Just when we thought all we had to worry about was the Cascadia Subduction Zone (here, here and here) - from the Seismological Society of America:
Large Quakes Along Olympic Mountain Faults
A comprehensive study of faults along the north side of the Olympic Mountains of Washington State emphasizes the substantial seismic hazard to the northern Puget Lowland region. The study examined the Lake Creek-Boundary Creek and Sadie Creek faults along the north flank of the Olympic Mountains, and concludes that there were three to five large, surface-rupturing earthquakes along the faults within the last 13,000 years.
The study published September 26 in the Bulletin of the Seismological Society of America estimates that the two most recent earthquakes on the faults, one occurring around 2900 years ago and one occurring 1300 years ago, were likely of magnitude 7 and magnitude 6 to 7, respectively. Based on an analysis of fault scarps mapped with airborne lidar imagery (a remote sensing method used to examine the Earth’s surface) and the dating of earthquake stratigraphy in trenches, fault slip rates are about one to two millimeters per year, and as much as 56 kilometer lengths of the faults may have ruptured during earthquakes.
While the presence of large earthquakes in the region is not surprising, given the ongoing tectonic deformation in the region, said Alan Nelson and Steve Personius of the U.S. Geological Survey, the Lake Creek-Boundary Creek fault, and other young, active faults like it, pose a significant earthquake hazard for the northern Puget Lowland region. The Puget Lowland includes Seattle and extends through western Washington from Bellingham in the north to Olympia and Tacoma in the south.
The threat of a magnitude 8 to 9 megathrust earthquake and tsunami in the Pacific Northwest at the Cascadia subduction zone offshore, where the Juan de Fuca tectonic plate is pushed underneath the North American plate, often steals the seismic hazard spotlight in the region. But much shallower, upper-plate earthquakes also can produce strong ground shaking and damage. At least nine active upper-plate faults, like the Lake Creek-Boundary Creek fault, have been documented in the Puget Lowland, said Nelson.
“If you consider the hazard from these upper-plate faults, whose earthquake epicenters are only 10 or 15 kilometers deep, future upper-plate earthquakes will be much closer to large population centers in the Puget Lowland region,” Nelson said, “than will larger earthquakes on the plate boundary of the Cascadia subduction zone.”
Bend over and kiss your ass goodbye. Still, looking at the long-term, no place is safe so worrying is useless. Preparing is not.
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