The United States Geological Survey recorded a Magnitude 3.0 Earthquake at a depth of 27 km, 2 km to the north of Indianola, on Puget Sound opposite Seattle, slightly before 0.10 am local time (slightly before 7.10 am GMT) on Thursday 5 September 2013. This is not a large quake, and no damage or injuries have been reported, but it was felt over a fairly wide area.
The approximate location of the 5 September 2013 Indianola Earthquake. 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.
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 (or if you wee in the area but did not, which is also useful information) you can report it to the USGS here.
See also Magnitude 6.2 Earthquake of the coast of British Columbia, Magnitude 3.6 Earthquake in Washington State, Magnitude 5.3 Earthquake shakes Vancouver Island, Magnitude 4.3 Earthquake in central Washington State and Scientists predict eruption of Axial Seamount.
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