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Hazards

Willamette Basin Earthquakes

On the basis of anecdotal science and geological records, Oregon has a long history of seismic events. For example, anecdotal and written records dating back to 1841 indicate there have been more than 6,000-recorded earthquakes in Oregon, most with a magnitude below three. One of the best summaries on the history of earthquakes in the Willamette Basin is provided in the Clackamas County Mitigation Plan.

Portland and its surrounding region is potentially the most seismically active area within Oregon. The Portland metropolitan region has encountered seventeen earthquakes of an estimated magnitude of four and greater, with major earthquakes in 1877 (magnitude 5.3), 1962 (magnitude 5.2), and 1993 (magnitude 5.6). Earthquakes frequently occur in the Willamette Basin, but they are just so small we can't feel them.

Why are there so many earthquakes? It is geology getting in the way of our lives again, this time by the Cascadia Subduction Zone (see figure below).

Cascadia Subduction Zone

Earthquakes pose a serious threat to communities in the Willamette Basin. Local and state governments, planners, and engineers have developed plans to deal with the risk.

Earthquake related hazards are many and include ground shaking, landslides, liquefaction, and amplification. Ground shaking is caused by seismic waves generated by the earthquake and is the primary cause of earthquake damage. Landslides are secondary earthquake hazards that occur from ground shaking. Ground shaking causes wet granular soils to "liquefy" or change from a solid state to a liquid state. The magnitude of the seismic waves can be "amplified" and is dependent on the thickness of geologic materials and their physical properties.

What is the probability of an earthquake occurring in the Willamette Basin? USGS provides information on probabities of earthquakes in specific regions.

Conterminous States Probabilistic Map

Conterminous States Probabilistic Map (Earthquake Hazards)
(US Geological Survey National & Regional Seismic Hazard Maps)

Additional Resources

Earthquake Magnitude Scale. UPSeis: an educational site for budding seismologists. [Accessed June 25th, 2007]

Clackamas County Emergence Management. Earthquake (pp. EQ1-8). University of Oregon’s Community Service Center(2012). [Accessed February 2015]

Significant Earthquakes Archive. USGS. Earthquake Center. [Accessed June 25th, 2007]

Planning for Natural hazards. Seismic TRG (PDF). Salem: Oregon Department of Land Conservation and Development July 2000. [Accessed June 25th, 2007]

Authored by Todd Jarvis, Senior Researcher, Institute for Water and Watersheds, Oregon State University (2007)

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