(Much of the information contained here was provided by the Geophysics Department at the University of Washington from their web site "Pacific Northwest Earthquake Information").
Roughly 1,000 earthquakes per year are recorded in Washington and Oregon. Between one and two dozen of these cause enough ground shaking to be felt by residents. Most are in the Puget Sound region, and few cause any damage. However, based on the history of past damaging earthquakes and an understanding of the geologic history of the Pacific Northwest, there is an increasing consensus among many scientists that damaging earthquakes (magnitude 6 or greater) will occur in the Pacific Northwest. However, there is no way to predict when these earthquakes will occur.
Earthquakes are driven by geologic processes which produce stresses in the earth. In the Pacific Northwest, oceanic crust is being pushed beneath the North American continent along a major boundary parallel to the coast of Washington and Oregon. This boundary, called the "Cascadia Subduction Zone" lies about 50 miles offshore and extends from the middle of Vancouver Island in British Columbia past Washington and Oregon to northern California.
• The two most recent damaging earthquakes in Washington, in 1965 (magnitude 6.5, located between Seattle and Tacoma), and in 1949 (magnitude 7.1, near Olympia), were roughly 40 miles deep and were in the oceanic plate where it lies beneath the continent. Both earthquakes caused serious damage, and were felt as far away as Montana. No aftershocks were felt following them.
• Other sizable events which were probably deep occurred in 1882, 1909, and 1939.
• Although no large earthquakes have happened along the offshore Cascadia Subduction Zone since our historic records began in 1790, similar subduction zones worldwide do produce "great" earthquakes - magnitude 8 or larger. These occur because the oceanic crust "sticks" as it is being pushed beneath the continent, rather than sliding smoothly. Over hundreds of years, large stresses build which are released suddenly in great earthquakes. Such earthquakes typically have a minute or more of strong ground shaking, and are quickly followed by damaging tsunamis and numerous large aftershocks.
• The Alaskan earthquake of 1964 was a great subduction zone earthquake.
• Geologic evidence shows that the Cascadia Subduction Zone has also generated great earthquakes, and that the most recent one was about 300 years ago.
• Large earthquakes also occur at the southern end of the Cascadia Subduction Zone (in northern California near the Oregon border) where it meets the San Andreas Fault system; including a magnitude 7.1 earthquake in 1992, and a magnitude 6.8 (estimated) earthquake in 1873.
• Although no large earthquakes have happened along the offshore Cascadia Subduction Zone since our historic records began in 1790, similar subduction zones worldwide do produce "great" earthquakes - magnitude 8 or larger. These occur because the oceanic crust "sticks" as it is being pushed beneath the continent, rather than sliding smoothly. Over hundreds of years, large stresses build which are released suddenly in great earthquakes. Such earthquakes typically have a minute or more of strong ground shaking, and are quickly followed by damaging tsunamis and numerous large aftershocks.
• The Alaskan earthquake of 1964 was a great subduction zone earthquake.
• Geologic evidence shows that the Cascadia Subduction Zone has also generated great earthquakes, and that the most recent one was about 300 years ago.
• Large earthquakes also occur at the southern end of the Cascadia Subduction Zone (in northern California near the Oregon border) where it meets the San Andreas Fault system; including a magnitude 7.1 earthquake in 1992, and a magnitude 6.8 (estimated) earthquake in 1873.
Seattle is earthquake country. Older residents can remember when in 1949 and 1965, earthquakes of magnitude 7.1 and 6.5, respectively, hit the Puget Sound region. Eight people were killed in each earthquake. Since 1965 the earth has moved a few times in Seattle, but strong, sustained ground motions have not been felt. Damage, from events such as the one that hit near Duvall in 1996 disrupting a Mariners game, has been very slight.
Despite a lack of larger, damaging earthquakes, earth scientists and engineers now understand that earthquake hazards in the Seattle area are greater than previously known. In addition to deep earthquakes like those in 1949 and 1965, in the early 1990s scientists and engineers accepted the geologic evidence that great subduction zone earthquakes, of magnitude 8 to 9, repeatedly strike along the Washington coast.
For more on the New Madrid Fault, visit theSaint Louis University Earthquake Centeror visit theUSGS New Madrid Fact Sheet "The Mississippi Valley -Whole Lotta Shakin' Goin' On".
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