1958 Lituya Bay earthquake and megatsunami

Mega tsunamis are defined as a wave reaching more than 100m (328 feet) in the deep ocean. The highest wave ever recorded occurred on July 9, 1958 in Lituya Bay, reaching a height of 516m (1,720ft), 470ft taller than the Empire State building.

Lituya Bay is a fjord located on the Fairweather Fault in the northeastern part of the Gulf of Alaska. It is a T-shaped bay with a width of two miles and a length of seven miles.[1] Lituya Bay is an ice-scoured tidal inlet with a maximum depth of 220m (722ft). At the narrow entrance of the bay is has a depth of only 10m (33ft).[2] The two arms that create the top of the T-shape of the bay are the Gilbert and Crillon inlets and are a part of a trench on the Fairweather Fault.[3] In the past 150 years Lituya Bay has generated five giant waves. The last event, before this 1958 mega tsunami occurred was on October 27, 1936. This wave reached a height of 150m (492 feet), and was not created by an earthquake.[4]

• Photo from the back of Lituya Bay
  Photo from the back of Lituya Bay
• 
  Damage cause by the 1,720ft megatsunami

Near the crest of the Fairweather Mountains, sits the Lituya and the North Crillon glaciers that are each about 12 miles long and one mile wide, with an elevation of 4000ft (1,220m). The retreats of these glaciers now form the present T-shape of this bay, the Gilbert and Crillon inlets.[5]

It has been known in scientific literature that waves with significant energy are produced by different mechanisms associated with large earthquakes in regions of subduction, to landslides, to large masses of water added suddenly to a body of water, and to volcanic and nuclear explosions. How this giant 1958 wave, that reached a maximum of 1,720ft, was generated remains a mystery to scientists to this day. However, they have come up with a combination of disturbances that could have triggered this mega tsunami.[6]

The major earthquake that struck on the Fairweather Fault with 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 58.6'N., longitude 137.1`W. 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. September 4, 1899 is the date of the Cape Yakataga earthquake, with a reading of 8.2 on the Richter scale. The shock was felt in southeastern Alaska cities within 400,000 square miles, as far south as Seattle, Washington and as far east as Whitehorse, Y.T., Canada.[7]

A major factor in this massive wave was the sub aerial rock fall that occurred right after the earthquake in the Gilbert Inlet.[8] This landslide put 30 million cubic meters of rock to fall into the bay.[9] However, no landslide has ever been recorded to produce a wave even close to the one in Lituya Bay. The largest slide-generated waves on record occurred in Norway. Even these waves were eight times smaller than the 1,720ft Lituya bay wave. Scientists have concluded that a simple displacement of water by material is not near enough energy to be the cause of this 1,720ft run-up on the other side of the Gilbert Inlet.[10]

Likewise, tectonic movement could not have been the major contributing mechanism of creating this mega tsunami that reached the height of 1,720ft. The Fairweather Fault line lies near the northeast side of the Gilbert and Crillon inlets, and is about 280 kilometers long (174 miles). The tectonic displacements, caused by the earthquake, were primarily in the horizontal plain. These displacements resulted in the upward movement of 3.5 ft and a horizontal movement of 21ft.[11]

After the earth quake there was an observation made on the subglacial lake, located northwest of the bend in the Lituya Glacier at the head of Lituya Bay. This subglacial lake had dropped 100ft. So this proposes another possible cause to the production of the giant 1,720ft wave. It is possible that a good amount of water drained from the glacial lake through a glacial tunnel flowing directly in front of the glacier, though neither the rate of drainage nor the volume of water drained could produce a wave run-off to be 1,720ft.[12] After all, even if a large enough drainage were to take place in front of the Gilbert Glacier, the run-off would have been projected to be on the opposite side in Crillon inlet. After these considerations glacial drainage was not the mechanism that caused the giant wave.[13]

July 9, 1958 at 10:15 p.m. the earthquake with a magnitude of 7.9 struck the Lituya Bay area, which is still daylight for Lituya Bay during this time of year. 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:[14]

“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 can’t ordinarily see that glacier from where I was anchored. People shake their heads when I tell them I saw it that night. I can’t help it if they don’t believe me. I know the glacier is hidden by the point when you’re 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 don’t 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 while—its hard to tell just how long—and 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.’’[15]