Sunday's 6.8 quake off the coast of Eureka sparked momentary concerns about a tsunami.
Now, the Los Angeles Times reports that danger, and the potential for a huge quake in that area, are much greater than previously thought.
Scientists had believed the Cascadia subduction zone could only produce a quake up to 7.5 on the Richter Scale.
Now, they believe there was a magnitude 9 in the year 1700, which caused sections of the coastline to drop by 5 feet.
A tsunami caused by such a quake today, researchers say, would kill as many as 10,000 people, destroy Highway 101, and cause $70 billion in damages up and down the Pacific coast.