- A magnitude 7.8 magnitude rocked the Seattle region 1,100 years ago
- Experts determined that the quake came from two fault lines that ruptured simultaneously
- READ MORE: At magnitude 7.5, a tsunami with waves of up to 42 feet could hit Seattle within three minutes
A massive magnitude 7.8 “megaquake” struck 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 includes Seattle, Tacoma and Olympia, and have ruptured in the past, but the old quake was 30 times stronger 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 and found that it triggered landslides and a local tsunami that would be catastrophic if it hit the region where more than four million people now live.
A 2005 scenario found that even a magnitude 6.7 earthquake would kill more than 1,600 people, destroy nearly 10,000 buildings and cause total economic damage of $50 billion.
A team of scientists led by the University of Arizona uncovered the natural disaster and found that it triggered landslides and a local tsunami that would be catastrophic if it hit the region where more than four million people now live
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 retrieved fossilized Douglas firs from six Puget Sound sites associated with the Seattle and Saddle Mountain fault zones that are believed to have been destroyed by seismic activity.
The team then measured the radiocarbon concentration in the wood and was able to determine the year each tree fell from its roots.
All trees were killed within six months, between 923 and 924.
These results explain two scenarios that could have occurred.
The faults are located in Puget Sound, which includes Seattle, Tacoma and Olympia, and have ruptured in the past, but the old quake was 30 times stronger than current warning signs suggest is possible from the fault zones
In these first cases, the two faults ruptured when two earthquakes occurred hours to months apart – there was a magnitude 7.5 quake on the Seattle Fault Zone and a magnitude 7.3 quake on the Saddle Mountain Fault Zone.
“The second possibility is a single, larger, multi-fault earthquake rupturing both the Seattle and Saddle Mountain fault zones with an estimated mean magnitude of 7.8,” the study says.
The team noted in the study that ruptures have occurred simultaneously in other places in our modern world, including a magnitude 7.9 quake in southern Alaska in 2001, a magnitude 7.3 earthquake in Southern California in 1992, and a magnitude 7.8 earthquake that struck New Zealand in 2016 – but that event affected more than 20 different faults.
In the case of Puget Sound, scientists believe the two faults may be kinematically linked.
A 2022 study by researchers at the Washington State Department of Natural Resources also examined a megaquake in Seattle, but it only had a magnitude of 7.5.
The model showed that 42-foot waves would engulf the city within three minutes.
The 42-foot waves would swirl around the Seattle Great Wheel landmark and reach inland to Lumen Field stadium, home of the Seattle Seahawks, and T-Mobile Park, where the Seattle Mariners baseball team plays.
Thirty miles south of downtown, waves could be seen in Tacoma Harbor reaching up to three miles inland.