Considered the world’s largest, subduction zone earthquakes range in magnitude from 8.0 to more than 9.0.
Such quakes happen when the stress building between two giant underground plates “breaks,” causing them to slip. The result is a “megathrust.” The action takes place about 20 miles below the surface.
The quakes trigger prolonged shaking, tsunamis and numerous aftershocks, many that can be about 7.0 in magnitude.
Chile experienced the largest recorded subduction zone quake in 1960 at 9.5 magnitude. More recently in 2004 the Great Indonesian quake and tsunami killed 250,000 people; in 2011 the east Japan earthquake killed 16,000 people.
Premium content for only $0.99
For the most comprehensive local coverage, subscribe today.
The Cascadia subduction zone lies largely offshore of the Pacific Northwest. It is about 700 miles long and covers the area from Vancouver Island, British Columbia, down the West Coast to Cape Mendocino, California.
The last time a subduction zone quake happened on the Cascadia fault was January 1700. Scientist studied geological landscapes and found evidence on land and on the seafloor to determine the quake was a 9.0 magnitude. A tsunami followed within minutes.
Research indicates these types of quakes happen about every 500 years along the Cascadia fault. So South Sound residents might see one before the year 2200.
In such a quake, people living east of the Cascade Mountain range can expect little to no damage.
Those to the west can expect everything from landslides and rising water levels in Puget Sound to the collapse of major bridges and roadways and disruption to the power grid that could leave people without electricity for weeks or longer.
Officials expect the worst-hit areas to be on Washington’s coast, where injuries could total 5,000 and deaths could hit 9,100, according to the Federal Emergency Management Agency.
An exercise set to begin Tuesday details the worst-case secnario if a 9.0 magnitude Cascadia subduction zone earthquake were to hit 95 miles west of Eugene, Oregon.
If this were to happen, Washington would experience less intense shaking than Oregon, said Bill Steele of the University of Washington’s Pacific Northwest Seismic Network.
Even so, the duration would be longer than any major earthquake felt in the Pacific Northwest in more than 300 years.
“Ours is going to produce strong shaking for about two minutes and then milder shaking for almost five minutes,” Steele said. “Two minutes of strong shaking is a long time for strong shaking and gives longer time for bad things to happen.”
By comparison, the 2001 Nisqually earthquake lasted about 40 seconds. The magnitude 6.8 quake, occurring about 31 miles below the surface, was caused by the slipping, or fracturing, on a small part of the subducting plate.
Washington is much more likely to experience another quake like the Nisqually quake than a subduction zone quake, Steele said.
Deep quakes like the Nisqually quake happen every 20 to 30 years, he said. A magnitude 7.1 quake under Olympia hit in 1949 and there was a 6.5 magnitude quake under Federal Way in 1965.
Damage was the worst in the Tacoma and Puyallup areas, where flooding and liquefaction — where soils separate and liquefy — disrupted the Puyallup Valley, the Port of Tacoma and Commencement Bay, Steele said.
Preparing for a massive earthquake like the 9.0 detailed in the FEMA exercise increases people’s awareness about the need to be prepared for any earthquake, Steele said.
“We need to think longer term than we used to as far as being prepared,” he said. “We used to say three days, and now we’re talking about ‘It’d be great if you could be ready to be on your own for a couple of weeks.’”
If a major earthquake hits the Pacific Northwest, emergency management officials estimate it could be at least two weeks before lesser hit areas receive assistance.
People should create emergency kits with enough supplies such as food, water and medication to last this long.
Emergency kits should include a “grab and go” bucket for evacuations and people should have separate kits for home, work and car. Suggested contents include:
▪ Flashlight and radio with extra batteries.
▪ Water and non-perishable food.
▪ First aid kit.
▪ Copies of driver’s licenses, insurance information, out-of-area contacts stored in watertight bag.
▪ Spare home, car keys.
▪ Change of clothes.
▪ Diapers or feminine hygiene products
A more extensive list of what to include in a kit and other earthquake preparedness tips can be found at bit.ly/statekit
#CascadiaEQ: Join the conversation
Local, state and federal scientists and preparedness experts will be on Twitter at noon Monday to answer questions about a Cascadia subduction zone earthquake and tsunami. Follow @shakeout and use #CascadiaEQ to ask questions and follow the conversation.