Seattle Earthquake Could Generate 42-Foot-Tsunami, Demolish City for Miles
A tidal wave of massive proportions could devastate Seattle if an earthquake hits along the Seattle Fault, according to a new report.
The report from the Washington Department of Natural Resources stated that “tsunami waves would reach the shoreline in fewer than three minutes.”
The Seattle Fault goes east-west through downtown Seattle and Puget Sound.
The study noted that the last earthquake on the fault took place about 1,100 years ago. This is key, since a major (Magnitude 6.5 or more) earthquake has occurred on average every 584 years since 1500 BC. The fault is overdue.
Over the past 3,500 years, five additional earthquakes estimated to have a magnitude of 6.5 took place along the Seattle Fault.
For the sake of worst-case scenario planning, the study looked at the impact of a 7.5-magnitude earthquake.
Flooding from a tsunami would exceed 20 feet along the shoreline of the Seattle area and could generate waves as high as 42 feet tall, according to Fox News.
Maximilian Dixon, the hazards and outreach program supervisor for the Washington Emergency Management Division, said the department does not want to provoke panic, but urges individuals to be prepared.
“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,” he said.
The study said that tsunami waves would hit the eastern side of Bainbridge Island, Elliott Bay and Alki Point, and could last for more than three hours.
The Port of Tacoma would face six feet of inundation with waves going as far as three miles inland, according to the study.
A long history of earthquakes on faults in the Puget Sound
“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,” Commissioner of Public Lands Hilary Franz said.
“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.”
A 2001 earthquake caused $36 million worth of damage in Seattle from the impact on buildings, roads and other infrastructure, according to the city’s website.
The city noted that damage from an earthquake would also include landslides throughout the Seattle region. Further, the city estimated there are 1,100 un-reinforced buildings in Seattle that would be prone to extensive damage in an earthquake. About 15% of Seattle’s total area is soil that is prone to ground failure in earthquakes. The Duwamish Valley, Interbay, and Rainier Valley are vulnerable to ground failure and shaking because of the liquefiable soils in these areas.
And that is only one type of quake the area suffers from.
The second type would be a megathrust quake along the Cascadia fault line. Megathrust earthquakes are the greatest risk to the broader west coast region. A megathrust earthquake could reach M9.0+ and affect an area from Canada to northern California. A Cascadia megathrust earthquake could rank as one of the largest earthquakes ever recorded, but because Seattle is several hundred miles from the source seismic waves would weaken slightly before they reach Seattle. Shaking would be violent and prolonged, but possibly not as intense as in a Seattle Fault quake.
Mount Rainier Will Erupt Again, Say Researchers Studying Volcano’s Magma Flows
Then there’s that 800-lb gorilla known as Mt. Rainier.
Mount Rainier last erupted in the 19th century. It is the tallest volcano and the fifth-highest peak in the contiguous U.S.. The volcano is about 14,410 feet tall and located about 58 miles southeast of Seattle.
The U.S. Geological Survey has described Mount Rainier as “an active volcano that will erupt again.” Sitting atop volcanic flows as much as 36 million years old, Rainier has erupted explosively dozens of times during the past 11,000 years, spewing ash and pumice.
There are FIVE active volcanoes in Washington State. The last one to erupt was Mt. St. Helen (1980.) If you’re not old enough to remember, you can see videos of it on YouTube and elsewhere. Seattle disaster planners claim it is ‘unlikely’ that a lahar would reach Seattle itself. Tell that to the people around St. Helens.
Envision that there would be solid blocks the size of Volkswagens and fine grain material being blasted into the atmosphere and then falling back on Rainier’s surface. It would be hot, and would melt the ice and snow on Rainier’s flanks And tumble over cliffs.
The lava flows encounter those very steep slopes and make avalanches of hot rocks and gas that are hurtling down the mountain maybe 100 miles per hour or so.
The lava would stop flowing near the boundaries of the national park.
But the snow water it melted would create a much bigger hazard: A flash flood that would look like concrete and chew up everything in its path.
It would pull down trees. Giant boulders would bounce on its surface, cracking as they collide with each other.
Scientists say Orting will probably have plenty of warning before an eruption. But just in case, there’s a backup plan, a siren that gives people in Orting roughly 40 minutes warning before the lahar hits.
This nightmare mud flow is called a lahar. And it would sound like “a rocket launching. Or maybe a train barreling down a track where no railroad tracks exist.”
How would it affect Seattle-Tacoma?
An earthquake on the Seattle fault by itself would cause a massive catastrophe. So would a Seattle-facing Mt. St. Helen eruption. If they occurred at nearly the same time? Likely everything from Olympia north to Vancouver island would be in ruins with many thousands dead or injured.
On the upside, Redmond (and therefore Microsoft) would disappear.
Note: data for this story comes from investigative news reports and the Seattle and WA state government websites.