In February, a devastating 7.8-magnitude earthquake rocked the Turkey-Syria border, followed closely by another significant tremor. These shallow fault ruptures, occurring less than 18 miles beneath the surface, unleashed destructive, focused quakes that claimed countless buildings and lives. While such seismic events are well-documented globally, a recent study led by the University of Arizona has unveiled a seismic episode of comparable significance in the Puget Lowlands of western Washington, dating back approximately 1,000 years.
The researchers turned to tree rings to precisely pinpoint the timing of this ancient earthquake, which is estimated to have occurred in late A.D. 923 or early 924. This discovery carries profound implications, suggesting that the region, now home to over 4 million people, including cities like Seattle, Tacoma, and Olympia, is at risk of experiencing a similar event in the future. These findings have been detailed in the journal Science Advances.
The ancient earthquake in the Puget Lowlands is believed to have been the result of either simultaneous ruptures of all shallow faults in the region, generating an estimated 7.8-magnitude earthquake, or a sequence of twin quakes occurring in close succession, with estimated magnitudes of 7.5 and 7.3. Shallow faults are known for producing more intense and focused shaking compared to other geological configurations.
While earthquakes are not uncommon in the Pacific Northwest, this study sheds light on the interconnectedness of events on these shallow faults, either through underground connections or the transfer of stress between faults. Surprisingly, current regional hazard models, which inform engineering design and policies, do not account for this possibility. This oversight warrants a reconsideration of seismic preparedness and infrastructure resilience.
The research focused on identifying the timing and nature of the last seismic activity on these shallow faults, including the Seattle Fault, Saddle Mountain Fault, Tacoma Fault, and Olympia Fault. These four faults had previously displayed evidence of rupturing approximately 1,000 years ago in a cluster of earthquakes referred to as the millennial cluster. The impacts of this cluster included dramatic geological shifts, such as a 25-foot cliff thrusting into the air in West Seattle, triggering a local tsunami, and causing landslides that swept entire forests into nearby lakes Washington and Sammamish.
To unravel the precise timing and potential interconnectedness of these quakes, the researchers turned to trees for answers. Trees add rings to their trunks each year, with the width of the rings influenced by climate conditions. These annual growth patterns create time-specific bar codes in tree rings, unique to a particular region.
Dendrochronologists can match growth patterns in dead trees with patterns from living ones, allowing them to establish the exact dates of death for the deceased trees. In this case, the researchers examined trees that had died due to the formation of a lake resulting from the Saddle Mountain Fault, as well as trees killed during a rock avalanche that impounded a stream. Sections from trees drowned in landslides following a large earthquake on the Seattle Fault more than 30 years ago were also analyzed.
By comparing growth patterns, the researchers identified that the trees died in the exact same year along both the Saddle Mountain and Seattle faults, and this occurred during the dormant season, narrowing the timeframe to late fall through early spring. To further refine the calendar year of death, the team constructed a 1,300-year chronology from extremely old living trees. This chronology, when matched with the earthquake-killed trees, pinpointed the dormant season of death as late 923 to early 924.
The findings indicate that these trees from across the region died simultaneously, confirming a linked event. This discovery has significant implications, as it narrows uncertainties surrounding these two faults, indicating that earthquakes on these faults can occur in rapid succession or synchronously. This realization challenges existing hazard models, which do not account for linked faulting, potentially underestimating the region’s seismic threat.
In conclusion, while earthquakes of this magnitude would pose a significant threat to the Pacific Northwest, they are fortunately relatively rare. Nonetheless, the research underscores the importance of reassessing seismic risk and preparedness, particularly given the potential for rapid succession or simultaneous ruptures of shallow faults, with consequences that could rival historically devastating earthquakes like the 1906 San Francisco quake or the recent quakes in Turkey.