How a shallow fault running directly below Seattle went undiscovered for a millennium
And why it a major quake there could be more devastating than a similar quake along the Cascadia Subduction Zone
About 1,100 years ago, a major earthquake rocked Puget Sound, suddenly shooting what is now Restoration Point on Bainbridge Island up about 23 feet while Seattle's West Point sunk more than three feet.
The quake, which would likely have had a magnitude greater than 7.5, rattled along the Seattle Fault Zone, a several-mile-wide fault that runs east-west, from Issaquah to Hood Canal, passing through the stadiums and Alki on its way.
It's almost unimaginable that one of Seattle's biggest threats runs right through the middle of the city -- and it was just discovered less than 30 years ago.
One would think it might be difficult to miss such a feature -- a 45-mile fracture in the Earth running directly through a major city -- but discovery of what was dubbed the Seattle Fault Zone was shocking, because few suspected the fault existed before then.
Five papers described the Seattle Fault Zone for the first time in an issue of the journal Science published on Dec. 4, 1992. Scientists also detailed the serendipitous way it was discovered.
Geologist Robert Bucknam was investigating Restoration Point when he found the first evidence of the fault's existence. In an analysis of sediment, which included humus, peat, charcoal and single-cell organisms buried in the rock, two corresponding pieces of land appeared at different elevations, one above and one below sea level. Avalanche data also supported this finding.
But the final piece of evidence was in the trees.
Three groves of Douglas fir trees slid from their original habitats to grounds 90 feet underwater, located near Mercer Island, Seward Park and St. Edward Park on Lake Washington. Radiocarbon dating indicated the trees were between 1,000 and 1,300 years old. Growth rings showed all trees from the three groves died in the same year and season.
What's more -- the examination of these trees showed striking similarity to a tsunami-driven log found on Magnolia beach above Elliott Bay in February 1992. The incident would have been catastrophic to the Emerald City that sits along the fault zone today -- similar to major quakes recorded in Northridge, California and Kobe, Japan.
Earthquake danger is often in the news for Seattleites these days, especially with talks of the Cascadia Subduction Zone being the source of the region's next "Big One," which could feasibly reach a magnitude of 9.0 or higher.
But a major quake along the Seattle Fault Zone could be devastating. First of all, quakes produced by the Seattle Fault Zone are likely to originate closer to the Earth's surface; shallow quakes have the potential to cause more damage because they do not have as far to travel.
"A shallow earthquake in the middle of a populated area is no small thing. The region should take this as a warning that this is big-time earthquake country," Craig Weaver, coordinator of the U.S. Geological Survey's earthquake hazards research in the Pacific Northwest, told a Seattle Post-Intelligencer reporter in 1992.
To see how a major quake would impact Seattle, click through the slideshow above.