There have been two major earthquakes in the region since 1949, but neither matched the April 13, 1949 geological upheaval.
One came the morning of April 29, 1965. It rattled the ground for 45 seconds, killing seven and causing $12.5 million in damages.
That temblor was a 6.7 magnitude.
After that was the Nisqually, a 6.8 quake which struck Feb. 28, 2001 and shook the earth for 40 seconds. It injured about 400 people, but there were no fatalities. Damages surpassed $2 billion.
Although the magnitudes of the 1949 and 2001 earthquakes were similar, the Nisqually was only one-third as strong as the 1949 quake in terms of energy released, according to the Pacific Northwest Geodetic Array at Central Washington University.
“For their strength, they actually were less damaging than you might expect,” Tobin said.
That’s because all three of the major temblors were deep, about 40 to 70 miles below the surface, and the depth helped to buffer some of their force.
Deep earthquakes like in 1949, 1965 and 2001 typically occur every 20 to 50 years.
Those quakes were triggered by the Juan de Fuca plate being pushed beneath the North American plate. Based on historical patterns, another is possible by 2030.
Most likely to wreak havoc in the Pacific Northwest is a crustal earthquake, officials say.
That’s because several big cities sit on top of or near shallow crustal faults like the Seattle Fault, which runs beneath CenturyLink Field, west to Bainbridge Island to Hood Canal and east past Issaquah, roughly following Interstate 90.
The Seattle Fault last let go about 1,100 years ago.
There’s an 84 percent chance Western Washington is hit by a similar crustal earthquake within the next 50 years, Dixon said.
The most powerful and dangerous is a subduction zone quake, which can range in magnitude from 8.0 to 9.0 and happen roughly 20 miles below the surface.
Those can cause prolonged shaking, tsunamis and large aftershocks.
A subduction zone quake hasn’t struck the offshore Cascadia fault since January 1700. Scientists believe the quake was a 9.0, triggering a tsunami within minutes.
“When we think about the potential for the big one, the highest probability is a quake like in 1949, 1965 and 2001,” Tobin said. “That’s the most activity we’re seeing seismically.”
Types of earthquakes in Washington state
▪ Deep — The Juan de Fuca plate is being forced beneath North America and into the Earth’s mantle. The plate’s crust is made of basalt and marine sediments and when the slab bends and breaks, it causes quakes every 20 to 50 years.
▪ Crustal — These occur in the North American plate at shallow depths of 20 miles or less.
▪ Subduction zone — Strain is building where the oceanic Juan de Fuca and continental North American plates are locked and will result in major future quakes. Usually occur every 500 to 600 years.
▪ Volcanic — Triggered by changes to magma inside of and underneath volcanoes. When magma levels change, surrounding rock shifts and rattles the earth.
If a major earthquake hits the Pacific Northwest, emergency management officials estimate it could be at least two weeks before some areas receive assistance.
People should create emergency kits with enough supplies of food, water and medication to last this long.
Emergency kits should be portable and can be kept at home, work and in the 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 and out-of-area contacts stored in watertight bag.
▪ Spare home and car keys.
▪ Change of clothes.
▪ Blankets or sleeping bags.
▪ Diapers or feminine hygiene products