Statewide Seismic Needs Assessment Home

Frequently Asked Questions

1. Why is this report being released now?
2. Are earthquakes a problem in Oregon?
3. What do the results of the Seismic Needs Assessment mean?
4. Where can I go to learn more about Oregon’s earthquake risk?
5. What does “risk” mean and what are the different risk categories in the report?
6. What does “maximum considered earthquake” mean?
7. How do you define “collapse” in this report?
8. How can soil affect the score?
9. What is meant by the term “ground motion”?
10. Is my child safe in a high risk school?

1. Why is this report being released now?

In 2005, the Oregon Legislature directed the Oregon Dept Of Geology and Mineral Industries (DOGAMI) to develop a statewide seismic needs assessment (Senate Bill 2, 2005) that includes seismic safety surveys of K-12 public school buildings and community college buildings that have a capacity of 250 or more persons, hospital buildings with acute inpatient care facilities, fire stations, police stations, sheriffs' offices and other law enforcement agency buildings. DOGAMI was asked to finish the report and release the results by July 1st, 2007.

Senate Bill 2 (2005) (html) (pdf), provides the first step in a pre-disaster mitigation strategy that is further defined in Senate Bills 3-5 (2005). Senate Bill 3 (2005) (html) (pdf) directs the Oregon Emergency Management office to create a Seismic Rehabilitation Grant Program. Senate Bill 4 (2005) (html) (pdf) and Senate Bill 5 (2005) (html) (pdf) direct the state treasurer to issue voter approved bonds.

A Seismic Rehabilitation Grant Program will help school, fire and police districts address these issues – with this program the state will share the costs with these districts. This report is only the first step in a multi-part process that will eventually create a pool of state money that can be used to seismically retrofit eligible buildings. The general process of creating the granting committee and issuing funds has been written into law but the specific process of how the money gets allocated has not yet been defined. It is recognized that upgrading so many buildings built before information on earthquake readiness was available will involve significant levels of investment and years to complete.

2. Are earthquakes a problem in Oregon?

Earthquakes from three different sources threaten communities in Oregon and the Pacific Northwest. These sources are the 600 mile long Cascadia Subduction Zone that sits off the northwest coast, intraplate earthquakes and crustal earthquakes.

Great subduction zone earthquakes occur around the world where the tectonic plates that make up the surface of the earth collide. When these plates collide, one plate slides (subducts) beneath the other, where it is re-absorbed into the mantle of the earth. This dipping interface between the two plates is the site of some of the most powerful earthquakes ever recorded, often having magnitudes of 8 to 9 or larger. The 1964 Great Alaskan earthquake (magnitude 9.2) and the 2004 Sumatra/Indian Ocean earthquake (magnitude 9.3) were subduction zone earthquakes.

Deeper intraplate earthquakes occur within the remains of the ocean floor that is being subducted beneath North America. Intraplate earthquakes caused damage in the Puget Sound region in 1949, 1965 and 2001. This type of earthquake can occur beneath much of the Northwest at depths of 25-37 miles (40-60 km).

The most common types of earthquakes in Oregon are crustal earthquakes, which typically occur along faults, or breaks in the earth’s crust, at shallow depths of 6-12 miles (10-20 km) below the surface. The 1993 Scotts Mills earthquake (magnitude 5.6) caused over $30 million in direct physical damage in the Portland-Salem area. The 1993 Klamath Falls earthquakes, whose main shocks (magnitude 5.9 and magnitude 6.0) occurred with 30 minutes of each other, caused over $10 million in damage and killed 2 people.

Since the late 1980s there has been a significant increase in the understanding of earthquake hazards in the Pacific Northwest. The Cascadia Subduction Zone, which lies about 75 miles off the Oregon and Washington coasts, has been identified as capable of generating great earthquakes and destructive tsunamis, even though there have been no subduction zone earthquakes during our short 200-year historical record. In the past 2 decades, a variety of studies have found widespread evidence that great earthquakes have occurred repeatedly in the past, most recently about 300 years ago in January, 1700. The best available evidence indicates that these earthquakes occur, on average, every 500 to 600 years with an interval between individual events that ranges from 100-300 years to about 1000 years. There is every reason to believe that they will continue to occur in the future.

