1958 Lituya Bay Megatsunami

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A megatsunami is defined as a wave reaching more than 100 meters (328 feet) in the deep ocean. The highest wave ever recorded occurred on July 9, 1958 in Lituya Bay, Alaska reaching a height of 524 meters (1,720 feet), 250 feet taller than the Empire State Building. Near the crest of the Fairweather Mountains sit the Lituya and the North Crillon glaciers. They are each about 12 miles long and one mile wide with an elevation of 4000 ft (1,220m). The retreats of these glaciers form the present T shape of the bay, the Gilbert and Crillon inlets. The major earthquake that struck on the Fairweather Fault had a Richter scale reading of 7.9, and some sources have reported it to be as much as 8.3. The epicenter of the quake was at latitude 58.6N., longitude 137.1W. near the Fairweather Range, 7.5 miles east of the surface trace of the Fairweather fault, and 13 miles southeast of Lituya Bay. This earthquake had been the strongest in over 50 years for this region. The earthquake caused a subaerial rock fall in the Gilbert Inlet. This landslide caused 30 million cubic meters of rock to fall into the bay, creating the megatsunami. At 10:15 p.m. PST on July 9, 1958, which is still daylight at that time of year, an earthquake with a magnitude of 7.9 struck the Lituya Bay area. The tide was ebbing at about plus 1.5m and the weather was clear. Anchored in Anchorage Cove, near the west side of the entrance of the bay, Bill and Vivian Swanson were on their boat fishing when the unthinkable happened: "With the first jolt, I tumbled out of the bunk and looked toward the head of the bay where all the noise was coming from. The mountains were shaking something awful, with slide of rock and snow, but what I noticed mostly was the glacier, the north glacier, the one they call Lituya Glacier. I know you canít ordinarily see that glacier from where I was anchored. People shake their heads when I tell them I saw it that night. I canít help it if they donít believe me. I know the glacier is hidden by the point when youíre in Anchorage Cove, but I know what I saw that night, too. The glacier had risen in the air and moved forward so it was in sight. It must have risen several hundred feet. I donít mean it was just hanging in the air. It seems to be solid, but it was jumping and shaking like crazy. Big chunks of ice were falling off the face of it and down into the water. That was six miles away and they still looked like big chunks. They came off the glacier like a big load of rocks spilling out of a dump truck. That went on for a little whileóits hard to tell just how longóand then suddenly the glacier dropped back out of sight and there was a big wall of water going over the point. The wave started for us right after that and I was too busy to tell what else was happening up there." Based on this description, it is possible that the quake had caused the entire glacier (or a large portion of it) to slide over the cliff. What the fisherman may have seen, therefore, could have been that section breaking off and falling into the bay. This might account for the vast displacement of water, while leaving little or no evidence once the ice melted. The height of the wave, however, was accurately measured at 1,720 feet, based on the elevation extent of the damage caused to the foliage up the headlands around the bay. [READ THE REST OF THIS ARTICLE]





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