A massive 7.8-magnitude ‘megaquake’ rocked the Seattle area 1,100 years ago when two fault lines ruptured simultaneously – and experts fear it could happen again.
The faults are located in Puget Sound, which contains Seattle, Tacoma and Olympia, and have ruptured in the past, but the ancient tremor was 30 times more powerful than current warning signs suggest is possible from the fault zones.
A team of scientists led by the University of Arizona uncovered the natural disaster, determining it triggered landslides and a local tsunami that would be catastrophic if it hit the region, home to more than four million people today.
A 2005 scenario found that even a magnitude 6.7 earthquake would kill more than 1,600 people, destroy nearly 10,000 buildings and produce total economic losses of $50 billion.
A team of scientists led by the University of Arizona uncovered the natural disaster, determining it triggered landslides and a local tsunami that would be catastrophic if it hit the region – home to more than four million people today
The fault lines in question are the Seattle Fault Zone (SFZ), located beneath and around the city, and another from the Saddle Mountain Fault Zone (SMFZ) in southwest Washington.
Researchers pulled fossilized Douglas-fir trees from six Puget Sound sites associated with the Seattle and Saddle Mountain fault zones, which were believed to have been destroyed by seismic activity.
The team then measured radiocarbon concentrations in the wood, allowing them to pinpoint the year each tree fell from its roots.
All the trees were killed within six months, between 923 and 924.
These findings explain two scenarios that could have happened.
The faults are located in Puget Sound, which contains Seattle, Tacoma and Olympia, and have ruptured in the past, but the ancient tremor was 30 times more powerful than current warning signs suggest is possible from the fault zones
READ MORE: Experts on red alert for mega-earthquake off the US coast – after discovering a crack in 600-mile long fault line at the bottom of the Pacific
The hole spewing hot liquid sits 50 miles off the shoreline of Oregon, on the boundary of the dipping fault known as Cascadia Subduction Zone, which spans from Northern California into Canada.
In these first cases, the two faults ruptured as two earthquakes separated by hours to months – the Seattle fault zone had a 7.5 magnitude earthquake, and the Saddle Mountain fault zone had a 7.3 magnitude quake.
‘The second possibility is a single, larger, multifault earthquake rupturing both the Seattle and Saddle Mountain fault zones with a median magnitude estimated at a magnitude of 7.8,’ the study published in Science Advances reads.
The team noted in the study that simultaneous ruptures had occurred elsewhere in our modern world, including a 2001 7.9 magnitude quake in southern Alaska, a 1992 7.3 magnitude in Southern California and a 7.8 magnitude earthquake that hit New Zealand in 2016 – but this event involved more than 20 distinct faults.
In the case of Puget Sound, scientists believe the two faults may be kinematically connected.
A 2022 study by researchers from the Washington State Department of Natural Resources also looked at a megaquake hitting Seattle, but this one was only a 7.5-magnitude.
The model showed that 42-foot waves would engulf the city within three minutes.
The 42-foot waves would swirl around the landmark Seattle Great Wheel, reaching inland as far as Lumen Field stadium, home of the Seattle Seahawks, and T-Mobile Park, where the Seattle Mariners baseball team plays.
Thirty miles south of downtown, the port of Tacoma could see waves reach as far as three miles inland.