Weekend Long Reads: A Whole Lot of Sloshing Going On! What a Tsunami Would Do in Puget Sound

by Kevin Schofield

If you've lived here in the Pacific Northwest for a while, you've probably heard of the Cascadia Subduction Zone, a massive earthquake fault off the coast of British Columbia, Washington, and Oregon where the seismic plate holding up the land is slipping underneath the one at the bottom of the ocean. Pressure builds up for centuries along the area where they overlap and rub against each other, and every 500 years a major "rip" occurs where the mainland plate moves farther west and down, and the ocean plate is pushed up (and potentially east). The resulting earthquake is around magnitude 9.0 — about 100 times stronger than the 2001 Nisqually earthquake, our last big seismic event here in Seattle. In addition to the earth-movement damage that it would cause, the uplifting and dropping of the ocean floor along the fault line is expected to cause a tsunami wave.

The last major "rip" of the Cascadia Subduction Zone was in 1700 AD. Scientists and historians roughly estimated when it occurred based upon coastal soil samples, tree fossils, and oral histories from local tribes who lived along the coast at the time. But then were later able to pinpoint the exact date because the tsunami generated by the quake traveled west across the entire Pacific Ocean and caused unusual coastal flooding in Japan that was documented in their copious records. Of course a tsunami also headed east and hit the Pacific Northwest coast — hard. Recently the state deployed a tsunami advance-warning system to warn people if the "big one" hits.

But a tsunami from the Cascadia Subduction Zone would also enter Puget Sound and work its way around the inner waters, so scientists have been working on complex models to estimate what flooding might look like in various populated areas. Earlier this month, the State's Department of Natural Resource published a new set of predictions for waves and flooding.

Predicting wave propagation is very tricky business, especially when it comes to what happens when a wave hits land. The factors involved include the shape and strength of the waves, the depth of the water offshore, the roughness of the terrain along the coast, tidal changes, and how much the land itself shifts during the earthquake. It's further complicated by the ragged coastlines and hundreds of islands within Puget Sound: Waves also bounce off the land, just as they ping-pong back and forth in a bathtub. When the next tsunami hits, it won't cause the same waves everywhere, and it won't hit everywhere at the same time. But the tsunami also will hit the same places with multiple waves over several hours as the water sloshes around the Sound — and the largest wave might not be the first one to hit.

The State's latest modeling focuses on the coast along the eastern and southern portions of Puget Sound and models what the waves might look like in 43 different locations. Admiralty Inlet, a straight shot in from the Pacific Ocean, will get hit hard, with a peak wave as high as 12 feet, so would the west side of Whidbey Island and Deception Pass. Interestingly, the southern end of Vashon Island could get hit even harder. The good news for downtown Seattle is that the peak wave would be much lower, perhaps closer to four feet — though the raised sea level could sustain for over an hour.

We also would have a bit of advance warning: Admiralty Inlet, Whidbey Island, and Deception Pass would see a big wave starting about 90 minutes after the quake; it would take another hour or so for the first wave to hit Seattle. But in almost all cases, we'd see sea level drop before it rises: Water will rush out to sea to fill in where the ocean floor drops before it sloshes back in. It's a bit ironic, but if you see a sudden drop in sea level (beyond regular tidal changes), run for higher ground. Also, some of the sea level drops could be substantial: Three hours after the quake hits, Deception Pass could see a drop of 10 to 12 feet. If that happens at low tide, a lot of boats could end up grounded — or if it happens after the wave hits, it could deposit a lot of boats on land.

Tsunamis are serious stuff, and we do much less preparation for them in the inner waters of Puget Sound than our coastal communities do. With these latest models, cities and towns may need to rethink that.

Tsunami hazard maps of the Puget Sound and adjacent waters

Featured image belongs to the public domain.