A recent research study has raised concerns about increased earthquake risks in the Seattle area. The findings emphasize the necessity of revising local hazard assessments to accommodate the potential occurrence of more destructive "multi-fault" earthquakes.

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SEATTLE, WASHINGTON - MARCH 13: The Space Needle stands over the Seattle skyline on March 13, 2022 in Seattle, Washington. The iconic observation tower was constructed in 1962 for the World's Fair and was once, at 605 feet, the tallest structure west of the Mississippi River. 

Facing Risks of Enhanced Earthquakes

According to municipal authorities in Seattle, deep earthquakes are the predominant large seismic events in the Puget Sound region. Throughout history, Interesting Engineering reported that earthquakes of magnitude greater than six have occurred in 1909, 1939, 1946, 1949, 1965, and 2001.

The study reveals that over a millennium ago, a cluster of earthquakes in the Puget Sound region triggered landslides, the uplifting of fault remnants, and a localized tsunami. This event led to the submergence and burial of numerous local trees across an extensive area spanning 193 square miles.

While this discovery confirmed the widespread impact of the earthquake roughly 1,100 years ago, it did not pinpoint the exact calendar year of the event or establish connections with occurrences on distant fault lines.

Interesting Engineering reported that the researchers conducted a thorough examination of tree mortality in six distinct sites spanning the Seattle and Saddle Mountain fault zones, situated within the Puget Sound region.

To ensure the accuracy of their earthquake-related tree-ring chronologies, they integrated this data with an absolute dating chronology derived from live trees. This meticulous approach allowed them to establish precise timelines of tree death caused by seismic activity in the region.

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A previous dendrochronological study conducted on these submerged trees revealed a fascinating correlation: a tree that plunged into Lake Washington during a landslide shared the same timeline as a tree that got buried in a tsunami along the Puget Sound shoreline.

Conducting Tests

A group of divers was enlisted to retrieve a tree sample from the depths of Price Lake in Washington. This tree sample, originally downed by an earthquake, held valuable clues.

By employing a combination of radiocarbon dating and tree-ring analysis, Science reported that the researchers were able to pinpoint the exact timeframe when these trees perished.

Their findings indicated that the trees met their demise between 923 and 924 AD, all within a six-month window. This crucial evidence suggests that the earthquakes that affected the Puget Sound region occurred in rapid succession. 

The research team's calculations suggest that a solitary multi-fault earthquake could have reached a magnitude of approximately 7.8. In contrast, if there were two separate earthquakes within that six-month timeframe, each of them would have surpassed 7.3 in magnitude, as stated in the press release.

These findings introduce scenarios that are currently not accounted for in existing hazard models. They also underscore the necessity for seismic preparedness and engineering design to accommodate larger earthquake sizes than previously considered.

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