An earthquake is the sudden release of stored energy; most earthquakes occur along a fracture within the earth, called a fault. The shaking caused by this sudden shift is often very small, but occasionally large earthquakes produce very strong ground shaking. It is this strong shaking and its consequences – ground failure, landslides, liquefaction – that damages buildings and structures and upsets the regional economy.
Washington, especially the Puget Sound basin, has a history of frequent earthquakes. More than 1,000 earthquakes occur in the state each year. A dozen or more are strong enough that people feel ground shaking; occasionally, earthquakes cause damage. Large earthquakes in 1946 (magnitude 5.8), 1949 (magnitude 7.1) and 1965 (magnitude 6.5) killed 15 people and caused more than $200 million (1984 dollars) in damage throughout several counties. The state experienced at least 20 damaging events in the last 125 years.
The Nisqually earthquake on February 28, 2001, was a deep, magnitude 6.8 earthquake 10 miles northeast of Olympia. One person died of a heart attack, more than 700 people were injured, and various estimates place damage at between $1 billion and $4 billion; exact figures are not available, as insurance claims information is not available.
The earthquake threat in Washington is not uniform. While most earthquakes occur in Western Washington, some damaging events, such as the 1872 magnitude 6.8 (est.) quake, do occur east of the Cascades. Geologic evidence documents prehistoric magnitude 8 to 9.5 earthquakes along the outer coast, and events of magnitude 7 or greater along shallow crustal faults in the urban areas of Puget Sound.
Washington’s earthquake hazards reflect its tectonic setting. The Pacific Northwest is at a convergent continental margin, the collision boundary between two tectonic plates of the earth’s crust. The Cascadia subduction zone, the fault boundary between the North America plate and the Juan de Fuca plate, lies offshore from northern California to southern British Columbia. The two plates are converging at a rate of about 2 inches per year. In addition, the northward-moving Pacific plate is pushing the Juan de Fuca plate north, causing complex seismic strain to accumulate. The abrupt release of this slowly accumulated strain causes earthquakes.