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Massive Seattle Earthquake 1,100 Years Ago Triggered by Hidden Faults, Possibility of Recurrence

A new study reveals that a massive 7.8 magnitude earthquake hit Seattle 1,100 years ago, stronger than current predictions.

A groundbreaking study has revealed that Seattle experienced a devastating earthquake approximately 1,100 years ago, caused by the simultaneous eruption of multiple hidden faults. This earthquake, measuring a destructive 7.8 magnitude, surpassed the predictions made by current fault zone warnings.

The region encompassing Puget Sound, including Olympia, Tacoma, and Seattle, has a history of being struck by powerful earthquakes. Geological evidence suggests that these earthquakes occurred between A.D. 900 and A.D. 930, with magnitudes reaching the low 7s.

While previous estimates indicated that there were two major earthquakes during this time period, the new study suggests that each fault may have ruptured simultaneously or within a very short timeframe. If this simultaneous rupture did occur, the resulting earthquake would have been stronger than any earthquake previously simulated.

To conduct their research, scientists analyzed fossilized Douglas fir trees from six different sites in Puget Sound. By using radiocarbon dating and examining tree ring counts, they were able to estimate that these trees died between A.D. 923 and A.D. 924. This evidence suggests that tectonic plate movements likely triggered the earthquakes, increasing the likelihood of simultaneous ruptures. However, even if the ruptures did not occur simultaneously, the individual earthquakes would still have had magnitudes ranging from 7.3 to 7.5. It is important to note that earthquake magnitudes are measured on a logarithmic scale, meaning that a magnitude 8 earthquake is ten times stronger than a magnitude 7 earthquake.

The simultaneous ruptures likely resulted in the most powerful earthquake that the fault zones in the region could produce. This raises concerns for the four million individuals living in Puget Sound, as current warning models may need to be updated to account for the potential of a similarly powerful earthquake.

While these findings are significant, it is important to note that Seattle's most dangerous seismic threat comes from the Cascadia Subduction Zone (CSZ). This massive fault line stretches approximately 1,000 kilometers from Vancouver to Northern California and has the potential to generate magnitude 9 earthquakes.

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