Washington is earthquake country. More than 1,000 earthquakes are recorded in the state each year; a dozen or more of these produce significant shaking or damage. Large earthquakes in 1949 and 1965 killed 15 people and caused more than $200 million (1984 dollars) property damage.

Earth scientists believe that most earthquakes are caused by slow movements inside the Earth that push against the Earth's brittle, relatively thin outer layer, causing the rocks to break suddenly. This outer layer is fragmented into a number of pieces, called plates. Most earthquakes occur at the boundaries of these plates. In Washington, the small Juan de Fuca plate off the coast of Washington, Oregon, and northern California is slowly moving eastward beneath a much larger plate that includes both the North American continent and the land beneath part of the Atlantic Ocean. Plate motions in the Pacific Northwest result in shallow earthquakes widely distributed over Washington and deep earthquakes in the western parts of Washington and Oregon. The movement of the Juan de Fuca plate beneath the North America plate is in many respects similar to the movements of plates in South America, Mexico, Japan, and Alaska, where the world's largest earthquakes occur.

We cannot predict precisely where, when, and how large the next destructive earthquake will be in Washington, but seismological and geological evidence supports several possibilities. Large earthquakes reported historically in Washington have most frequently occurred deep beneath the Puget Sound region. The most recent and best documented of these were the 1949 Olympia earthquake and the 1965 Seattle-Tacoma earthquake. The pattern of earthquake occurrence observed in Washington so far indicates that large earthquakes similar to the 1965 Seattle-Tacoma earthquake are likely to occur about every 35 years and large earthquakes similar to the 1949 Olympia earthquake about every 110 years. Such large earthquakes deep beneath the Puget Sound area will happen again.

The largest earthquake reported in the state did not occur in the Puget Sound region, but rather at a shallow depth under the North Cascade Mountains. Recent studies in the southern Cascades near Mount St. Helens indicate that other areas in the Cascades may produce large, shallow earthquakes, comparable in size to the 1949 and 1965 Puget Sound earthquakes. The average interval of time between occurrences of such earthquakes in the Cascade Mountains is uncertain because they have occurred infrequently. However, the 1872 North Cascade earthquake and earthquake activity in the southern Cascades are reminders that Puget Sound is not the only region in Washington having significant earthquake hazards.

The largest earthquake now considered a possibility in the Pacific Northwest is a shallow subduction-style earthquake similar to recent destructive earthquakes in Alaska and Mexico, which had magnitudes greater than 8. An earthquake this large would be expected to occur along the coast of Washington or Oregon. Although we have no record of such large earthquakes in the Pacific Northwest within the last 150 years, some scientists believe that rocks and sediments exposed along the coasts of Washington and Oregon show evidence that as many as eight such earthquakes have occurred in the last several thousand years. This evidence indicates an average interval of time between subduction earthquakes of several hundred years. A magnitude 8 subduction earthquake would not only cause widespread dangerous ground shaking but would also likely produce water waves capable of inundating coastal areas in a matter of minutes.

Earthquake damage is primarily caused by ground shaking. However, wood frame houses, well attached to their foundations and built on firm ground, generally sustain little structural damage during earthquakes. In contrast, unreinforced brick buildings commonly suffer severe damage. Ground shaking may also displace and distort the non-structural parts of a building-including windows, ceiling tiles, partitions and furniture-producing property damage and endangering life. Other hazards such as ground liquefaction are commonly triggered by strong ground shaking.

Future injuries and property losses from earthquake hazards can be reduced by considering these hazards when making decisions about land use, by designing structures that can undergo ground shaking without collapse, by securely attaching the non-structural elements of a building, and by educating the public about what to do before, during, and after an earthquake to protect life and property. Many businesses and corporations without earthquake emergency contingency plans fail following earthquake disasters. Earthquakes cannot be prevented, but practicing appropriate response actions and the mitigation of hazards will reduce their impact on people.

Now is the time to prepare. Establishing earthquake safety policy, both governmental and organizational, will provide guidelines for the development of earthquake safety programs and ensure that programs are consistently carried out. Businesses that have earthquake emergency plans are less likely to fail following earthquake disasters.

More detailed information about earthquakes, local geology, and earthquake safety in Washington can be obtained from the following:

Washington State Department of Natural Resources
Division of Geology and Earth Resources
P.O. Box 47007
Olympia, WA 98504-7007
(360) 902-1450

Washington Military Department
Emergency Management Division
PO Box 40955
Olympia, WA 98504-0955
(360) 459-9191

Federal Emergency Management Agency Region 10
130 - 228th Street SW
Bothell, WA 98021-9796
(206) 487-4600

Branch of Distribution
U.S. Geological Survey
Box 25046, Building 810, Federal Center
MS stop 504
Denver, CO 80225-0046
(303) 202-4700 Customer Service
(303) 202-4600 Information Service
(303) 202-4200 Earth Science Information Center

Earth Science Information Center (ESIC)
U.S. Geological Survey
MS 532
345 Middlefield Road
Menlo Park, CA 94025
(415) 329-4309