In terms of sheer numbers, previous reports estimated that Washington state would have a major quake every 500 years on average. Goldfinger’s team now shows them to occur about every 430 years. And northern Oregon, which used to receive a quake about every 430 years, instead faces one every 350 years.

This means that there is about a 20 percent chance that northern Oregon—a region that includes Portland and Astoria—will be hit by a magnitude-8.0-or-higher quake in the next 50 years. (Previously, the area was estimated to have about a 12 percent chance of facing such a quake.) Washington state has between a 14 and 17 percent chance of facing a big one, up from an 8 to 14 percent chance.

These numbers still pale in comparison to earthquake risks to the south. For instance, the San Francisco Bay area has a 50 percent chance of experiencing a magnitude-7.0 earthquake in the next 30 years. Los Angeles faces a 93 percent chance. But scientists expect a major rupture in Cascadia would be much more powerful than one of these Californian quakes.

“Geology is a badly edited tape, and we lose lots of evidence along the way,” says Mika McKinnon, a geophysicist and disaster researcher who was not associated with the study. “We lose evidence of drowned forests, we lose evidence of faults moving, we bury everything in new volcanic floes, we run glaciers over everything 15,000 years ago and completely erased the older history.”

This means that geologists looking for clear evidence turn to the Holocene, the past 10,000 years that followed the end of the last Ice Age.“Yet even in that little, limited time span, we found [earthquakes] as close together as 100 years, and as far apart as 1,000 years. Averaging doesn’t work very well when you’re using very small numbers,” she said.

“It is not surprising that we are finding more evidence of more earthquakes,” she added.

Goldfinger’s team conducted this research by taking core soil samples from underwater sites off the Pacific coast that showed evidence of submarine landslides. By comparing how those core sample hold up to a variety of tests—including density, resistivity, and radiocarbon aging—they can identify whether multiple landslides occurred simultaneously. If different samples from different parts of the ocean bear the same signature, it’s a give-away for an earthquake.

This isn’t a new technique in the hunt for Cascadian earthquakes, but Goldfinger and the team deployed it much more broadly than before. Previous studies had only examined 12 core samples; their research analyzed 195 different deposits.

For geologists and geophysicists, the headline of the report isn’t so much the newly discovered earthquakes—it’s the increased confidence around the basic science of the Cascadian subduction zone. By going back 10,000 years, the study makes it even clearer that enormous ruptures in the Pacific Northwest happen regularly and strongly.