Because Oregon sits in a relatively quiet zone for earthquakes compared to California and Washington, it’s sometimes difficult to imagine large earthquakes interrupting our lives, but we now know this region may experience damaging earthquakes much larger than any that have occurred in the historical past. Because of this threat, planning to respond to earthquake disasters and strengthening homes, buildings, utility lifelines and communications systems can greatly reduce the impact of an earthquake. These measures should be based on the best possible forecast of the amount and distribution of future earthquake damage.

3. What do the results of the Seismic Needs Assessment mean?

By giving each building a score and then using the score to rank the building in one of 4 collapse potential categories – Low, Moderate, High, and Very High –DOGAMI has assigned to each building how the building will behave in a maximum considered earthquake. In other words, there is a Low, Moderate, High and Very High probability that the building will collapse, or fail, if ground motions occur that are equal to or exceed the maximum considered earthquake at that location. It is important to note that these collapse estimates in this report are based upon limited observed and analytical data, and the probability of collapse is therefore approximate. More detailed structural investigation by qualified and experienced engineers is required to fully assess the seismic risks and rehabilitation issues of any one building. The term “probability of collapse”, which is used in the report, is a term used by the Federal Emergency Management Agency (FEMA) and does not necessarily mean the complete collapse of a building. A building may collapse or “fail” in many different ways during an earthquake. There is no way to forecast what this failure will be.

The legislative directives include an instruction for DOGAMI to rank the visual screening results in risk categories based on need, and other categories. The other categories listed were:
• Importance of the building to the community,
• Risk to the building posed by its location,
• Risk posed to the community by the collapse of the building during a seismic event,
• Projected cost of the necessary seismic rehabilitation

We were not able to determine the relative importance of any one building to one community versus the importance of another building to another community, and an assessment of the projected cost of necessary seismic rehabilitation of each building is far beyond the scope and funding of this project. We did assess the risk posed to buildings by their proximity to the tsunami inundation zone. Instead, we have focused on need, defined as “To be in Need or Want,” as in “To need money.”

The logic behind this is that we anticipate competition for State-sourced seismic rehabilitation bond funds. By identifying the relative fiscal need of various districts in advance, especially as compared to their relative seismic risk, DOGAMI is giving the Grant Committee additional information for their deliberations.

4. Where can I go to learn more about Oregon’s earthquake risk?

You can go online and learn about Oregon’s earthquake risk at: http://www.oregongeology.com/sub/earthquakes/EQs.htm. 2 additional sources on line are the Federal Emergency Management Agency (FEMA) at: http://www.fema.gov/hazard/earthquake/index.shtm and the U.S. Geological Survey (USGS) at: http://earthquake.usgs.gov/learning/.

Your library also has many informative books on Oregon’s seismic history and risk including Living With Earthquakes in the Pacific Northwest by Robert S. Yeats. The Red Cross, Salvation Army, and your local Emergency Manager also have earthquake information.

5. What does “risk” mean and what are the different risk categories in the report?

Risk is the likelihood (chance) of harm if someone or something is exposed to a hazard, and generally quantified by three terms: likelihood (chance), a level of hazard (loss), and exposure (time). For example, in health sciences, risk can be defined as the likelihood (probability) of getting cancer if an average daily dose of a hazardous substance (hazard level) is taken over a 70-year lifetime (exposure). In the financial world, risk can be the probability of losing a certain amount of money (loss) over a period. Seismic hazard and risk are also fundamentally different and are commonly defined as:

Seismic Hazard: Earthquakes of a certain magnitude or the phenomena generated by the earthquakes, such as surface rupture, ground motion, ground motion amplification, liquefaction, and induced-landslides, that have potential to cause harm. Seismic Risk: Likelihood (chance) of experiencing a level of seismic hazard for a given exposure (time and asset).

For the purposes of this report, DOGAMI ranked all buildings into four risk categories: Very High, High, Moderate and Low Seismic Risk. In the report’s Summary of Seismic Risk for all Qualifying Sites & Buildings a score of greater than 2.0 implies there is less than a 1 in 100 chance that the building will collapse or a low seismic risk. A score between 2.0 and 1.0 indicates a moderate seismic risk. A score between 1.0 and 0.0 indicates a high seismic risk. A score of 0.0 implies there is a 100% chance of collapse or very high seismic risk. FEMA recommends that all buildings with a score of 2.0 or less should be considered to have inadequate performance during the anticipated maximum considered earthquake.

