Before the Big One: Geologists dig into smaller fault lines to predict Washington's future earthquakes

The faults under Eastern Washington aren't as famous as the feared Cascadia Subduction Zone, but they still pose a threat to communities like Walla Walla, Tri-Cities and Yakima.

A geologist digs at a fault line

USGS volunteer John Lasher helps dig volcanic ash out of the trench in this dig area southeast of Pasco. (Courtney Flatt/Northwest Public Broadcasting)

Just off the highway near Washington’s Tri-Cities, 20 miles southeast of Pasco, Steve Angster steps into a giant trench. In it, the U.S. Geological Survey geologist is hoping to find evidence of a fault line.

He looks out through the morning fog. It’s hard to see the surrounding landscape, but Angster knows it well. 

“On a clear day you could see Rattlesnake Mountain, and then, we have what we call the rattles, which are those little buttes along the way,” Angster said.

Those buttes and the basin south were formed thousands of years ago from a fault – a fault geologists know very little about. 

But they’re trying to learn more. 

Worried about Cascadia, but …

“Everybody’s worried about [the] Cascadia [Subduction Zone],” Angster said of the fault that’s predicted to one day rupture and cause “The Big One.” It’s predicted to catastrophically damage land and infrastructure west of Interstate 5 in Washington and Oregon.

“But we have a lot of faults closer to home that pose a pretty significant hazard to the [nearby] communities” in the Inland Northwest, Angster said. “Being that this is really close to the Tri-Cities area, we want to understand this better. How big of an earthquake can we expect from a fault like this?”

Unlike the Cascadia Subduction Zone, the faults in Eastern Washington are in the upper crust, (the outermost layer of the Earth). That means they’re closer to the surface and to communities. They may not produce as large of an earthquake as the subduction zone off the coast, but the earthquakes these inland faults can produce could significantly damage  infrastructure in nearby communities.

A geologist looks at flags inside a scarp
USGS geologist Steve Angster scrapes volcanic ash out of the trench and collects it in a plastic baggie. He can use the ash to figure more about when this scarp formed. (Courtney Flatt/Northwest Public Broadcasting)

Despite all the warnings about The Big One, among the largest earthquakes to happen in Washington have shaken communities east of the Cascades: a magnitude 6.8 near Entiat in Chelan County in 1872 and a magnitude 6.0 near Walla Walla in 1936.

The fault Angster is searching for could be a trace of that 1936 earthquake.


Back at the dig site near the Tri-Cities, you can actually see where the fault line might be. It’s what geologists call a scarp. To the average eye, it looks like a slight rolling bump in the ground, but it’s actually where one side of the fault line has slid up relative to the other side.

With the help of LIDAR laser-imaging technology, Angster can follow this bump, or scarp, a little more than 12 miles. That’s from Wallula Gap toward Walla Walla along the base of Horse Heaven Hills.

“Since it’s a [suspected] fault scarp, we want to dig across it and figure out when did this rupture,” Angster said.

They’ve dug a 90-by-12-foot trench across the scarp. It’s mapped with pink, yellow and blue flags.

“We clean it and then we study it for three weeks — we were sitting in this thing through the wind and everything. And this is pretty dusty material,” Angster said. “So we had some pretty crazy days.”

To Angster’s eyes, the barcode-like sediments inside this trench are like a history book. He can identify different features from the sediments, flagging important pieces of the trench wall. They know the fault scarp formed sometime after the Missoula floods, when torrents of glacial floodwaters swept through the area some 15,000 years ago.

Geologists are hoping to learn more about fault lines east of the Cascades, like the one Angster is searching for.

They know of at least 10 fault zones in Eastern Washington that are 40 miles or longer, according to Megan Anderson, an earthquake geophysicist with the Washington Geological Survey.

“Those are the faults that could rupture to produce an earthquake magnitude that is damaging to communities and of particular concern. That isn’t to say there are some out there that we don’t know about,” Anderson said.

Her goal is to one day map all the fault lines in the state. That way geologists can provide a risk assessment for people: the probability of when a fault line in each area is expected to rupture. Right now, they can mostly tell when a fault last ruptured on a geological time scale, over thousands of years.

But a risk assessment “[is] so much easier to understand than this one fault having an earthquake once every 2,000 years. How do we prepare for that? How do we even absorb that in our everyday lives? You know, useful for us geologists, but a little less useful for everybody else,” Anderson said. 

A backhoe digs near a fault scarp
After the geologists are finished studying the fault scarp in the trench, they fill it back in. They plan to dig another trench in 2020 to study this geologic feature even more. (Courtney Flatt/Northwest Public Broadcasting)

In California, that sort of mapping took almost 100 years, but it should be faster in Washington because technology is better now.

Some of the big faults geologists know about on the dry side of the state are near the Tri-Cities, Walla Walla, Chelan, Ellensburg and Yakima. You might recognize these landmarks they formed: Saddle Mountain, Umtanum Ridge and Rattlesnake Ridge.

“All of those ridges were uplifted. All of that topography was made by the faults that lie underneath them,” Anderson said.

They started work on a major mapping project this year in the Ellensburg area. The project could take up to five years.

“We’re in the middle of trying to understand how [faults] all work together, how active they are, all these different things,” Anderson said. “It’s a really exciting time in science because it’s a really a big phase of discovery.”

And it’s important for people to be prepared, even if they’re not in danger of damage from The Big One, Anderson said. Emergency managers say you need to be at least two weeks ready when a disaster hits, with enough food, water and medicine stocked away.

Carbon dating

Outside of the Tri-Cities, geologist Steve Angster opens up a plastic baggie and scrapes tiny bits of dirt inside. He’s collecting ash from volcanic eruptions. The team hopes to date the carbon to know more about when this scarp formed.

They think it could date back 10,000 years.

“That puts this fault on our list as pretty active and potentially hazardous fault for this area,” Angster said.

They didn’t find the fault line in this trench, but Angster said they plan to keep on looking, hopefully digging again next summer.

“What we’ve found really in this trench is really good evidence for strong shaking,” he said. “We are at the very eastern end of this rupture or this fault trace. So it could be that we just have very little movement here. And so it could be very hard to see.”

This story was originally published on Northwest Public Broadcasting on Dec. 6, 2019.

Please support independent local news for all.

We rely on donations from readers like you to sustain Crosscut's in-depth reporting on issues critical to the PNW.


About the Authors & Contributors