Earthquakes

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Mercer Slough Oct. 2017.JPG
 

Washington State is situated in a highly active geologic area which contains shifting tectonic plates, crustal faults, and a subduction zone that all produce frequent earthquakes. Earthquakes occur here almost on a daily basis, and many of them go unnoticed. There are three types of earthquakes which may occur in our region: megathrust, intraplate (deep), and crustal (shallow).

Crustal faults are caused by the deformation within the North American Plate due to compression by neighboring tectonic plates. Crustal earthquakes occur no deeper than 18 miles below the surface, and can produce earthquakes as large as M7.5. Several major population hubs are situated directly atop such faults, such as the Seattle Fault which runs directly beneath the Cities of Seattle and Bellevue. The most recent Seattle Fault earthquake occurred about 1,100 years ago, and has been active 3-4 times over the past 3,000 years.

The most frequent types of earthquakes in Washington are Interplate (deep). These earthquakes rupture faults within the subducting Juan de Fuca Plate, and are usually less than M7.5. Because these earthquakes are so deep (18-42 miles beneath the surface), the seismic wave energy spreads over a much larger area than a shallow quake, thus shaking is felt over a larger geographic extent. Historically, interplate earthquakes occur every 10 to 30 years. The USGS estimates that there is an 84% chance of an interplate earthquake of M6.5 or great occurring within the next 50 years. Deep earthquakes larger than M6.0 occurred in 1909, 1939, 1946, 1949, 1965, and most recently on February 28, 2001 with the epicenter under the Nisqually Delta.

Megathrust earthquakes occur at subduction zones, and produce earthquakes as powerful as M9.0. In the Pacific Northwest, megathrust earthquakes occur along the Cascadian Subduction Zone. Historically, these earthquakes reoccur at a 500-year interval.

Seismic Scenario Catalog

The Washington State Seismic Scenario Catalog contains estimates of earthquake damage for a variety of faults in the State of Washington. Explore the catalog to visualize the impacts of different earthquake scenarios.

Recent earthquakes around the world

image of map of recent earthquakes around the world
Earthquakes from the last two weeks provided by the IRIS Seismic Monitor.

Sources

Understanding Earthquake Hazards in Washington State
Deep Earthquakes

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