Scientists Warn of Potential Repeat of 7.8-Magnitude ‘Megaquake’ that Rocked Seattle 1,100 Years Ago
Seattle, Washington – A devastating 7.8-magnitude earthquake, known as a “megaquake,” struck the Seattle region 1,100 years ago, causing widespread destruction and triggering landslides and a local tsunami. Scientists fear that a similar event could occur again in the future.
Research conducted by a team led by the University of Arizona discovered that the ancient megaquake was the result of two fault lines, the Seattle Fault Zone (SFZ) and the Saddle Mountain Fault Zone (SMFZ), rupturing simultaneously. The tremor was estimated to be 30 times more powerful than what current warning signs suggest is possible from these fault zones.
The Puget Sound region, which includes the cities of Seattle, Tacoma, and Olympia, is home to over four million people. If a similar megaquake were to occur today, the consequences would be catastrophic. In 2005, a scenario simulation showed that even a magnitude 6.7 earthquake would result in over 1,600 fatalities, the destruction of nearly 10,000 buildings, and total economic losses of $50 billion.
Scientists reconstructed the timeline of the ancient megaquake by analyzing fossilized Douglas-fir trees from six Puget Sound sites associated with the SFZ and SMFZ. By measuring radiocarbon concentrations in the wood, they were able to determine the year each tree fell from its roots, all of which were killed within a six-month period between 923 and 924.
The study published in Science Advances proposes two possible scenarios for the megaquake. The first involves separate earthquakes occurring in the SFZ (7.5 magnitude) and the SMFZ (7.3 magnitude), with a time gap between them. The second scenario suggests a single, larger earthquake (7.8 magnitude) rupturing both fault zones simultaneously.
While such simultaneous ruptures have been observed in other parts of the world, including Alaska, Southern California, and New Zealand, the Puget Sound case is unique. The SFZ and SMFZ may be kinematically connected, according to the researchers.
A separate study conducted by the Washington State Department of Natural Resources in 2022 also predicted a megaquake hitting Seattle, but with a magnitude of 7.5. This study revealed that the city would be engulfed by 42-foot waves within three minutes of the quake. The waves would reach popular landmarks such as the Seattle Great Wheel, Lumen Field stadium, and T-Mobile Park, as well as the port of Tacoma located south of downtown.
Given the potential devastation, scientists and authorities are urging residents and local governments to be prepared and to take necessary precautions to mitigate the impact of a future megaquake. The research findings serve as a reminder of the region’s vulnerability to powerful seismic events and the importance of proactive measures to ensure the safety and resilience of communities.