Massive Two-Fault Earthquake May Have Struck the Pacific Northwest 1,100 Years Ago
New Study Reveals Ancient Earthquake Intensity
Scientists have uncovered evidence of a massive earthquake—or two in quick succession—that struck the Puget Sound region in the Pacific Northwest nearly 1,100 years ago. The study, published in Science Advances, used tree ring dating to determine that two fault zones near Seattle ruptured within a six-month period between 923 and 924 C.E.
Earthquake Risk Underestimated
This discovery challenges previous research that had indicated the area was susceptible to smaller earthquakes. The new findings suggest that the region, which includes Seattle, Tacoma, and Olympia, Washington, could experience more extreme earthquakes than previously thought.
Multiple Fault Rupture Earthquakes
Earthquakes involving multiple fault zones at once are often the most damaging type of quake an area can experience. However, they are rare and difficult to observe in the historical record. The earthquake described in the new study was likely the result of a multiple fault rupture, making it particularly hazardous.
Tree Rings Reveal Earthquake Timing
To determine the timing of the ancient earthquake, researchers analyzed Douglas Fir tree rings from six sites around Puget Sound. By comparing the trees’ rings to each other and to a reference chronology, they determined that the trees died between October 923 and March 924. This allowed them to pinpoint the earthquake(s) to that six-month window.
Miyake Event Confirms Dating
To bolster their confidence in the finding, the team searched the tree rings for a distinctive sign that aids in dating: a spike in stellar radiation called a Miyake event. This event, which could have been a solar flare or an exploding star, would have left a signature jump in carbon-14 concentration within the trees. They located such an event between 774 and 775 C.E. and used that as a reference point in dating, confirming the last tree ring across the sampled sites was 923 C.E.
Magnitude and Energy Release
Based on the results, the researchers estimate that during the six-month window, either two closely spaced earthquakes of magnitude 7.5 and 7.3 or a single, multi-fault earthquake around magnitude 7.8 struck the area. The single earthquake scenario is around three times as likely as the two separate quakes.
A 7.8-magnitude earthquake like the one described in the new paper would release 38 times as much energy as a 6.7-magnitude earthquake, which was the basis for previous hazard models. This would likely cause a local tsunami in addition to widespread damage and casualties.
Hazard Models Need Updating
The study’s findings underscore the need to update hazard models to account for the possibility of earthquakes of this magnitude. Current models do not consider quakes of that strength, which could lead to underestimating the potential risks and inadequate disaster preparedness.
Preparing for the Worst
While the chance of such an extreme event occurring in any given year is low, the study highlights the importance of being prepared for catastrophic earthquakes. Building codes and emergency response plans should be reviewed and updated to reflect the potential for larger earthquakes than previously anticipated.
Implications for the Pacific Northwest
The earthquake described in the new study was likely the region’s most intense in the last 16,000 years. This underscores the importance of ongoing research and monitoring to better understand the earthquake risks facing the Pacific Northwest and to develop effective mitigation strategies.