The seismology lab at the University of Washington records roughly 1,000 earthquakes per year 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 our understanding of the geologic history of the Pacific Northwest, we are certain that damaging earthquakes (magnitude 6 or greater) will recur in our area, although we have no way to predict whether this is more likely to be today or years from now. A map and list of selected historic earthquakes shows that while the most damaging events were fairly deep, many moderate sized earthquakes are fairly shallow. Several detailed Earthquake Scenarios, projecting likely effects from large quakes, are available.

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.

Deep Earthquakes: The three most recent damaging earthquakes in Washington, in 2001 (magnitude 6.8, near Olympia), 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. Each earthquake caused serious damage, and was felt as far away as Montana. No aftershocks were felt following the 1965 and 1949 earthquakes, and only 2 small aftershocks were felt after the 2001 quake. Other sizable events which were probably deep occurred in 1882, 1909, and 1939.

Shallow crustal earthquakes: The largest historic earthquake in Washington or Oregon occurred in 1872 in the North Cascades. This earthquake had an estimated magnitude of 7.4 and was followed by many aftershocks. It was probably at a depth of 10 miles or less within the continental crust. In 1993, a magnitude 5.6 earthquake in the Willamette Valley caused $28 million in damages (including damage to the Oregon State Capitol in Salem), and a pair of earthquakes near Klamath Falls, OR (magnitudes 5.9 and 6.0) caused two fatalities and $7 million in damages. Many other crustal sources in Washington and Oregon could also produce damaging earthquakes. Recent studies have found geologic evidence for large shallow earthquakes 1,100 years ago within the central Puget Basin. Massive block landslides into Lake Washington, marsh subsidence and tsunami deposits at West Point in Seattle, tsunami deposits at Cultus Bay on Whidbey Island, and large rock avalanches on the southeastern Olympic Peninsula have all been dated to approximately 1,100 years ago.

Subduction Zone earthquakes: 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 scientists have tried for decades to predict earthquakes, no one has discovered a method which can be applied with regular success. For some areas with well-understood patterns of seismicity, it may be possible to forecast decades-long time windows when large earthquakes are likely to occur. However, the Pacific Northwest has only been monitored for a couple of decades; not long enough to allow us to see what patterns, if any, exist here. Seismologists are still trying to understand what types of earthquakes are possible here, and what kind of shaking we will experience from future earthquakes (depending on the earthquake location and size, and the site geology and topography). Earthquake hazards can be reduced by advance preparation; such as coordinating emergency communications and activities across jurisdictional lines, preparing personal emergency plans, and considering seismic hazards in land use plans, building codes, and planning for medical, utility, and emergency facilities.

The Federal Emergency Management Agency (FEMA) has information about earthquake hazard mitigation and preparedness. For more detailed information about earthquakes and earthquake hazards in the Pacific Northwest, there is a list of resources, the best of which is Information Circular 85 "Washington State Earthquake Hazards" available for $1.00 (postage and handling) from the Washington State Dept. of Natural Resources, POB 47007, Olympia, WA 98504-7007