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Violent earthquake threat in the Pacific Northwest revealed in tree rings

A recent study led by the University of Arizona has revealed a new kind of earthquake threat to the Pacific Northwest. The examination of tree rings helped the researchers pinpoint a significant seismic event in the Puget Lowlands in western Washington around late A.D. 923 or early 924.

Devastating seismic activity

The team discovered that similar shallow faults, located less than 18 miles beneath the surface, ruptured approximately 1,000 years ago, mirroring the recent earthquakes that devastated the Turkey-Syria border. 

The 7.8-magnitude earthquake in Turkey and Syria was followed by one nearly as large nine hours later. Shallow faults less than 18 miles beneath the surface buckled. The violent earthquakes leveled thousands of buildings and killed more than 50,000 people. 

Earthquake threat

The new earthquake threat that has been uncovered by the Arizona team poses a significant risk to over 4 million residents across the Pacific Northwest, including major cities like Seattle, Tacoma, and Olympia. 

The researchers noted that the ancient quake in this region could have either been a solitary event with an estimated magnitude of 7.8 or a sequence of twin quakes occurring consecutively, with magnitudes of about 7.5 and 7.3.

Linked faulting

One of the striking revelations of the study was the interconnectedness of the shallow faults. According to study first author Professor Bryan Black, these events on shallow faults are intricately linked, either through subterranean connections or through the transference of stress from one fault to another. 

Regional hazard models, used to develop engineering design and policies, don’t currently reflect the possibility of this earthquake threat – but should, said Professor Black. 

Shallow faults

Scientists have been finding shallow faults in the region since the 1960s, when the Seattle Fault was first discovered, followed by the Saddle Mountain Fault, which runs along the eastern foothills of the Olympic Mountains, and the Tacoma and Olympia faults.

“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. “A 25-foot cliff was thrusted into the air from west Seattle out to Puget Sound. It also triggered a local tsunami and landslides that stripped mountainsides of whole forests and discarded them into nearby lakes Washington and Sammamish.”

Prior to this study, it was not known when and how these faults last gave out. “These quakes could have ruptured at the same time, hours apart or centuries apart,” said Black. “We weren’t sure.”

Synchronous timing of death

To investigate, the researchers explored the Pacific Northwest’s rugged terrain, collecting samples from tree stumps that perished during previous seismic activities. 

The team utilized underwater chainsaws, powered by generators placed on makeshift barges, to extract samples from trees that died during the millennial cluster of earthquakes. 

The experts compared the growth patterns of these ancient trees with those still living, discovering a synchronicity in the time of death across trees situated along different fault lines.

The tree deaths, occurring during their dormant season, provided crucial insight into the timing of the ancient seismic events, narrowed down to late fall through early spring of late 923 to early 924. 

Hazardous conditions 

“Combined, the evidence showed us that these trees from across the region died together, and this was in fact a linked event,” said Black. 

“We’ve taken uncertainties around these two faults that used to span decades or centuries and narrowed it down to within one season. It’s a much different scenario if we have earthquakes on these two faults separated by 100 years versus 100 hours.”

“Demonstrating that these faults can rupture synchronously or in very rapid succession has really changed what we understand about the hazard in the region.”

However, current hazard models don’t currently recognize that linked faulting is possible, he said.

Rapid succession 

“If Saddle Mountain and Seattle faults went together it would be on par with the 1906 earthquake in San Francisco,” Black said. “Or, like the quakes in Turkey, they can also go at rapid succession.” 

“If that were the case, the infrastructure and landforms already weakened by one quake are then going to get the knockout punch with a second quake. It will still be quite destructive: thrusting up water mains, severing roads, triggering landslides and local tsunamis.”

Luckily, he said, the bigger and more severe the quake, the less frequent it is. Earthquakes of this size are relatively uncommon.

The research is published in the journal Science Advances.

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