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Earthquake  This web page broadly identifies and describes the earthquake hazard that Oregon faces. It also highlights previous plans, assessment tools, and resources that have been developed to identify, profile, and assess the vulnerability of risk from earthquake events in Oregon.Page Index: EARTHQUAKE OVERVIEW: An earthquake is a sudden movement of the Earth, caused by the abrupt release of strain that has accumulated over a long time. Sometimes the movement is gradual. At other times, the plates are locked together, unable to release the accumulating energy. When the accumulated energy grows strong enough, the plates break free. If the earthquake occurs near populated areas, it may cause many deaths and injuries, and extensive property damage. Oregon is affected by the Cascadia Subduction Zone where the Juan de Fuca plate slides underneath the North American plate. While earthquakes along this zone occur infrequently, plate movement can produce major earthquakes. In addition, Western Oregon is underlain by a large and complex system of faults that can produce damaging earthquakes; these smaller faults produce lower magnitude events, but their ground shaking can be strong and damage can be great to structures nearby.  Source: Shoreland Solutions. Chronic Coastal Natural Hazards Model Overlay Zone. Salem, OR: Oregon Department of Land Conservation and Development (1998) Technical Guide-3. Earthquakes can trigger other geologic and soils failures that contribute to damage. While surface fault rupture can produce damage to facilities and infrastructure astride the fault, losses from this are minor compared to those resulting from strong ground shaking and associated ground failures. These include landslides and slope failures, lateral spreading and slumping, and liquefaction. Back to index SUMMARY OF OREGON'S VULNERABILITY TO EARTHQUAKES: The occurrence of a major earthquake or seismic event would be the most catastrophic of all hazards to Oregon. Based on geologic studies, Oregon is prone to major seismic events (magnitude 8 to 9 earthquakes) every 500 to 600 years on average. Intervals between individual catastrophic seismic events range from 150 to 1000 years. Oregon is quite vulnerable to earthquakes (and tsunamis, which often accompany major seismic events) because of the state’s proximity to the Cascadia Subduction Zone just off the Pacific Coast. Also, Oregon is vulnerable to damage because of its topography and geology; many of its local soil profiles are prone to liquefaction during the shaking that would occur during a Cascadia event. Depending on the epicenter, areas receiving major damage from an 8.0 – 9.0 magnitude earthquake would include most of the counties in Western Oregon; the heavily populated metropolitan areas of Portland, Salem, and Eugene would certainly experience major damage. A major earthquake (> 8.5) would be devastating if it occurred in the Portland metropolitan area. The Northern Willamette Valley/Portland Metro Region is the most densely populated with a total population of almost 1.5 million people. A major earthquake would likely do extensive damage to many of the region’s 1382 bridges and overpasses as few bridges have been retrofitted to withstand this type of event. In addition, many structures are located on soils likely to experience liquefaction from the shaking that would occur. Most of the state’s major medical facilities exist in this region. That is good only if they can be accessed after such an event. This region also has 49 dams that could be affected by a major earthquake. Depending on where the epicenter is located, this magnitude of earthquake would likely cause extensive damage to structures and infrastructure in the Mid/Southern Willamette Valley Region as well. The city of Salem, Oregon’s state capital, is only 46 miles south of Portland. To gain a perspective of the potential damage from a major earthquake, 169 of the state’s 380 facilities are located in or near Salem. To replace these state facilities would cost over $850 million dollars. Marion County, where Salem is located, has over 20 dams and 400 bridges that could also be affected. The long-term effects from a major earthquake would be felt for years. Major damage would likely occur to most of the region’s public and private buildings, its vast road network, to its rail lines and power transmission lines, and to the state’s most important employment centers. A major earthquake that occurs in the southern, central, or eastern areas of Oregon would be catastrophic to that region. It may also be catastrophic to the state economically if key facilities and infrastructure (i.e., highways, bridges, rail lines, power transmission lines, and dams) are damaged to the degree that links with the Portland metropolitan region and the rest of the state could not quickly be repaired. However, the length of time for the state to recover from such a disaster occurring in an area away from the Portland metropolitan area should be much shorter than if the same event occurred near Portland. Back to index
Back to index Maintained by: Oregon Partnership for Disaster Resilience Community Service Center University of Oregon Last Updated 07/02/2007 |
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