The 4.0 magnitude earthquake that rattled the San Francisco Bay Area early Tuesday occurred along the Hayward Fault, which is part of the larger San Andreas Fault system.
There's another West Coast fault that isn't as well known as the San Andreas. It's called the Cascadia subduction zone. Scientists say it could produce a devastating earthquake in the Pacific Northwest.
Chris Goldfinger is a geology professor at Oregon State University.
On Insight with Beth Ruyak today he said part of what made discovering the Cascadia difficult is that it doesn't trigger smaller quakes.
"So the paradigm," says Goldfinger, "went from 'we're not sure what's going on here,' to essentially, 'oh oh, we have a very large, maybe M9 producer of earthquakes."
...meaning a magnitude 9.0 earthquake, like the one that hit Tohoku Japan and triggered a tsunami in 2011.
Scientists say the odds of the Cascadia triggering a major quake in the next 50 years are roughly one-in-three. Goldfinger says the probability varies up and down the subduction zone.
"So in Washington it's relatively modest, 10-15 percent," says Goldfiner. "But in Northern California and southern most Oregon it's as much as 37 percent. And so that's where the one-in-three number comes from."
The Cascadia subduction zone runs 700 miles along the coasts of California, Oregon and Washington.
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