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Study: Megaquake looms over Seattle

If new findings are accurate, the Cascadia fault will rupture within 68 miles of downtown Seattle, pouring seismic energy into a densely populated urban area.
Image: Quake map
The white line in the above graphic is a previous estimate for nine-meter slip. The red line shows an area of the nine-meter slip 25 kilometers deep in the crust, as predicted by James Chapman and Timothy Melbourne of Central Washington University. The green line runs down where the slip dissipates to zero as the fault deepens to the east.Larry O'Hanlon
/ Source: Discovery Channel

The Cascadia thrust fault, one of the most dangerous and powerful faults on Earth, will hit even closer to home than anyone thought possible, according to a new study.

If the new findings are accurate, the fault will rupture within 68 miles of downtown Seattle, pouring seismic energy into a densely populated urban area, threatening to knock down buildings both large and small, and endangering the lives of millions.

The threat of earthquakes is just a part of life for those living in Seattle, Vancouver and throughout the Puget Sound region. Scientists know it is just a matter of time before Cascadia lets loose a devastating quake on the order of magnitude 9.0.

For decades, scientists, urban planners and emergency responders have taken small comfort in the fact that the tremor would stop near the coast of the Olympic Peninsula, perhaps as far as 118 miles west of Seattle.

If a huge quake hit, shaking in the city would be strong but low-frequency, the thinking went. There's a risk that buildings might collapse under that scenario, but those less than 10 stories would be spared.

However, the fault's behavior tells a different story.

Since 1997, Timothy Melbourne of Central Washington University in Ellensburg has been watching the Olympic Peninsula deform and listening to tiny quakes deep in Cascadia's bowels.

In an article due to be published in Geophysical Research Letters, he and colleague James Chapman show that the fault is locked down to a depth of 15 miles (25 kilometers), 6 miles deeper than previously thought.

"Normally, this is academic, and nobody gives a rip," Melbourne said. "But this is a shallow-dipping fault. As it goes down, it heads east from off the coast towards Seattle and Vancouver. At 25 kilometers' depth, that brings you right to the western edge of the Seattle metropolitan area."

That spells trouble for millions of people who call the area home.

As far as scientists know, large quakes of around magnitude 9.0 occur every 550 years or so on Cascadia. The last temblor was in 1700 so it could be some time before the next big one, but the fault already has enough pent up energy to slip 30 feet in the blink of an eye.

With tectonic plates shattering so close by, the city of Seattle could be rocked by a shotgun blast of high and low-frequency energy, Melbourne suggested. Ground shaking could be up to five times stronger than anyone has planned for. Everything from small buildings to skyscrapers would be at risk of collapse.

"What this really means is emergency service planners need to take a long, hard look at the policies they have in place for dealing with an earthquake," Melbourne said. "Whenever this happens, it's not going to be good. It's going to get ugly."

No one disagrees with the danger Cascadia poses to the region. But Garry Rogers of the Geological Survey of Canada said that Melbourne and Chapman's findings aren't new.

Previous studies suggested that rupturing could spread deep along the fault. To be safe, those estimates have already been built into hazard maps in both Canada and the United States.

"There might need to be a few tweaks to the maps, but it won't be the dramatic shift that they claim," Rogers said. He added that he also thinks it's unlikely that much high-frequency energy will affect Seattle or Vancouver.

Instead, Rogers applauded the researchers for confirming what scientists had previously only been able to guess at: When (not if) Cascadia ruptures, it's going to break deep into the crust, releasing devastating amounts of energy into a densely populated area.