The United States Geological Survey recorded a Magnitude 3.6 Earthquake at a depth of 11.7 km beneath the Key Peninsula in Washington State, roughly 40 km southwest of Seattle, slightly after 11.40 am local time (slightly after 6.40 pm GMT) on Tuesday 20 August 2013. There are no reports of any damage or injuries resulting from the quake, but it was reportedly felt over quite a wide area, including Olympia, Seattle, Everett and parts of the Pacific Coast of the State.
The approximate location of the 20 August 2013 Key Peninsula quake. Google Maps.
Washington State is located on the western margin of the North American Plate, to the west the Juan de Fuca Plate is being subducted along the Cascadia Subduction Zone, passing under Washington State as it sinks into the Earth. This is not a smooth process, and the two plates frequently stick together then break apart again as the pressure builds up, causing Earthquakes in the process.
In addition the heat and pressure within the Earth also slowly melts the subducting plate, liquifying more volatile minerals which then rise through the overlying North American Plate as magma, fueling the volcanoes of America's Pacific Northwest.
Witness accounts of quakes can help geologists to understand these events and the rock structures that cause them. If you felt this quake you can report it to the USGS here.
See also Magnitude 5.3 Earthquake shakes Vancouver Island, Magnitude 4.3 Earthquake in central Washington State, Major landslip on Whidbey Island, Washington State and Scientists predict eruption of Axial Seamount.
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