Seattle’s Seismic Past Unveiled: New Evidence Suggests Simultaneous Fault Ruptures and Potential for a Major Earthquake

A recent study suggests that the Seattle region experienced a significant earthquake, possibly with a magnitude of 7.8, around 1,100 years ago. Contrary to previous assumptions of multiple smaller quakes, researchers found evidence indicating that hidden faults simultaneously ruptured, potentially leading to a more powerful earthquake than previously predicted for these fault zones. The study, published in Science Advances on September 27, analyzed fossilized tree remains, concluding that the Seattle Fault Zone (SFZ) and the Saddle Mountain Fault Zone (SMFZ) likely ruptured at the same time or in very close succession.

Geological evidence had previously suggested earthquakes with magnitudes in the low 7s occurring between 900 and 930 A.D., attributed to the SFZ and SMFZ. However, the new findings propose a simultaneous rupture, challenging existing seismic hazard models. The study focused on fossilized Douglas fir trees across Puget Sound, estimating their demise between 923 and 924 A.D. The shallow nature of SFZ and SMFZ, akin to fault lines responsible for devastating earthquakes in other regions, underscores the potential destructive force of simultaneous ruptures.

The implications of this discovery are significant for the approximately 4 million people residing in Puget Sound. While the study does not dismiss the possibility of individual earthquakes with magnitudes between 7.3 and 7.5, it highlights that a simultaneous rupture could result in the most potent quake these fault zones can produce. The logarithmic nature of earthquake measurements emphasizes the substantial difference in power between magnitudes, with a magnitude 8 being ten times more powerful than a magnitude 7.

This revelation prompts a reconsideration of current seismic warning models, as the study suggests a potential repeat of a simultaneous rupture. Despite these findings, the Cascadia Subduction Zone (CSZ) remains the most threatening seismic hazard for Seattle, capable of producing magnitude 9 earthquakes, exemplified by the historic 1700 event known as “the big one.” Additional discoveries this year, such as evidence on Bainbridge Island, underscore the ongoing research into Puget Sound fault zones.