I bet you didn’t know that Seattle is safe from earthquakes. According to a University of North Carolina geologist Collier Cobb it is. He had this to say to a meeting of the Chamber of Commerce: “Other sections may suffer earthquakes, but Seattle, set in the deepest glacial drift yet discovered on this continent, has a shock absorber which makes the city immune from disaster from quakes.” Sadly he was wrong but then again, Dr. Cobb made his comment in October 1920.
Cobb arrived in Seattle after spending six weeks exploring the shorelines of Alaska and Puget Sound. In a his speech to the Chamber, he noted that Seattle’s “wonderful harbor, unmatched anywhere on this continent” had been carved out less than 2,000 years ago by glacial action. During this process, the ice generated 900 feet of glacial drift, which he defined as “the scrapings of the best top soil of other sections.” This glacial action was the “most recent on the face of the globe.” Because of the great depth of the drift, its shock absorbing quality meant that “there are no sharp faults, which can make for a serious seismic disturbance here…Seattle is secure for all time.”
We now know that Cobb was wrong on all accounts. The glacial ice and water flowing under it did carve out the region but that was roughly 16,500 years ago. We also know that three major faults zones affect Seattle. 1. The deep Benioff zone quakes, which moved in 1949, 1965, and 2001. 2. Cascadia Subduction, which last moved in January 1700, had a magnitude of more than 9.0, and generated a tsunami that hit Japan. 3. The Seattle Fault Zone, which last moved about 1,100 years ago and produced more than 20 feet of uplift. Oh well. It was a nice thought.