Over a millennium ago, Puget Sound experienced the effects of a powerful earthquake or possibly even two.

Approximately 1,100 years ago, the Puget Sound area, where Seattle now stands, experienced strong ground movement due to seismic activity. This event caused landslides, raised cliffs, flooded forests, and created lakes. The land still shows evidence of this seismic event through various faults. However, it has not been determined whether these faults ruptured within a short timeframe of hours, months, years, or even a century of each other. Recent research using tree ring analysis has identified a 6-month period in which two adjacent faults were active. It is possible that these faults were triggered by two closely spaced earthquakes or a single earthquake on both faults that exceeded the expected magnitude for the Puget Sound region.

Rich Briggs, a research geologist at the U.S. Geological Survey (USGS), who was not part of the study, described the findings as a significant paleoseismic discovery. Typically, scientists use radiocarbon dating on plants or charred wood to gather information about the timing of past earthquakes. However, it can be challenging to establish a direct correlation between these records and an earthquake, according to Briggs. He also noted that there is often a margin of error spanning several decades to centuries when it comes to dating fossil earthquakes.

Instead, this group examined the yearly growth patterns of trees that perished during the Puget Sound catastrophe, either by being buried in mud or submerged in water. The team collected Douglas fir samples from five locations near the Seattle and Saddle Mountain Faults, as previous studies suggested these faults were responsible for the seismic activity. In addition, divers at Price Lake in North Cascades National Park used a chainsaw in extremely low visibility to extract sections of wood that had been submerged and covered with mud.

Based on their positions relative to the faults, it was determined that five of the tree sites along the Seattle Fault could have been destroyed by seismic activity, while the trees at Price Lake may have only been affected by the Saddle Mountain Fault.

Unfortunately, a large number of the specimens included the outer layer of the tree known as bark. The bark, found on the outermost growth ring, signifies the conclusion of a tree’s yearly data. According to Bryan Black, a dendrochronologist from the University of Arizona and primary contributor to the research, this allowed for precise determination of the tree’s date of death.

The researchers, led by Black, utilized the growth rings in trees as a unique identifier of their lifespan. These rings’ widths are affected by climate conditions and can be compared to a barcode. Through their study at six locations, they discovered that the ring patterns were consistent, indicating that the trees lived and died in close proximity.

During the year 774-775 CE, there was a cosmic occurrence, potentially a solar storm, which resulted in a worldwide increase in radiocarbon levels. This provided evidence for the accuracy of the dates determined by the team through analyzing multiple trees. By comparing the patterns with a sample from an unaffected fir tree in Vancouver, the team concluded that the trees affected by earthquakes must have died after their growth season. This suggests that the seismic activity occurred within a time frame of 6 months, spanning the end of 923 to the beginning of 924 CE. The team, in their publication in Science Advances, noted that since all six sites experienced simultaneous tree deaths, it can be inferred that both the Seattle and Saddle Mountain Faults were active during this period.

Revamping Earthquake Preparedness Strategies

Using the fault sizes as a reference, the team calculated that there were either two earthquakes measuring 7.5 and 7.3 within a span of 6 months, or one earthquake measuring 7.8 that affected both faults. Black commented that this is a significantly powerful earthquake, comparable to the one that occurred in San Francisco in 1906.

Diego Melgar, a seismologist at the University of Oregon in Eugene, stated that this research holds significant importance. He noted that concerns about the Seattle Fault have been ongoing for many years. The study reveals that significant earthquakes have occurred in the past and are likely to occur again in the future. Understanding the timing and severity of these quakes could aid scientists in predicting future hazards.

According to Briggs, both situations described by the team are concerning. The impact of a single strong earthquake must be taken into account when constructing buildings. Additionally, if a significant earthquake occurs shortly after another one, it could cause further damage to already weakened structures and infrastructure, as seen in Türkiye and Syria earlier this year.

“Neither of these is a better option,” Briggs said. “They’re both different options that need to be understood and modeled.” Simulations based on the physics of earthquakes could help to reveal what these faults are capable of.

The research has sparked numerous inquiries for future investigation, and the implications for building design are still uncertain, according to Susan Chang, a geotechnical engineer at the Seattle Department of Construction and Inspections.

Chang stated that if there are multiple events or a sequence of events rather than a single event, there are significant consequences for emergency management and building performance.

New designs for tall buildings may need to factor in the possibility of a large shock or back-to-back quakes. Chang said she expects this new information will also be considered the next time that USGS scientists revise their ground motion maps, which the city bases its building codes on.

—Carolyn Wilke (@CarolynMWilke), Science Writer

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Reference: Wilke, C. (2023). The Puget Sound was shaken by one or more powerful earthquakes 1,100 years ago. Eos, 104. Retrieved from https://doi.org/10.1029/2023EO230405 on October 27, 2023.

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