Earthquake Hazards in Oregon
Excerpted from The Oregonian staff and wire reports, DOGAMI website.
Say earthquake and many Americans immediately think of California. But the Northwest is one of the most seismically active regions in the world and has a history of strong quakes.
Nearly 17,000 earthquakes of magnitude 1.0 to 6.0 have been recorded in Oregon and Washington since 1970. About 15-20 quakes a year are felt in the Northwest. Earthquakes are usually felt if they are at least magnitude 3 to 4.
Know the Risk. Do you know what your area’s earthquake risks are?
Although many earthquakes occur on previously unknown faults, you can find out if there are any faults mapped near you and whether they are considered active (there are many known faults that geologists no longer expect to move).
Different rock and soil types behave differently in earthquakes. Are you on bedrock? If so, earthquake waves may be dampened in your area. Are you on alluvium in a river valley? If so, waves may be amplified; unfortunately, this is the case for much of the Willamette Valley.
The Oregon Department of Geology and Mineral Industries (DOGAMI) has produced maps for many communities that specifically address earthquakes and a wealth of information can be found on their website: www.OregonGeology.org
, or by calling their Portland office at (971) 673-1555.
Non-Structural vs Structural Risks
Earthquake risks are divided into two categories: non-structural and structural.
Non-Structural risks relate to the danger of falling objects or other risks not connected to the building’s seismic safety. Many earthquake injuries are caused by objects falling such as poorly secured bookcases falling in classrooms or parapets falling off the top of buildings. View a PowerPoint about non-structural risks
Structural risks relate to the safety of the buildings themselves. How likely is a building to collapse or suffer serious damage during an earthquake? When was a building constructed? What type of soil is the building situated on? What types of seismic upgrades have been made to make the buildings? See below for information on the Statewide Seismic Needs Assessment and more information on structural risks.
How Does Your School Rank Regarding Seismic Safety?
In June 2007, the Oregon Department of Geology and Mineral Industries (DOGAMI) released the full results of their Statewide Seismic Needs Assessment
. Among other buildings, this study reviewed the majority of Oregon’s public school buildings to assess their likelihood of collapse during a very severe earthquake. Your school’s degree of collapse risk (low, moderate, high or very high) is recorded as part of the assessment.
View a report on your school
. Building specific reports are available for all screened buildings. Simply click on the link above, search by county and then by school.
How were the buildings assessed?
The process DOGAMI used to assess these buildings is referred to as Rapid Visual Screening or RVS. During RVS, or sidewalk surveys, surveyors looked for potential seismic hazards and identified, inventoried, and ranked buildings that are potentially at risk of collapse from a very serious earthquake.
- The five key parameters that determine the relative seismic risk of a building are the:
- Seismic Zone (how hard the ground is expected to shake in a given area)
- Building Structural Type (wood frame, reinforced masonry, steel frame)
- Building Irregularities (the shape of the building)
- Original Construction Date, and Soil Type (softer soils amplify the severity of ground motion) Based on a review of these five elements, buildings were classified as being at Very High, High, Moderate or Low Seismic Risk.
Of the 2182 K-12 school buildings reviewed using RVS, 274 were ranked as at Very High Seismic Risk and 744 were ranked as High Risk. There were 497 buildings that were Moderate Risk and 667 that were Low risk. The map to the left shows the location of the 2,109 sites assessed as part of the Statewide Seismic Needs Assessment.
Does this mean that these buildings are unsafe?
First, keep in mind that these were Rapid Visual Screenings. These assessments looked at a building’s structure, the soil type, construction type, construction date etc. This was not a comprehensive structural assessment and it may not have captured all of the local steps taken in recent years to upgrade or retrofit older facilities. If you are concerned about the safety of a school in your community, check with your local school district about steps they may have taken to mitigate the risks. School districts with high or very high risk ratings can work with local experts or engineers to perform a more detailed assessment.
Second, also keep in mind that the risk of collapse is based on the occurrence of a “maximum considered earthquake” – the strongest possible earthquake considered possible in an area.
Preparedness and Next Steps
10 simple things schools and communities can do to improve the seismic safety of their schools.
1. Do your homework - Get familiar with the Seismic Needs Assessment view the Report Executive Summary. View the Full Report.
Notify DOGAMI if there are mistakes in the report about your schools. Most districts already know which schools are the most vulnerable, but this report can help you to prioritize.
2. Ask the experts – If your school rated high or very high on the seismic needs assessment, consider working with structural engineers or local experts to conduct a more comprehensive review and map out needed improvements.
3. Prioritize – Helping to ensure a school survives an earthquake doesn’t have to mean a complete redo. Identify the most vulnerable areas of the school and fix them first. Make sure that the exits to your school are safe and structurally sound. A blocked or dilapidated exit can be catastrophic in an emergency.
4. Look ahead - Look at seismic rehabilitation as a long term project. This is not something that will be fixed overnight. As you make other needed upgrades and remodels to your buildings, keep seismic safety in mind and make needed improvements at that time.
5. Learn from others – There are many school districts that have already been working on improving their buildings and making them more earthquake resistant. These districts can provide wonderful models of what works best in their planning and retrofitting. Learn from and share with neighboring school districts.
6. Plan – Work with the school district and community to review and update your emergency plan so that everyone knows what to do in the event of an emergency.
7. Don’t overlook the small stuff – There are many non-structural fixes you can make that will help tremendously in the case of an earthquake. Many injuries in earthquakes are from non-structural risk – bookcases falling over, lights falling from the ceiling, or chimneys or parapets collapsing and sending debris raining down. Securing potentially dangerous furniture and building features can prevent injury and even save lives.
9. Vote for Change - Work as a community to pass bonds that include improvements to the seismic safety of your schools.
10. Ask for help – Let your community organizations, parents, school board, and state leaders know what you need from them.
In The Zone
- Seismic Safety piece in the Portland Monthly
General Earthquake Resources:
Seismic Building Rehabilitation
Earthquake Education Resources: Teaching your students about earthquakes
Masters of Disaster
(earthquake curriculum as well as other disasters from the American Red Cross)