Pacific Northwest faces new kind of earthquake risk, tree rings reveal

And that, researchers say, means the potential is there for a repeat event in an area which includes the cities of Seattle, Tacoma and Olympia, that is now home to some 4 million people.

“The evidence showed us that these trees from across the region died together, and this was in fact a linked event,” said the paper’s author Bryan Black, an associate professor of dendrochronology at the University of Arizona’s Laboratory for Tree-Ring Research.

“We’ve taken uncertainties around these two faults that used to span decades or centuries and narrowed it down to within one season,” Black said. “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."

Black cautioned that current quake models do not take into account the possibility that shallow fault activities in the area could be linked, either by underground connections or through the transfer of stress from one to another, which has implications for engineering design and policies. For example, a second quake could provide the “knockout punch” that ends up destroying water mains and roads and triggering landslides and tsunamis.