'The Big One': How Cascadia Tsunami Would Impact Puget Sound

OLYMPIA, WA — It may be this decade, or it could be hundreds of years from now, but at some point the Cascadia subduction zone will give way, causing a massive earthquake centered right off the coast of Washington.

For years, local leaders have been studying the subduction zone to predict, and hopefully mitigate, some of the extensive damage that quake is likely to cause.

The latest of those studies, released this week, comes from the Washington State Department of Natural Resources (DNR), and is giving a greater look at how a tsunami caused by "The Big One" will impact communities across the Puget Sound region.

“Millions of people live along the Salish Sea, and knowing what to expect is critical to creating response plans so that we can be ready when – not if – an inevitable Cascadia mega-quake hits,” said Commissioner of Public Lands Hilary Franz. “It’s our duty to put the training and knowledge of our highly-trained geologists to work to make sure our communities can be prepared and safe.”

The study can be alarming, but while it is possible that the quake could happen soon, it's also not very likely. The DNR estimates there is between a 10 to 17 percent chance for a Cascadia earthquake in the next 50 years. The last Cascadia rupture was 321 years ago.

In their study, the DNR simulated a magnitude 9.0 quake on the Cascadia subduction zone, and tracked the projected impact from Blaine, near the Canadian Border, down to Olympia in Thurston County.

One key takeaway from the study: the tsunami will come on quick. Researchers say the first waves would hit Whidbey Island within 90 minutes of the quake. The rest of Puget Sound would be hit between two and four hours after the earthquake.

After the tsunami hits, the water will stick around for some time. The DNR says some shorelines will be inundated for 14 hours or so, but inland waters will be soaked with strong currents for more than a day. How much flooding each area will see varies significantly, from up to 13 feet at the Vashon Island Ferry Terminal to just six inches at the Port of Olympia.

In the Seattle region, Harbor Island will be hit the hardest with about 3.7 feet of flooding.

In the south Sound, the Port of Tacoma would also see 3.5 feet of water.

Other places that would be inundated with water include:

• Bellingham: 10.7 ft
• Deception Pass State Park: 11.7 ft
• Oak Harbor: 6.7 ft
• Oyster Bay: 5.7 ft

The speed of the tsunami will also vary from less than 1 knot to more than 9. The DNR says anywhere hit with 9 knots or higher could suffer extreme damage. Locations of concern named by the study include Gig Harbor, Deception Pass and the Portage Canal.

The following image shows predicted water speeds near Vashon Island:

'The Big One' off the coast is not the only quake that could cause a tsunami in the Puget Sound region — researchers are also looking at how the smaller-but-closer Seattle and Tacoma faults might cause waves.

Here's the modeled inundation for the Tacoma-area from a Seattle Fault Tsunami:

Inundation from the Tacoma Fault or a Tacoma-Rosedale Fault tsunami would be less severe, though the property damage would be significantly higher as the fault is centered just north of Tacoma. The same is true for Seattle and the Seattle Fault.

When a tsunami hits, one of the biggest warning signs is when water recedes rapidly, which the DNR estimates will happen gradually for all the Sound's inner coastal waterways.

>> Learn more about the DNR's models and the state's tsunami response plans by visiting the DNR's website.