A recent study presented at the American Geophysical Union's annual meeting shows Seattle may suffer from a future megathrust earthquake. Previously, Seattle was thought to be relatively safe. But University of Washington earth scientist Kenneth Creager reveals otherwise. According to Creager, new evidence “continues to paint the picture of the possibility that a megathrust earthquake can occur closer to the Puget Sound region than was thought just a few years ago."
Scientists report that megathrust quakes occur roughly every 500 years along the Cascadia subduction zone. This zone runs from Northern California to Vancouver Island in British Columbia, just where the Juan de Fuca tectonic plate dips beneath the North American plate.
According to scientists, this is a locked area that can produce a megathrust earthquake, causing widespread heavy damage. Massive magnitude 9 earthquakes occur about every 500 years. An earthquake of this magnitude would be comparable to the 2004 Indian Ocean earthquake which caused a devastating tsunami.
How have scientists come to this conclusion? Rumblings have been recently detected in the Cascadia zone. According to Creager, "There appear to be tremor swarms that repeat, both in terms of their duration and in where they are." These rumblings last anywhere from one to 70 hours, and they occur in semi-regular patterns between the "tremor and slip" events that scientists were already monitoring.
Apparently, the two tectonic plates are locked together. Consequently, stress builds at their interface as they collide with each other at a rate of about 4 centimeters, or 1.6 inches a year. These tremor and slip events occur at the interface of the plates as they gradually descend beneath the surface, at depths of about 19 to 28 miles.
By studying these small tremors, scientists are getting a better idea of just how a megathrust earthquake would affect Seattle and its outlying areas. Hopes remain that the study will enable engineers to revise building codes in preparation. Buildings would be able to withstand a huge quake, especially if the source is substantially closer to the city than it was previously expected to be.
But don’t stock up on batteries quiet just yet. The last megathrust earthquake occurred in 1700, which makes the Pacific Northwest due another one sometime around 2200.