1,100 years ago, double earthquake hit Seattle region, researchers prove

Geological evidence shows that earthquakes along two fault lines in the Puget Sound area occurred around the same time in late 923 CE or early 924 CE. The new information should help Seattle, Tacoma and Olympia prepare for the “big one.”

Sending divers and chainsaws to underwater ghost forests, geologists collected tree-ring data to understand that a compound earthquake along two adjacent fault zones struck the Seattle region around 1,100 years ago. Knowing more about the quakes, which either took place at the same time or in quick succession, can help communities prepare for a “big one.”

Contributing to a University of Arizona study published in Science Advances, Brian Sherrod, ’98, a UW affiliate assistant professor and U.S. Geological Survey geologist, helped identify and sample trees from ancient forests that were submerged in Washington lakes and felled by a specific earthquake event.

The researchers collected Douglas fir rings from several sites around Western Washington and factored in traces of ancient solar storms captured in the tree rings to determine the quakes’ timing. They found evidence that the Saddle Mountain Fault on the southeast flank of the Olympic Mountains experienced a ground rupture at about the same time as the Seattle Fault Zone, resulting in an estimated magnitude 7.8 earthquake.

Previous earthquake modeling has focused primarily on the Seattle fault and the impacts that a 7.5-magnitude earthquake could have on the region. But this new research shows evidence of quakes from distinct faults interacting with each other, says Harold Tobin, director of the Pacific Northwest Seismic Network at the UW. While Tobin didn’t participate in the study, he is interested in the results. “Those two faults could have a simultaneous single earthquake that puts us in a realm of a significantly larger earthquake than just the Seattle fault [alone]. It doesn’t sound like a big difference, but 7.8 is double to triple the strength [of a single 7.5 quake].”

The findings could inform contemporary hazard models and preparations for future seismic events in the Northwest. Emergency planning had not previously accounted for the possibility of two simultaneous earthquakes that could compromise critical infrastructure, bridges and buildings that were not built to withstand multiple, successive ruptures. While the agencies considering these scenarios will have to take a double earthquake into account, changes to earthquake preparedness won’t happen overnight, Tobin says. Planning for event disasters involves “many different moving parts from state emergency agencies to FEMA to building codes to city ordinances and other municipalities and regions.”

Though earthquakes happen on all these fault lines, they rarely happen at the same time. “We just have to prepare,” Tobin says. “And we can prepare to be resilient.”