Flooding

Flooding is a hazard that is often triggered by earthquakes. Flooding can be caused by related natural hazards, such as tsunamis and landslides. It can also be triggered by damage to manmade structures, such as failed dams or broken water pipes.

Large earthquakes originating offshore can generate tsunamis that flood low-lying coastal areas (Tsunamis). Tsunamis are extremely long waves caused by the large and sudden displacement of the seafloor due to seismic activity. Tsunamis in the Pacific Northwest can be caused by both distant and local earthquakes. Scientists and emergency managers expect that a Cascadia Subduction Zone earthquake would generate tsunami waves averaging 40 to 60 feet high along the coast, with some waves even reaching 100 feet tall. But smaller earthquakes along shallow crustal faults can generate tsunamis in the Pacific Northwest as well. A large Seattle Fault earthquake, for example, could trigger a tsunami up to 16 feet high that would flood the Seattle shoreline within minutes. Likewise, an earthquake along the Tacoma Fault could cause a tsunami and flooding in the Puyallup River delta.

If you feel shaking or receive an earthquake early warning alert when you are near the coast, take action immediately. Drop, Cover, and Hold On until shaking stops. Then move inland to higher ground immediately. Immediate evacuation should also be taken if you are at the coast and see a sudden rise or fall in the ocean or hear a loud roar. Tsunami inundation and evacuation maps are available for both Washington and Oregon.

Flooding can also be caused by landslides and seiches. Seiches are standing waves in inland bodies of water, caused by earthquake shaking, whose oscillations can last hours and cause damaging flooding along the shorelines. Even though seiche waves are usually only a few feet high, they can still flood or wash away houses. Large inland landslides can also temporarily fill river valleys, creating unstable dams that flood large areas upstream of the obstruction. Eventually, however, the dam is likely to fail, creating even more flooding hazards downstream.

Damage to flood control and water distribution systems can also cause flooding after an earthquake. Most water pipelines, dams, and other infrastructure in the Pacific Northwest were built prior to modern seismic design codes. Earthquakes can crack or buckle these structures and potentially cause them to release damaging volumes of water.

Although earthquakes can cause dams to fail, dams in the Pacific Northwest are unlikely to completely collapse during a major earthquake. Only one concrete dam in modern history has failed due to seismic activity: the Shih-Kang Dam in Taiwan. This likely occurred because the rupturing fault was located directly below the dam. A bigger concern may be damage to dam spillway gates. Spillway gates control the amount of water that passes from a reservoir into a river downstream. The Army Corps of Engineers has noted that a Cascadia earthquake could cause spillway gates on the Detroit Dam in Oregon to buckle. This would lead to devastating flooding in the Santiam Canyon and hundreds of thousands of people downstream, including residents of Salem.

Localized flooding can also occur due to broken water pipelines. Ground displacement and liquefaction can cause buried water lines to snap or pull apart at the joints. This creates hundreds of thousands of leaks that can drain entire water systems. The Environmental Protection Agency has developed an earthquake resilience guide for water and wastewater utilities with recommendations for how these risks can be addressed.

ShakeAlert-powered technology is one such tool. The ShakeAlert Earthquake Early Warning System (Earthquake Early Warning) can detect significant earthquakes quickly so alerts can be sent to people in affected areas before they feel shaking. ShakeAlert is managed by the U.S. Geological Survey, and the Pacific Northwest Seismic Network operates the system’s seismometers in Oregon and Washington. Community lifelines and businesses can integrate earthquake early warning technology into their control systems to trigger pre-programmed actions upon earthquake detection. The City of Grants Pass, for example, uses ShakeAlert-powered technology to close output valves for its five-million-gallon water reservoir upon earthquake detection to mitigate flooding and conserve water for human consumption.