Written by by Darren Orf at popularmechanics.com
A secret seismic threat has emerged near the Emerald City.
In the six-month period between the fall of 923 CE and the spring of 924 CE, hanging around the Puget Sound area in northwestern Washington was a bad idea. According to new dendrochronology research (aka the study of tree rings) by scientists at the University of Arizona, not one, but two seismic clashes dramatically altered the landscape and produced an earthquake that rivaled some of the most famous in recorded history.
And all evidence suggests that earthquakes along these same faults, known as the Saddle Mountain and Seattle faults, could very well happen again. The results of the study containing this information were published this week in the journal Science Advances.
“These are four shallow faults that had shown evidence of having ruptured roughly 1,000 years ago in a cluster of earthquakes called the millennial cluster,” Black said in a press statement. “These quakes could have ruptured at the same time, hours apart, or centuries apart. We weren’t sure.”
To figure out the answer, Black and his team turned to trees—specifically, to ones that died when the Saddle Mountain earthquake blocked a stream that in turn flooded a forest. After creating a makeshift raft out of a sheet of plywood and two canoes to hold a generator, the dendrochronologists used underwater chainsaws to free tree ring samples from long-dead trees along the bottom of Price Lake in the eastern Olympic mountains.
Comparing these tree samples to other samples collected near the Seattle Fault revealed that the two sets of trees died during the dormant season (aka the six-month period from fall to spring) of the same year. When the researchers further compared the samples to old-yet-still-living trees, they were able to tag the two synchronous earthquakes to late 923 or early 924 CE. And to independently confirm that timeline, scientists had an cosmic ace up their sleeve.
“Our team was also lucky that there was a massive solar storm between the years 774 and 775, which caused a sudden global spike in radiocarbon,” co-author Charlotte Pearsons said in the press statement. “We measured radiocarbon in the rings of earthquake-killed trees to show that this spike occurred right where we thought it should.”
Knowing that these two earthquakes occurred mere hours apart (and not years or even centuries) gives a more detailed picture of the seismic threat facing the Puget Sound area, which is home to more than four million people in the cities of Tacoma, Seattle, and Olympia.