The 1958 Lituya Bay Alaska Tsunami
One of the largest-known landslide-induced tsunamis struck Lituya Bay of southeastern Alaska on July 9, 1958. Lituya Bay is located about 150 miles (240 km) southeast of Juneau and is a steep-sided, seven-mile-(11-km-) long, glacially carved fjord with T-shaped arms at the head of the bay where the Lituya and Cril-lon Glaciers flow down to the sea. The glaciers are rapidly retreating, a rock spit known as La Chausse spit blocks the entrance to the bay, and a large island, Cenotaph Island, rests in the center of the fjord. Forest-covered mountains rise 6,000 feet (1,800 m) out of the water. As the fjord is a glacially carved valley, it has a rounded floor under the sea, with a depth of only 720 feet (220 m). The rocks surrounding the bay are part of the Pacific plate, and the Fairweather fault, the boundary between the Pacific and North American Plates, lies just inboard of the bay.
At 10:15 p.m. on July 9, 1958, a magnitude 7.9-8.3 earthquake struck the region along the Fair-weather fault 13 miles (20.8 km) Southeast of the bay. The ground surface was displaced by up to 3.5 feet (1.1 m) vertically and 20 feet (6.3 m) horizontally in Crillon and Lituya Bays. In some locations, ground accelerations in a horizontal direction exceeded twice the force of gravity and approached the force of gravity in the vertical direction. The earthquake sent a huge mass of rock plunging into the water below, released from about 3,000 feet (900 m) up the cliffs near the head of the bay next to Lituya Glacier. This material landed in the water at the head of Lituya Bay and created a huge semicircular crater 800 feet (250 m) across, circling the rockfall. The force of the impact of the rockfall was so great that it tore off the outer 1,300 feet (400 m) of Lituya Glacier and threw it high into the air (an observer at sea reported seeing the glacier rise above the surrounding ridges). The wave generated by this massive collapse was enormous. The first wave (really a splash) soared up to 1,720 feet (524 m) on the opposite side of the bay, removing trees and soil with the force of the wave and the backwash. This splash reached heights that were three times deeper than the water in the bay. It washed over Cenotaph Island in the middle of the bay, destroying a government research station and killing two geologists stationed there who happened to be investigating the possibilities of tsunami hazards in the bay. A 100-170-foot- (30-51-m-) high tsunami was generated that moved at 96-130 miles per hour (155-210 km/hr) out toward the mouth of the bay, erasing shoreline features along its path and shooting a fishing troller out of the bay into open water.
The size of the tsunami produced by this landslide was exceptional, being eight times higher than the next highest known landslide- or rockfall-induced tsunami from a Norwegian fjord. Most landslides that fall into the water do not produce large tsunamis because only about 4 percent of the energy from the rockfall is transferred to the water to form waves in these events. Therefore, some geologists have suggested that the Lituya Bay tsunami may have had help from an additional source, such as a huge surge of water from an ice-dammed lake on Lituya glacier that may have been suddenly released during the earthquake, and the rockfall may have landed on this huge surge of water. However, even if this speculative release of water occurred, it still would not be enough to create such a large wave. A better understanding of this wave generation phenomenon is needed and awaits further study.
Lituya Bay and others like it have experienced numerous tsunamis as shown by distinctive scour marks and debris deposits found in the bay. Studies have shown that tsunamis in the bay in 1853 and 1936 produced run-up heights of 400 and 500 feet (120 and 150 m) above sea level. This phenomenon was also well known to the native Tlingit, who had legends of spirits who lived in the bay who would send huge waves out to punish those who angered them.
Continue reading here: Giant landslideinduced Tsunamis in Hawaii
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