Tsunamis

A tsunami is a wave generated in a body of water by a disturbance that moves the whole water column, from seafloor to ocean surface. Any physical force that displaces a massive amount of water can generate a tsunami, including earthquakes, landslides, volcanic eruptions, and even the impact of cosmic bodies such as meteorites. However, major subduction zone earthquakes (Cascadia Subduction Zone Megathrust) are the leading cause of tsunamis. Tsunamis can impact coastlines, causing devastating property damage and loss of life.

Earth’s largest earthquakes occur along the boundaries of tectonic plates. At some plate boundaries, known as subduction zones, dense oceanic plates force their way underneath less-dense continental plates. Even as plate tectonics slowly, constantly grinds along around the world, at some subduction zones the contact surface between the two plates stick together between earthquakes. As the plates continue to move over time, the stuck area builds up stress and the plates bend under the force. Eventually, the stress exceeds the strength of the fault and it ruptures, causing the edge of the continental plate to rebound and push upward into the ocean. Check out the short video below to visualize this cycle:

The upward motion of the crust raises the ocean over a massive area, initiating the tsunami. The waves begin to travel away in all directions from the earthquake source region, similar to ripples in a pond.

In the open ocean, even the largest tsunami waves are barely perceptible to people and boats because the vertical displacement of the ocean surface takes place over wavelengths of tens of kilometers. Tsunamis travel at about the speed of a commercial airplane in the open ocean, significantly slower than seismic waves. As they move into shallower, coastal waters, they slow down and their amplitudes increase. At the shoreline, tsunamis do not look like massive, crashing waves that tower above buildings. Instead, in the description of geophysicist Steven N. Ward, “the ocean turns into a river” that flows over the land. 

Tsunami waves are incredibly powerful, such that even a one-foot-high wave can easily knock an adult off their feet. Tsunamis carry not only the strength of a flowing ocean but also a massive amount of debris that they pick up along the way. 

If a tsunami is generated by an earthquake far from the coast, waves may arrive after several hours. But when a tsunami is generated locally, there may be as little time as ten minutes to evacuate. In the case of a local tsunami, the shaking from the earthquake is your best warning that a tsunami is on the way. If you are on the coast and you feel strong shaking, do not wait for a tsunami warning; get to high ground as soon as it is safe to move.

The only way to be safe from a tsunami is to be on high ground. The coastal communities of the Pacific Northwest work with scientists and emergency managers to develop and update evacuation routes. Residents and visitors can practice following these evacuation routes to better prepare themselves when a tsunami strikes. Here are the evacuation maps for Washington and Oregon.

When high ground is not immediately accessible based on geographical factors, it may be built by engineers, in the form of a vertical evacuation structure (VES). These structures are built to withstand the impact of a tsunami, with enough space and supplies for local communities to ride out the waves until they recede.

The Cascadia Subduction Zone (CSZ) produces great earthquakes that are capable of generating large tsunamis that periodically threaten the coasts of British Columbia, Washington, Oregon, and Northern California. The average recurrence interval for these great earthquakes is between 300 and 500 years. Most, perhaps all, of these CSZ earthquakes produced tsunami waves. The last great earthquake here occurred on January 26, 1700 and produced a significant tsunami (1700 Cascadia Subduction Zone Earthquake).

The National Tsunami Hazard Mitigation Program (NTHMP), formed in 1995 by Congress, directed the National Oceanic and Atmospheric Administration (NOAA) to form and lead a Federal/State working group to address the general lack of tsunami preparedness and hazard assessment for the U.S. West Coast and the need for significant improvement in tsunami detection and forecasting. The NTHMP is a partnership between NOAA, the United States Geological Survey (USGS), the Federal Emergency Management Agency (FEMA), the National Science Foundation (NSF), and the 28 U.S. Coastal States, Territories, and Commonwealths. The NTHMP produced tsunami hazard maps for the West Coast, leading to the establishment of evacuation routes and signage in coastal areas and greater local hazard awareness. The program also contributed badly needed financial resources to states and local communities to run tsunami education programs in communities at risk.

In Washington and Oregon, several state agencies work with local communities to draw tsunami inundation maps and reduce risk of losses from future tsunamis, including: 

A seiche is a wave phenomenon in a body of water that is similar to, but distinct from, a tsunami.