An earthquake along the Cascadia Subduction Zone, which stretches from northern California to northern Vancouver Island, could bring tsunami waves of 10 feet or more.
That information is not new but was illustrated in a series of videos from the Washington State Department of Natural Resources released last week, showing wave simulations for the Washington coast and southern Vancouver Island, with more detailed views for Bellingham and the San Juan Islands. The simulations show a tsunami after what is often referred to as “the Big One,” a 9.0 earthquake along the full length of the Cascadia Subduction Zone.
The first waves reach the outer coast and western Vancouver Island in about 15 minutes. The tsunami then travels through the Strait of Juan de Fuca and into Puget Sound, reaching the Tacoma waterfront about two hours and 30 minutes after the earthquake.
The two-minute video represents six hours in real-time. Washington Geological Survey’s Daniel Eungard stressed tsunamis are multiple-hour events, sometimes with high waves lasting 14 hours or more.
The videos are not meant to alarm, but to educate, with state officials and local disaster preparedness experts encouraging residents of the region to be prepared for earthquakes, including the full rupture of the Cascadia Subduction Zone, and resulting tsunamis.
“The community should start preparing — and I know they are in many ways — for when Cascadia happens,” Eungard said in a phone interview with the Vashon-Maury Island Beachcomber. “It is not if, but when.”
Along with having ample food, water, medicine and family disaster plans, Eungard said planning should include being ready to run to higher ground when an earthquake hits.
“The shaking of the earthquake is your warning,” he stressed.
He noted the duration of shaking is indicative of the quake’s severity. A quake of five to 10 seconds is likely minor, he said, while a quake of 30 seconds to a minute is major, and a quake of several minutes is likely a rupture of the Cascadia fault.
“Any amount of shaking should be your indication to get out of harm’s way,” he said.
That means people on low lying areas should move — on foot — to higher ground, and people in boats should move to deeper water or at least separate themselves from other vessels so they do not collide. Once safe, people can then check their phones or connect with emergency management to learn about the quake and determine if it is safe to return.
In tsunami areas, Eungard said, it is important to move quickly first — and then determine if heading to high ground had been necessary.
The simulation videos do not show how far inland the water would travel but modelling is in progress and is expected to be completed later this year. Eungard shared general information about that risk, noting that steep cliffs, of course, stop waves. But lower-sloping areas could allow the water to funnel up and reach higher levels than the height of the waves themselves, meaning a 10-foot wave could reach 12 to 18 feet inland.
Tsunami damage would come not just from high water, but low water as well. In fact, Eungard noted that in all the inland waters, what people would see first in a tsunami is the water receding, being sucked out of the bay and away from the shoreline.
This action presents a major danger to the maritime community, including for ferries, shipping vessels and recreational boaters, with the possibility for grounding and vessels being unable to move before the wave comes.
Currently, Eungard said, Washington’s Emergency Management Division is working on a plan for the maritime community so that large entities, such as the Coast Guard, Washington State Ferries and the ports of Seattle and Tacoma all know what to do when a tsunami alert is issued.