It lasted four minutes and thirty-eight seconds.
On 27 March 1964 at 5:36 pm local time the Prince William Sound region of Alaska experienced a magnitude 9.2 earthquake. It was the most powerful earthquake in North America to be recorded and the second-most powerful in the world.
Geological surveys taken immediately afterward showed parts of the Alaskan coast sank up to eight feet, other parts rose up to 38 feet and much of the coast moved 50 feet towards the ocean. Coastal forests plunged below sea level and were destroyed by salt water.
Scientists discovered afterwards that the Earth’s surface shifted vertically over “an area two-thirds the size of California.”
According to the U.S.Geological Survey, “the prevailing theories of the day could not explain” such a massive earthquake. Scientists now understand that “the Pacific Plate is being overridden by the North American Plate, it descends, or subducts, into the Earth’s mantle.”
The earthquake and resultant tsunamis resulted in 129 fatalities and an estimated $2.83 billion in property losses (in 2022 dollars).
The PNW is due
The Cascadia Subduction Zone (Cascadia SZ), a submarine fault, runs for 700 miles off the Pacific coast, beginning near Cape Mendocino, California, and ending near north Vancouver Island, British Columbia, Canada. It separates the Juan de Fuca and North America plates.
The Juan de Fuca plate is about 300 miles off shore from Seattle and is sliding under the North American plate, about 20-30 miles below Seattle and Portland. The subduction zone is approximately 60 miles offshore.
On 26 January 1700, the Cascadia earthquake took place; its magnitude is estimated at 9.0 (8.7–9.2). For comparison, that’s about 30 times more powerful than the largest earthquake predicted for the San Andreas fault, probably the best-known fault in America, which will max out at 8.0. (The World Series earthquake, 17 October 1989, was only a 6.9 magnitude quake.)
The timing of the 1700 Cascadia earthquake has been deduced from, among other things, Japanese records of an “orphan” tsunami.
We know this age with such precision—unique in paleoseismology—because of several lines of geologic proxy evidence that coalesce around that date, in addition to Japanese historical records describing an “orphan tsunami” (a tsunami with no corresponding local earthquake) on that particular date.
Geologists estimate that a “big one” happens in the Cascadia SZ about every 500 years (range of 300-900). The next big one here “poses one of the greatest natural hazards in North America and motivates diverse research across the Earth sciences.”
The odds of a Cascadia SZ magnitude 9.0 quake are about 1-in-7 (14%) over the next 50 years.
The Seattle Times science reporter Sandi Doughton wrote Full Rip 9.0: The Next Big Earthquake in the Pacific Northwest which describes the eventuality of a Cascadia mega-quake in frightening detail. She also explains the science behind the Cascadia SZ.
- A Grays Harbor County school built the first vertical tsunami refuge in the U.S. — with a gym strong enough so 1,000 people could stand on its roof through a tsunami surge.
- Doughton traveled to Japan to report on seafloor-monitoring technology that allows Japanese scientists to monitor the Nankai Trough, a submarine fault so similar to the Cascadia Subduction Zone it’s considered a “sister.”
- Experts say it’s possible 100-foot waves could slam the Pacific Northwest if the Cascadia fault snaps.
- When the “really big one” hits, Seattle’s bluffs and slopes are expected to slide, too. A University of Washington study found a big earthquake could could trigger as many as 30,000 landslides in Seattle.
Download an earthquake/tsunami warning app built on government data from around the world (iOS).
The Pacific Northwest’s last magnitude-9 event from the offshore subduction zone was in 1700. Only a few clues remain about how it unfolded… A research project by the University of Washington and the U.S. Geological Survey uses simulations of different magnitude-9 slips on the Cascadia fault to evaluate how the ShakeAlert system would perform in 30 possible scenarios.