It is important to recognize that a building with a very high collapse potential does not directly indicate that catastrophic building collapse will definitively occur. Different building types of varying vintage, shape and design will fail in different ways. More detailed structural investigation by qualified and experienced engineers is required to fully assess the seismic risks and rehabilitation issues of any one building.

6. What does “maximum considered earthquake” mean?

The maximum considered earthquake is defined as the maximum event considered likely in a reasonable amount of time. The phrase "reasonable amount of time" is defined by historical or geological records. For instance, the reasonable amount of time for the maximum considered earthquake in the Cascadia Subduction Zone is about 500 years, based on paleoseismic records. The maximum considered earthquake for any location is determined by the United States Geological Survey’s work, most recently updated in 2002. To learn more, go online to: http://earthquake.usgs.gov/research/hazmaps/.

7. How do you define “collapse” in this report?

The term “probability of collapse”, which is used in the report, is a term used by the Federal Emergency Management Agency (FEMA). It defines in a quantitative way, using the results of the FEMA 154 handbook, the likelihood or probability of a building sustaining major life threatening damage, given the occurrence of an earthquake. Different building construction types react in different ways to earthquake shaking and this does not necessarily mean the complete collapse of a building. A building may collapse or “fail” in many different ways during an earthquake. There is no way to forecast what this failure will be. Also collapse does not necessarily mean that injuries will occur.

To determine the true seismic safety or collapse potential of any given building, a much more detailed, site specific geotechnical and/or engineering survey is needed. For example, schools and school districts may also have completed seismic upgrades that were not noted in this assessment.

8. How can soil affect the score?

Some types of soil can actually amplify the shaking of an earthquake. Mud, sands, gravels, silts and artificial fill are considered “soft soils” and can make earthquake shaking stronger.
Records from many earthquakes show that ground conditions immediately beneath a structure affect how hard the structure shakes. For example, sites underlain by soft soils tend to shake more violently than those underlain by rock.

9. What is meant by the term “ground motion”?

Ground motion is the movement of the earth's surface from earthquakes or explosions. Ground motion is produced by waves that are generated by sudden slip on a fault or sudden pressure at the explosive source and travel through the earth and along its surface.

Most damage during an earthquake is caused by ground motion. A commonly measured ground motion is peak ground acceleration, which is expressed as a percentage of the acceleration of gravity. The larger an earthquake's magnitude, the stronger the ground motion it generates. The level of ground motion at a site depends on its distance from the epicenter -- the closer a site is to the epicenter, the stronger the ground motion, and vice versa. Strong ground motion could also induce secondary hazards such as ground-motion amplification, liquefaction, and landslide under certain site conditions.

10. Is my child safe in a high risk school?

This report is a general assessment of the seismic hazard potential of the public buildings DOGAMI was asked to survey. The assessment by DOGAMI was a “rapid visual screening (RVS),” not a comprehensive structural assessment. The evaluation may not have captured all local steps taken in recent years to upgrade older facilities based upon seismic needs. Many districts have completed either a local evaluation or local building upgrades and you should check with you local schools and school district for this information. Since many public buildings in Oregon were built before the heightened awareness of earthquake and tsunami potential in Oregon, public officials are aware that significant building upgrades are necessary to many structures.

To determine the true seismic safety of any given building, a much more detailed, site specific geotechnical and/or engineering survey is needed. Schools and school districts may also have completed seismic upgrades that were not noted in this assessment.
While many schools were built before the risks of earthquakes were know in Oregon, all schools are required to instruct and drill students on emergency procedures so that the students may respond to an emergency without confusion or panic. The emergency procedures include drills and instruction on earthquakes. In addition, schools at the coastal include tsunami drills and instruction as part of their earthquake awareness programs. Awareness of the threat of earthquake has been heightened for many years, and districts have complied with state laws that mandate regular earthquake drills for students. Schools and school districts have been taking action to prepare students for earthquake emergencies.
Knowing and understanding the risk, practicing drop, cover and hold drills and preparing at home by having a family emergency plan and emergency kit is a good first step. Talk with school officials about their mitigation and emergency plans and get involved in community efforts to raise seismic awareness.

Statewide Seismic Needs Assessment Home

For more information, contact Don Lewis, Assistant Director, Geologic Survey and Services Program, (971) 673-1541, or James Roddey, DOGAMI Earth Sciences Information Officer, (971) 673-1543.