Scientists tell us the Pacific Northwest is due for a very large earthquake—possibly as large as magnitude 9.2—from the Cascadia Subduction Zone (CSZ) off the Pacific coast, stretching from northern California to southern British Columbia.
320 years ago on Jan. 26, 1700, the Pacific Northwest experienced an estimated 9.0 magnitude earthquake. Today, seismologists predict the Pacific Northwest has a 40 percent chance of experiencing another very large earthquake during the next 50 years. This earthquake will test the region’s buildings, bridges and other infrastructure.
The U.S. Army Corps of Engineers has been evaluating potential impacts to its dams in the Willamette, Rogue and Columbia River regions to prepare for such an event.
Cascadia Earthquake Sources. Source: U.S. Geological Survey
Knowledge of dam performance during large earthquakes is informed by historical events in other parts of the world and by computer simulations of even more significant ground shaking. In general, dams historically have performed exceptionally well during seismic events. Only one concrete dam in modern history has ever failed as the result of a seismic event—Shih-Kang Dam in Taiwan. In this instance, dam failure was largely due to the fault that ran directly beneath the dam. Generally, concrete dams have sustained only minor damage in large earthquakes.
Earthen embankment dams, on the other hand, are dams composed of earth materials, such as rock, clay and sand. The performance of earthen embankment dams has been more mixed, when compared to concrete dams; however dam failure has been rare and remains unlikely.
During the March 11, 2011, 9.0 magnitude earthquake off the coast of Japan, over 400 embankment dams were within the area impacted by the earthquake and aftershocks. This earthquake was essentially the western Pacific version of a Cascadia event. All dams generally performed well, with one exception resulting in minor to moderate cracking. Fujinuma Dam, a small irrigation dam, completely failed and resulted in the loss of 8 lives. Forensic investigations suggest poor construction techniques in the dam’s foundation to be a major contributor to the dam’s failure.
During the 8.8 magnitude earthquake off the coast of Chile, Feb. 27, 2010, no embankment dams failed, and only a few suffered more than minor damage. The recent earthquakes in Chile and Japan both occurred above subduction zones very similar to the CSZ located below the North American Pacific coast.
As with many other structures, it is difficult to predict precisely how Portland District dams will perform during the next CSZ earthquake. Subduction zone earthquakes are the suspected cause of magnitude 8 to 9 earthquakes up and down the Pacific coast and can result in ground shaking for several minutes. The performance of dams during earthquakes depends on many factors, which include the earthquake’s size and location relative to the dam site, the quality of the original design and construction techniques, and the reservoir level at the time of ground shaking.
Hills Creek Dam near Oakridge, Ore. in the Willamette Valley
The Corps built Portland District’s dams to the standards of the time of construction, generally in the mid to late 1900s, prior to the knowledge of the CSZ. In partnership with experts on this subject, the Portland District is currently conducting studies of all its dams to better understand their vulnerability to a Cascadia earthquake. The studies indicate that Portland District dams should perform well in a vast majority of Cascadia simulations. However, there is potential for at least some of these dams to sustain deformation and other damage to the main dams, spillways, or powerhouses. Such damage may impair the Corps’ ability to manage downstream flows until after completing necessary inspections and repairs.
Graphic representation of general dam components
If an earthquake were to occur, the Corps’ emergency action plan is clear: dam operators and engineers would make immediate inspections, searching for evidence of damage or disturbance. If emergency conditions or changes to our operations and water flow were necessary, the Corps would immediately alert county and city emergency managers downstream, who are responsible for issuing alerts and evacuation notices as needed. The Corps regularly conducts emergency drills with county and city emergency managers to plan and rehearse coordinated response to emergency scenarios. Depending on the communication networks following the ground shaking, actions may be taken with little or no warning to people downstream. Following a major earthquake, the public is encouraged to avoid waterways and flood-prone areas.
Although dams pose some risk to residents downstream, this is a very small portion of the overall risk from all of the hazards we are exposed to. Even so, everyone should work to better understand the potential hazards in their community and build a plan to respond in case of an emergency.
The public is encouraged to prepare their households and families for potential hazards including earthquakes. Anyone living downstream of a dam should work with local emergency management agencies to understand the risk, learn how and where to get emergency alerts, and develop a plan to respond. Prepare for an earthquake event with resources found here: https://www.oregon.gov/oem/hazardsprep/Pages/2-Weeks-Ready.aspx.
The State of Oregon has information about natural hazards and preparedness. (https://www.oregon.gov/oem/hazardsprep/Pages/Cascadia-Subduction-Zone.aspx). Local county and city emergency managers can provide more specific information for your local area.