Earthquake near Seattle could produce massive 20ft tsunami within minutes, state warns

While unlikely, a quake in Puget Sound could send a 20ft wall of water into parts of the city

If a major earthquake were to hit off the coast of Seattle, the resulting tremors could send a 20-foot wall of water toward the city, a new state report finds.

The researchers found that with a large quake, the landscape could change dramatically, and massive waves would reach the shore within three minutes, with after-effects continuing for another few hours.

The results emphasize the potential tsunami risk in this tectonically-active area — even for somewhere like Seattle, which sits comfortably far from the ocean inside Puget Sound.

“Most often, when we think of tsunamis, we think of our outer coast and communities along the Pacific Ocean,” Washington’s Commissioner of Public Lands Hilary Franz said, via a press statement.

“But there’s a long history of earthquakes on faults in the Puget Sound.”

Like much of the Pacific rim, the Washington coast is riddled with fault lines, including the Seattle Fault, which runs right underneath the sound and the city. The last time an earthquake hit that fault was around 1,100 years ago, the state says, but the fault is still capable of shifting.

This study modelled an approximately magnitude-7.5 earthquake under Puget Sound along the Seattle fault as something of a worst-case scenario.

A magnitude-7.5 quake would be classified as a major earthquake, likely to cause intense damage.

Within minutes massive waves, 10 feet tall or higher, would crash into both Seattle on the east side of the sound and Bainbridge Island on the west side. Intensity would be highest in the fault area, but spread over the entire inner coastal region, from Bellingham in the north to Olympia in the south.

Maps show that a spot like Great Seattle Wheel on Elliott Bay could see over 40 feet of water.

In addition, some of that water would be moving extremely quickly — around 4 metres per second at its fastest, which could cause additional destruction.

Beyond the initial impact of the tsunami, water from the sound would continue to inundate parts of the city for hours. Waters could reach far inland in some places.

This study documents an unlikely, but potentially devastating, impact on one of the US’s metropolitan areas, with over 700,000 people living in Seattle alone. And the results show that many people near the coastline would have just minutes to respond if such a quake were to happen.

“Although the chances of this happening in our lifetime is low, it’s important for families to get prepared now,” Maximilian Dixon, from the Washington Emergency Management Division, said via the statement.

In addition to quakes along any number of the faults surrounding Puget Sound, the Pacific Northwest faces the potential danger of a major earthquake on the Cascadia subduction zone, where the Juan de Fuca and Gorda tectonic plates are slipping below the North American tectonic plate.

As Kathryn Schultz reported in the The New Yorker in 2015, the resulting “really big one” could be somewhere between magnitude 8.0 and 9.2.