New Study Details Impacts of Tsunami from Fault Running Through Seattle
July 7, 2022
Earthquake would produce tsunami waves that would hit state’s largest city in minutes
The Washington State Department of Natural Resources (DNR) has released a new study showing the impact of a tsunami produced by a major earthquake on the Seattle Fault.
This study, prepared by geologists within the Washington Geological Survey division of DNR, was conducted to help local and state emergency managers and planners develop and refine response and preparedness plans for a tsunami in the middle of Washington’s largest population center and economic hub.
The report finds tsunami waves would reach the shoreline in fewer than 3 minutes in many places on the eastern side of Bainbridge Island, Elliott Bay, and Alki Point. The report shows inundation from such a tsunami exceeding 20 feet along the shoreline of the greater Seattle area.
“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. “While the history of earthquakes and tsunamis along the Seattle Fault is less frequent than the Cascadia subduction zone, the impacts could be massive. That’s why it’s critical these communities have the information they need to prepare and respond.”
The last known earthquake on the Seattle Fault occurred about 1,100 years ago. However, geologic evidence shows five additional earthquakes of an estimated magnitude 6.5 occurred within the Seattle Fault zone during the last 3,500 years.
"Our highest responsibility as an administration is to keep our residents safe, and studies like this are a critical tool in that effort to analyze the data, understand risks and forecasts, and best prepare for future emergencies,” said Seattle Mayor Bruce Harrell. “We will continue to ensure our Office of Emergency Management -- and all our departments -- are best equipped to respond to emergencies and natural disasters, while we also strengthen our infrastructure and build a resilient city now and for the future."
“Although the chances of this happening in our lifetime is low, it's important for families to get prepared now,” said Maximilian Dixon, the hazards and outreach program supervisor for the Washington Emergency Management Division. “The ground shaking will be your warning that a tsunami may be on the way. Make sure you know where the closest high ground is and the quickest route to get there. Get signed up for tsunami and local alerts.”
The most recent earthquake was so great it thrust the beach at Restoration Point on Bainbridge Island upward by 23 feet while dropping land at Seattle’s West Point by 3 feet. Land level changes during the next earthquake may establish a new shoreline in many locations close to the Seattle Fault zone.
Study finds tsunami waves arrive in minutes at many locations
The Seattle Fault crosses east-west through Puget Sound and downtown Seattle, and has produced several earthquakes documented in the geologic record throughout the region. The earthquake scenario used in this modeling is for a very large, low-probability earthquake (~ magnitude 7.5). It produces the maximum-considered Seattle Fault-generated tsunami for emergency planning purposes.
Tsunami waves reach the shoreline in fewer than 3 minutes in many places on the eastern side of Bainbridge Island, Elliott Bay, and Alki Point. Tsunami inundation and strong currents may continue for more than 3 hours from the start of the earthquake.
While this study found the 6 feet of inundation at the Port of Tacoma would be lower than previous studies, it also found that waves may travel up to three miles inland in parts of the port.
While inundation from the tsunami will be greatest closer to the Seattle Fault, the study showed shoreline flooding and increased currents throughout the Salish Sea, from Blaine to Olympia.
The model does not account for tide stages or local tsunamis triggered by earthquake-induced landslides.
If you feel an earthquake drop, cover, and hold on, then evacuate to high ground and get as far inland as you can as quickly as possible.
This new study and maps are available at https://fortress.wa.gov/dnr/geologydata/tsunami_hazard_maps/ger_ms2022-03_tsunami_hazard_seattle_fault.zip
Video simulations can be found on the DNR YouTube channel at https://www.youtube.com/watch?v=XfGCk9WYP2Q&list=PLKpn_ilWjh52mzThOZk8ok1HmA_461pIa
More information about Washington tsunamis can be found online at: www.dnr.wa.gov/tsunami
Geology in the Public Interest
Under the leadership of Commissioner of Public Lands Hilary Franz, the Washington Geological Survey works to ensure the safety and economic well-being of Washington’s citizens from geologic events. The Survey is the primary source of geological products and services for Washington’s government agencies, businesses, and the public.