July 10, 1958 Tsunami: SE. Alaska, AK, United States

At least 8 separate tsunamis, generated by submarine and subaerial landslides and ice falls, followed this magnitude 7.9 earthquake. The epicenter was located on the northern coast of Cross Sound near Palma Bay. Underwater cables were broken in Juneau, Skagway, and Wrangell. A local submarine slump off Khantaak Island in Yakutat Bay caused 3 deaths and wrecked a boat. Lituya Bay, the site of repeated huge local tsunamis, experienced a massive landslide which generated a surge of water reaching 525 meters in height -- 400 feet higher than the Empire State Building. 2 boats were wrecked, and 2 deaths resulted. 30 million cubic meters of soil and loose material were washed away, along with ten square km of forest. Shellfish colonies, a cabin, and mining equipment were also destroyed by this massive surge. Total property damage --primarily from the three lost boats -- exceeded $100,000 (Reference #239).

The earthquake caused numerous rock avalanches and slides. In Disenchantment Bay, a "chunk" weighing several tons broke off from a glacier descending to the water; waves 6 m high arose on the surface of the bay. On Khantaak Island, the sea waves resulting from a coastal section submerging under water reached a height of 5-6 m. A collapse in the mouth of the Alsek River caused a wave 1-2 m high, which spread upriver. The water level in the lower reaches dropped approximately 1 m; several hours later, the dam was breached and the shores of the river were flooded. In Glacier Bay, rocks crashed down from the steep beaches, resulting in small waves no more than 1/2-1 m (2-3 feet) high. Similar waves were observed in Dixon Harbor several minutes after the earthquake. The most catastrophic events, unique in the history of strong earthquakes, occurred in Lituya Bay, which is the T-shaped bed of an ancient glacier that has been filled in with water. The distinctive feature of the bay is the strong tidal currents at the throat, reaching 12 knots in velocity. At the time of the earthquake, about 300 million square meters of rock collapsed into Gilbert Bay from the northeast shore. The water extruded from the bay by the collapsing mass, splashed up on the opposite shore, reaching a colossal height. The movement of the water was so fast, that it uprooted a whole forest in the flood zone and stripped the bark and branches from the trees. Besides this gigantic splash, the collapse of this huge mass gave rise to a wave, which crossed the entire Lituya Bay from Gilbert Bay to the sea with a velocity of 150-200 km/hr, that is, with the speed of gravity waves, and devastated the shores of Lituya Bay. The tectonic dislocations in the source of the earthquake, and possibly the wave emerging from Lituya Bay, gave rise to a small tsunami, which was registered on remote tide gauges (Reference #415).

The main mass of the slide, as outlined on figure 16, is a prism of rock that is roughly triangular in cross section, with dimensions of 2,400 feet and 3,000 foot along tho slope, a maximum thickness of about 300 feet normal to the slope, and a center of gravity at about 2,000 foot altitude. From those dimensions and an assumed specific gravity of 2.7, the volume and weight of tho rock mass are, respectively, 40 million cubic yards and 90 million tons. It is highly probable that this entire mass plunged into Gilbert Inlet as a unit at the time of the earthquake, although tho only known fact is that it fell between about noon on July 7 and about 10 .m on July 10. (reference #270)