances of a big Cascadia earthquake?
The USGS estimates about a 10 percent chance of a full-margin magnitude 9 earthquake in 50 years, or about 15 percent if the time since 1700 is taken into account. These are probabilities, not predictions.
How soon would a tsunami reach the Oregon and Washington coast?
Oregon's geological survey says roughly 10 to 30 minutes after shaking begins, depending on the location, and NOAA gives 15 to 30 minutes for some places. The long shaking itself is the warning to move to high ground.
Can Cascadia be predicted?
No. Scientists can estimate probabilities from the geologic record, but no one can say when it will rupture. Early warning systems such as ShakeAlert can give seconds of notice after a rupture begins, not days.
Sources and further reading
- Pacific Northwest Seismic Network: Cascadia Subduction Zone
- USGS Fact Sheet 2025-3050: Earthquake Probabilities and Hazards in the U.S. Pacific Northwest
- Oregon DOGAMI: Tsunami FAQ
- USGS: Cool earthquake facts
This guide is general information, not professional or emergency advice. Quake Tracker Live is not an official warning system; follow instructions from local authorities.