Puget Sound Earthquake Origin and Occurrence

Shallow earthquakes occur in the North American continent. The map below shows the cluster that has affected the Puget Sound region. The area numbered "3" in the cross section below shows generally where these quakes have occurred. Because of their shallow depth, large quakes of this type can cause great havoc in urban areas.

The cross section is larger in scale than the map to better show the three potential modes of earthquake generation under Puget Sound. Historical major subduction zone earthquakes (type number "1" in the cross section) have not occurred, but there is geologic evidence that a quake of this type occurred 300 years ago. Globally some quakes of this type have had magnitudes exceeding 9.0 and have caused wide-spread devastation.

Earthquakes that occurred in the down-going, oceanic rocks tend to cluster below Puget Sound. These earthquakes (locations shown on map below) are caused by the downward bending of the oceanic rocks, and have had magnitudes as large as 7.0. The area numbered "2" on the cross section above shows schematically where the quakes have occurred.