Ask geologists where the next big tsunami might strike, and the answer is a refrain: North America´s Pacific Northwest coastline. That´s the location of the Cascadia Subduction Zone, a 680-mile-long fault that hugs the shore from Northern California to Vancouver Island.

Akin to the tectonic deadlock that eventually snapped and spawned the Indian Ocean tsunami last December, Cascadia is a geological battleground where the Juan de Fuca and North American plates are duking it out. The subterranean stress building at the front lines could eventually rupture in exactly the same way it did in Southeast Asia-a seismic event geologists call a “megathrust earthquake,” so named because it occurs between a subduction plate and an overriding plate in a region known as the inner-plate thrust.

Quakes within this powerful inner-plate subduction zone can readily top magnitude 9.0 on the Richter scale, explains Eric Geist, a geophysicist with the USGS who has
created computer models that show how a Cascadia-generated tsunami might swamp Pacific Northwest communities.

“During one of these big earthquakes, the coastline will drop down one or two meters,” he says, noting that the collapse would happen instantly. “The modeling
suggests that the tsunami run-up could be as much as 20 meters or greater.” Towns such as Seaside, Oregon; Crescent City,
California; and Westport, Washington, could be swept away in minutes.

Predicting exactly when Cascadia will crack has stumped geologists. Because the fault has been quiet for so long, it´s tricky to calculate the frequency of quakes. Geological clues suggest that major temblors have occurred along Cascadia seven times in the past 3,500 years, leading scientists to believe that a quake-tsunami combo occurs there every three to five centuries.

Researchers recently unearthed evidence that a massive tsunami razed the Pacific Northwest coastline in 1700. They tracked down Japanese records from that same year documenting a major tsunami and