Earthquake Risk in the Pacific Northwest – Seattle, Puget Sound, Olympia, Cascadia

The Pacific Northwest is at risk for three kinds of tectonic earthquakes:
Magnitude 9 Cascadia megathrust quakes,
Magnitude 6 to 7 deep earthquakes within the Juan de Fuca subduction zone, and
Shallow crustal fault quakes up to magnitude 7.

We will discuss how a combination of factors produce these earthquakes: subduction of the Juan de Fuca plate, extension of the Basin and Range, invasion of crustal blocks from California, and the resistance of the Canadian crustal buttress.

Modeling of ground motions that would result from a magnitude 7.2 earthquake are sobering. Areas within a few miles of the Seattle fault, including much of Seattle, would experience 20 seconds of severe ground shaking. How does shaking from a shallow Seattle earthquake differ from a deep one like the 2001 Nisqually earthquake? Because the hypocenter at Nisqually was 32 miles deep, the seismic energy spread out beyond the Puget Sound, rather than being concentrated near the epicenter, as it would be for a shallow earthquake.

A subduction zone mega-thrust earthquake, like Japan experienced in 2011, will have the broadest impact. A magnitude 9 earthquake releases the equivalent energy of four magnitude 7 earthquakes every second over a rupturing interval of four minutes. Projected ground shaking for that event will be severe at the coast, but still very strong in the urban corridor, where ground shaking can last for six minutes, with slow back and forth motions, particularly challenging for tall and long structures. The tsunami comes ashore 15 to 20 minutes after ground shaking stops.

The Pacific Northwest is an area of high earthquake risk because of:
M-9 mega-thrust Cascadia earthquakes on the plate boundary,
Deep earthquakes within the Juan de Fuca subduction zone, and
Shallow crustal earthquakes throughout Cascadia, Olympia, Seattle and Puget Sound.

By putting them all together, scientists are able to create an earthquake hazard map for the region.

Amplified ground motions in the sediment filled basins of the urban corridor are a concern for all earthquakes. Exterior walls of un-reinforced masonry buildings constructed before modern building codes, often crumble during earthquakes. Instinctually, people run out of buildings during ground shaking and are often killed or injured by falling debris. A life-saving response is to drop-cover-and-hold-on until shaking stops.

Earthquake and tsunami education, along with construction of earthquake resistant buildings and infrastructure are vitally important in this seismically active region.

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