DESCRIPTION:
Cascade Range Earthquakes and Seismicity

The Cascade Range has been an active arc for about 36 million years as a result of plate convergence. ... The southern Washington Cascades are seismically active. Most earthquakes occur along the 100-kilometer-long, north-northwest trending St. Helens seismic zone, where most focal mechanisms show dextral slip parallel to the trend of the zone and consistent with the direction of plate convergence. Other crustal earthquakes concentrate just west of Mount Rainier and in the Portland (Oregon) area. Few earthquakes occur north of Mount Rainier or south of Mount Hood.

[Map,20K,InlineGIF]
Map, Plate Tectonics and the Cascade Range

Earthquakes occur when rocks break suddenly in response to various geologic forces. Magma moving in the Earth's crust may create sufficient force to produce volcanic earthquakes. More common, however, are tectonic earthquakes, which periodically strike parts of Oregon. These earthquakes, the result of fault movements driven by regional crustal stresses, typically have no direct connection to magma movement. Regardless of type, earthquake size is reported by magnitude, and many scientists and media describe earthquakes by the well-known Richter magnitude scale. ...

Tectonic earthquakes occur periodically in south-central and southeast Oregon, and they are capable of exceeding the magnitude of volcanic earthquakes. An example is the Klamath Falls earthquakes, a swarm that began in September 1993 with two large earthquakes of magnitude 5.9 and 6.0 that killed two people and and caused $7.5 million in property damage. Aftershocks as large as magnitude 5.1 continued to disturb residents for as much as six months. These earthquakes had no connection with volcanic processes.

Oregon Volcanoes Information - Menu

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 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 (magnitude 7.1) and the 1965 Seattle-Tacoma earthquake (magnitude 6.5). 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 ever 35 years and large earthquakes similar to the 1949 Olympia earthquake about ever 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. ...

Washington has features typical of convergent boundaries ... A zone of deep earthquakes near the probable boundary between the Juan de Fuca plate and the North America plate. The 1949 magnitude 7.1 Olympia earthquake and the 1965 magnitude 6.5 Seattle-Tacoma earthquake occurred within this deep zone. ... In sum, The subduction of the Juan de Fuca plate beneath the North America plate is believed to directly or indirectly cause most of the earthquakes and young geologic features in Washington and Oregon.

Washington Volcanoes Information - Menu

Information on Pacific Northwest earthquake activity and hazards is provided by the Pacific Northwest Seismograph Network (PNSN) which operates seismograph stations and locates earthquakes in Washington and Oregon. The PNSN is operated jointly by the University of Washington, the University of Oregon, and Oregon State University, and is funded by the U.S. Geological Survey, the Department of Energy, and the State of Washington. The PNSN is based at the Geophysics Program of the University of Washington in Seattle, and is a member of the Council of the National Seismic Systems (CNSS), a group of regional network operators who cooperate to locate and catalog earthquakes throughout the United States.

Pacific Northwest Seismograph Network -- Information, Maps, Links, etc.

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