Earthquakes occur in Oregon every day. Every few years, an earthquake is large enough for people to feel, and every few decades there is an earthquake that causes damage. Each year, the Pacific Northwest Seismic Network locates more than 1000 earthquakes greater than magnitude 1.0 in Washington and Oregon. Of these, approximately two dozen are large enough to feel. These noticeable events offer a subtle reminder that the Pacific Northwest is an earthquake-prone region.
The severity of an earthquake is dependent upon a number of factors including:
- The distance from the earthquake’s source (or epicenter);
- The ability of the soil and rock to conduct the earthquake’s seismic energy;
- The degree (i.e., angle) of slope materials;
- The composition of slope materials;
- The magnitude of the earthquake; and
- The type of earthquake.
Oregon and the Pacific Northwest in general are susceptible to three types of earthquakes:
Crustal Fault Earthquakes
These are the most common earthquakes and occur at relatively shallow depths of 6-12 miles below the surface. Oregon’s risk comes from slippage events within the North American Plate. When crustal faults slip, they can produce earthquakes of magnitudes up to 7.0. Although most crustal fault earthquakes are smaller than 4.0 and generally create little or no damage, some of them can cause extensive damage. Known active faults in the region include the Wallula, Hite, Mount Angel, and Lake of the Woods fault zones. Of the three earthquake types, crustal earthquakes are the greatest threat to Jackson County.
Deep Intraplate Earthquakes
Occurring at depths from 18 to 60 miles below the earth’s surface in the subducting oceanic crust, deep intraplate earthquakes can reach magnitude 7.5. This type of earthquake is more common in the Puget Sound; in Oregon these earthquakes occur at lower rates and have none have occurred at a damaging magnitude. The February 28, 2001 earthquake in Washington State was a deep intraplate earthquake. It produced a rolling motion that was felt from Vancouver, British Columbia to Coos Bay, Oregon and east to Salt Lake City, Utah.
Subduction Zone Earthquakes
The Pacific Northwest is located at a convergent continental plate boundary, where the Juan de Fuca and North American tectonic plates meet. The two plates are converging at a rate of about 1.5 inches per year. This boundary is called the Cascadia Subduction Zone (CSZ). It extends from British Columbia to northern California. Earthquakes are caused by the abrupt release of this slowly accumulated stress.
Although there have been no large recorded earthquakes along the offshore Cascadia Subduction Zone, similar subduction zones worldwide do produce "great" earthquakes with magnitudes of 8 or larger. They 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 numerous large aftershocks.
Subduction zones similar to the Cascadia Subduction Zone have produced earthquakes with magnitudes of 8.0 or larger. Historic subduction zone earthquakes include the 1960 Chile earthquake (magnitude 9.5), the 1964 southern Alaska (magnitude 9.2) earthquakes, the 2004 Indian Ocean earthquake (magnitude 9.0) and the 2011 Tohoku earthquake. Geologic evidence shows that the Cascadia Subduction Zone has generated great earthquakes, most recently about 300 years ago.
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.
While all three types of earthquakes have the potential to cause major damage, subduction zone earthquakes pose the greatest danger. A major CSZ event could generate an earthquake with a magnitude of 9.0 or greater resulting in devastating damage and loss of life. Such earthquakes may cause great damage to the coastal area of Oregon as well as inland areas in western Oregon including Jackson County. It is estimated that shaking from a large subduction zone earthquake could last up to five minutes. The specific hazards associated with an earthquake are explained below:
Ground shaking is the motion felt on the earth’s surface caused by seismic waves generated by the earthquake. Ground shaking is the primary cause of earthquake damage. The strength of ground shaking depends on the magnitude of the earthquake, the type of fault that is slipping, and distance from the epicenter (where the earthquake originates). Buildings on poorly consolidated and thick soils will typically see more damage than buildings on consolidated soils and bedrock.
Ground Shaking Amplification
Ground shaking amplification refers to the soils and soft sedimentary rocks near the surface that can modify ground shaking from an earthquake. Such factors can increase or decrease the amplification (i.e., strength) as well as the frequency of the shaking. The thickness of the geologic materials and their physical properties determine how much amplification will occur. Ground motion amplification increases the risk for buildings and structures built on soft and unconsolidated soils.
Surface faulting are planes or surfaces in Earth materials along which failure occurs. Such faults can be found deep within the earth or on the surface. Earthquakes occurring from deep lying faults usually create only ground shaking.
Liquefaction and Subsidence
Liquefaction occurs when ground shaking causes wet, granular soils to change from a solid state into a liquid state. This results in the loss of soil strength and the soil’s ability to support weight. When the ground can no longer support buildings and structures (subsidence), buildings and their occupants are at risk.
Earthquake-Induced Landslides and Rockfalls
Earthquake-induced landslides are secondary hazards that occur from ground shaking and can destroy roads, buildings, utilities and critical facilities necessary to recovery efforts after an earthquake. Some Benton County communities are built in areas with steep slopes. These areas often have a higher risk of landslides and rockfalls triggered by earthquakes.
Oregon Emergency Management maintains a comprehensive page on earthquake hazards and preparedness efforts. They have several helpful publications including Living on Shaky Ground, a guide to surviving earthquakes and tsunamis in Oregon.
Play the Shakeout game and learn how to ready your home an earthquake.