Seattle's Faults: Maps That Highlight Our Shaky Ground

KUOW's Deborah Wang produced this story as part of her 2010 series “On Shaky Ground.”

The rocks and mud of the Pacific Northwest tell the story of much larger earthquakes that have hit this region in the past – and that could hit us again in the future.

Geologist Brian Atwater is at the center of the geologic detective story. When he first arrived at the University of Washington in 1985, there was a very big question looming over the community of earth scientists.

A massive earthquake had just hit Mexico City. It had collapsed hundreds of buildings and killed about 10,000 people. The source of the 1985 quake was the subduction zone off the Mexican coast, a huge fault where two tectonic plates collide, and one slips under the other. Brian Atwater says it was just the kind of subduction zone that was known to exist off the Washington coast.

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Atwater: "And so there was great concern that a fault similar to the one in Mexico along our coast could send its seismic waves into the urban areas of the I-5 corridor."

Scientists knew that subduction zones were capable of producing really big earthquakes, called megathrust earthquakes. They can be greater than magnitude 9 with shaking that lasts for minutes rather than seconds.

Atwater calls himself a mud geologist, and he thought the tide flats and salt marshes along the Washington coast might actually hold the answer to that question.

Atwater: "I mean, it's a really bizarre thing that you could go out to a salt marsh and dig around in the mud and learn about the physical properties of a fault."

What he learned from the mud was that numerous times over the past few thousand years, something very dramatic had happened to the land. He saw signs that the land had dropped and places that had been dry enough to sustain plant life suddenly would be underwater.

There was evidence for that all along the coastline from Neah Bay to Willapa Bay. For him, it pointed to one thing – earthquakes. Scientists had seen just that kind of abrupt dropping of coastal land in previous subduction zone earthquakes around the world.

And Atwater discovered something else. He also found evidence that tsunamis had come ashore, depositing sheets of sand far inland. Another telltale sign of subduction zone earthquakes.

By the 1990s, a new consensus had formed. The Cascadia subduction zone was not only active, but it was thought to unleash a major earthquake every 500 years or so.

But that wasn't the end of the story. The more scientists looked into the geologic record, the more surprises they found.

In the early 1990s, scientists began examining rocks along the shoreline near Alki Point in West Seattle. They found evidence that sometime in the recent geologic past, those rocks had suddenly been thrust upwards by about 20 feet.

Scientists believe the cause of that big earthquake was a fault that lies right beneath our feet. The Seattle fault bisects the city west to east, from Alki Point in West Seattle, through downtown, along the I–90 corridor and all the way towards Issaquah.

Brian Sherrod, a geologist with the U.S. Geological Survey says thousands of years can separate the really big earthquakes, but smaller ones can recur every 600 or 700 years.

The last one was about 1,000 years ago.

“I just basically say that we are in the window of opportunity for a big earthquake,” he said. “It could occur tomorrow, it could occur 100 years from now. It could occur 1,000 years from now. We don't know."

And scientists say the Seattle fault is dangerous not just because of its location, running right through a major metropolitan area, but also because of its depth. It's right at the earth's surface.

In Puget Sound, we're used to experiencing deep earthquakes, like Nisqually. They originate miles underground, so their energy is dispersed by the time they reach us.

If the Seattle fault ruptures the way it has in the past, we'll feel the full force of the quake.

"That earthquake, if it reoccurred today, it would really impact the city, probably the worst seismic disaster in the country,” said Bill Steele, the information director for the Pacific Northwest Seismic Network. Several years ago, scientists, engineers and emergency planners came together to speculate what a magnitude 6.7 earthquake on the Seattle fault would look like. They produced a detailed 150 page report. It makes chilling reading.

Steele: "Ah, we find first of all east–west and north–south transportation severed. And severed for many weeks in some cases months. You lose I-90, 99, I-5 and your port, and suddenly you are kind of in a world of hurt and it's very difficult for aid to come into the area. "

The report projects more than 1,600 dead, more than 24,000 injured, and close to 10,000 buildings destroyed, including the total collapse of some older structures.

Seattle isn't the only place that faces that threat. In recent years, scientists have discovered similar crustal faults all over the state. Ten at latest count just in the Puget Sound region. The biggest one is called the South Whidbey Island fault, and it's thought to run from Victoria, B.C., through Woodinville and possibly beyond.

Sherrod says practically every place they look now, they find something new.

"We still often say that we are in kind of a reconnaissance mode. We're learning a lot right now and trying to see how everything fits together.

Scientists might be excited by the discovery, but for the rest of us, it's unsettling. The more they discover, the worse the picture gets. Scientists say their job is to let the public know about the hazards that are out there. Those giant, potentially devastating, but really rare earthquakes.

Now it's up to the rest of us to figure out what to do about them.