If a really big earthquake hits offshore from Seattle, the city’s shorelines could be struck with massive tsunami waves within a matter of three minutes. In a worst-case scenario, the waves hitting Seattle’s Magnolia Bluff neighborhood could crest at 33 feet high.
The takeaway: If the ground starts shaking in the greater Seattle area, quickly get to higher ground.
“Most often, when we think of tsunamis, we think of our outer coast and communities along the Pacific Ocean. But there’s a long history of earthquakes on faults in the Puget Sound,” said Commissioner of Public Lands Hilary Franz, in a statement announcing the report.
Franz emphasized the importance of making sure that at-risk communities are informed and develop response plans.
While much attention has historically been given to quakes generated by the Cascadia subduction zone, which is centered along a submarine fault just off the West Coast, the new study focuses on the Seattle Fault zone. This much shorter fault runs east-west under Puget Sound. It stretches through the naval town of Bremerton, across Bainbridge Island, through Seattle and east past Bellevue.
The last earthquake on this fault took place about 1,100 years ago and the fault is considered to still be active.
The research considered a worst-case, low-likelihood scenario triggered by a 7.5 magnitude quake. In this situation, tsunami waves would first hit in northern Elliott Bay, which is the body of water offshore of downtown Seattle; West Seattle’s Alki Point; and the eastern shoreline of Bainbridge Island. The waves could reach Olympia, the southern-most reach of Puget Sound, within an hour and extend north to Canada.
The large waves and strong currents generated by a quake could last for three hours. The tsunami would also cause extensive shoreline flooding and push flooding up rivers that empty into the sound.
The earthquake would shove some land masses up higher, a phenomenon known as uplift, while others would be pushed lower or subside.
The report was written by geologists within the Washington Geological Survey division of DNR and included researchers from the National Oceanic and Atmospheric Administration (NOAA) and the University of Washington.
The study drew on information from geologic records, oral histories from indigenous people, and scientific models.
Warning signs of an impending tsunami include the quake itself, water pulling away from shore or suddenly rising, or a roaring sound from the sea, the report states.
In January, the free MyShake app became widely available in Washington to provide early warnings that an earthquake is starting.
“It is important to be familiar with the natural warning signs of a tsunami,” said the reports authors. “Time may be too limited to wait for an official signal.”