Tsunami Historical Series: Aleutian Islands - 1946

On April 1, 1946 at 4:28 am (12:28 UTC), an 8.6 moment magnitude earthquake struck off the coast of Unimak Island in Alaska’s Aleutian Islands, generating a tsunami that caused the greatest damage and number of deaths in Hawaii’s history, leading to the creation of the United States’ first tsunami warning system. As is typical for dangerous tsunamis the greatest wave heights were nearest the epicenter. The waves reached as high as 42 m or about 138 ft. on Unimak Island and destroyed its lighthouse and killed the five people there. Elsewhere this tsunami caused the greatest damage and number of deaths on inhabited Pacific islands. In Hawaii the waves reached about 17 m or 55 ft. high and killed 158 people, most in the town of Hilo, while in the Marquesas Islands in French Polynesia the waves reached even higher to 20 m or 65 ft but killed only two people. Chile’s Easter Island also got nearly 9 m or 28 ft.while its Juan Fernandez Islands got nearly 3 m or 9 ft. high waves. Pitcairn Island also had 5 m or 16 ft. high waves, New Zealand had over 2 m or 8 ft. high waves, and Samoa had over 1 m or about 4 ft. high waves. In North America the highest waves were in California at over 2 m or over 8 ft. and killed one person there and in South America it killed one more person in Peru.

A tsunami warning system did not exist in 1946 and no one had any warning of the approaching dangerous waves. In response to this event the United States government set up its first tsunami warning operation at the Honolulu Magnetic and Seismic Observatory in 1948 to mitigate tsunami hazards in Hawaii. This facility would later be renamed the Pacific Tsunami Warning Center (PTWC) and expand its mission to include the rest of the Pacific Ocean and the Caribbean Sea.

Today, 70 years since the Unimak Island 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 (half-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.

The tsunami evacuation zones for Hawaii and Guam are available at http://tsunami.coast.noaa.gov.

Earthquake source used: Lopez, Alberto M., and Emile A. Okal. A seismological reassessment of the source of the 1946 Aleutian 'tsunami' earthquake. Geophysical Journal International, vol. 165, p. 835–849. (http://gji.oxfordjournals.org/content/165/3/835)