Tsunami Historical Series: Aleutian Islands - 1957

At 4:22 am on Saturday, March 9, 1957 (9 March, 14:22 Z UTC) the second great earthquake in 11 years struck Alaska’s Aleutian Islands. This earthquake had the same magnitude of the earlier earthquake--8.6 on the moment magnitude scale (Johnson et al. 1994)--but was to the west of the 1946 earthquake, near the Andreanof Islands. As with the earlier event it also caused a dangerous tsunami that caused significant damage in the Aleutian Islands and in Hawaii and was observed as far away as Chile. The greatest wave heights were in Alaska’s Aleutian Islands, with waves nearly 23 m or 75 ft. high coming ashore on Unimak Island. The tsunami would reach Hawaii a little over four hours later, with the largest waves striking the island of Kauai at over 11 m or 38 ft. high and would cause $5.3 million in damage statewide ($46 million in 2017), including the destruction of more than 80 homes. Elsewhere around the Pacific Ocean the tsunami waves would reach heights of 6 m or 20 ft. in the Marquesas Islands (French Polynesia), 3 m or 10 ft. in Japan, 1.5 m or 5 ft. in American Samoa, and over 1 m or 3 ft. in Mexico and Chile. Unlike the earlier event, however, it did not kill any people thanks to effective tsunami alerts from to the Honolulu Observatory and the Seismic Sea Wave Warning System. These efforts, established in 1948, would later become the Pacific Tsunami Warning Center (PTWC) and Pacific Tsunami Warning System.

Today, more than 60 years since this earthquake, PTWC will issue tsunami warnings in minutes after a major earthquake occurs, and will also forecast how large any resulting tsunami will be as it is still crossing the ocean. PTWC can also create an animation of a historical tsunami with the same tool that it uses to determine tsunami hazards in real time for any tsunami today: the Real-Time Forecasting of Tsunamis (RIFT) forecast model. The RIFT model takes earthquake information as input and calculates how the waves move through the world’s oceans, predicting their speed, wavelength, and amplitude. This animation shows these values through the simulated motion of the waves and as they travel through the world’s oceans one can also see the distance between successive wave crests (wavelength) as well as their height (amplitude) indicated by their color. More importantly, the model also shows what happens when these tsunami waves strike land, the very information that PTWC needs to issue tsunami hazard guidance for impacted coastlines. From the beginning the animation shows all coastlines covered by colored points. These are initially a blue color like the undisturbed ocean to indicate normal sea level, but as the tsunami waves reach them they will change color to represent the height of the waves coming ashore, and often these values are higher than they were in the deeper waters offshore. The color scheme is based on PTWC's warning criteria, with blue-to-green representing no hazard (less than 30 cm or ~1 ft.), yellow-to-orange indicating low hazard with a stay-off-the-beach recommendation (30 to 100 cm or ~1 to 3 ft.), light red-to-bright red indicating significant hazard requiring evacuation (1 to 3 m or ~3 to 10 ft.), and dark red indicating a severe hazard possibly requiring a second-tier evacuation (greater than 3 m or ~10 ft.).

Toward the end of this simulated 48 hours of activity the wave animation will transition to the "energy map" of a mathematical surface representing the maximum rise in sea-level on the open ocean caused by the tsunami, a pattern that indicates that the kinetic energy of the tsunami was not distributed evenly across the oceans but instead forms a highly directional "beam" such that the tsunami was far more severe in the middle of the "beam" of energy than on its sides. This pattern also generally correlates to the coastal impacts; note how those coastlines directly in the “beam” are hit by larger waves than those to either side of it.

You can also view a YouTube version of this animation.