Biggest Tsunami Ever

World's Biggest Tsunami

 

 

The largest recorded tsunami was a wave 1720 feet tall in Lituya Bay, Alaska

On the night of July 9, 1958 an earthquake along the Fairweather Fault in the Alaska Panhandle loosened about 40 million cubic yards (30.6 million cubic meters) of rock high above the northeastern shore of Lituya Bay. This mass of rock plunged from an altitude of approximately 3000 feet (914 meters) down into the waters of Gilbert Inlet (see map below). The impact generated a local tsunami  that crashed against the southwest shoreline of Gilbert Inlet. The wave hit with such power that it swept completely over the spur of land that separates Gilbert Inlet from the main body of Lituya Bay. The wave then contiuned down the entire length of Lituya Bay, over La Chaussee Spit and into the Gulf of Alaska. The force of the wave removed all trees and vegetation from elevations as high as 1720 feet (524 meters) above sea level. Millions of trees were uprooted and swept away by the wave. This is the highest wave that has ever been known.

Lituya Bay is an ice-scoured tidal inlet on the northeast shore of the Gulf of Alaska. It is about seven miles long (11.3 kilometers) and up to two miles wide (3.2 kilometers). It has a maximumdepth of about 720 feet (219 meters) but a sill of only 32 feet (9.7 meters) in depth separates it from the  Gulf of Alaska between La Chaussee Spit and Harbor Point.

The Fairweather Fault trends across the northeast end of the Bay and is responsible forthe T-shape of the bay.  Glacial scour has exploited the weak zone along the fault to produce a long linear trough known as the Fairweather Trench. The Lituya Glacier and North Crillon Glacier have scoured portions of the Fairweather Trench in the area of Lituya Bay.  Gilbert Inlet and Crillon Inlet occupy the Fairweather Trench on the northeast end of Lituya Bay.

The rock fall of July 9, 1958 occurred on steep cliffs above the northeast shore of Gilbert Inlet. It is marked on the map above in red.  The rocks fell from an elevation of about 3000 feet (914 meters). The impact of 40 million cubic yards  (30.6 million cubic meters) of rock hitting the water produced a local  tsunami that swept the entire length of the Lituya Bay and over the La Chaussee Spit. This wave stripped all vegetation and soil from along the edges of the bay. This damaged area is shown in yellow on the map above. The numbers are elevations (in feet) of the upper edge of the wave damage area and represent the approximate elevation of the wave as it traveled through the bay. Map redrawn from data included in United States Geological Survey Professional Paper 354-C.

Eyewitness Accounts from Survivors (As reported by Don J. Miller in United States Geological Survey Professional Paper 354-C, Giant Waves in Lituya Bay, Alaska, 1960) Account of Howard G. Ulrich Mr. Ulrich and his 7-year-old son, on the Edrie,entered Lituya Bay about 8:00 p.m. and anchored inabout 5 fathoms of water in a small cove on the southshore. Ulrich was awakened by the violentrocking of the boat, noted the time, and went on deckto watch the effects of the earthquake-described asviolent shaking and heaving, followed by avalanching inthe mountains at the head of the bay. An estimated2 1/2 minutes after the earthquake was first felt a deafeningcrash was heard at the head of the bay. Accordingto Ulrich, "The wave definitely started in Gilbert Inlet, just before theend of the quake. It was not a wave at first. It was like anexplosion, or a glacier sluff. The wave came out of the lowerpart, and looked like the smallest part of the whole thing. Thewave did not go up 1,800 feet, the water splashed there." Ulrich continued to watch the progress of the waveuntil it reached his boat about 2 1/2 to 3 minutes after itwas first sighted. Being unable to get the anchor loose,he let out all of the chain (about 40 fathoms) andstarted the engine. Midway between the head of thebay and Cenotaph Island the wave appeared to be astraight wall of water possibly 100 feet high, extendingfrom shore to shore. The wave was breaking as it camearound the north side of the island, but on the southside it had a smooth, even crest. As it approached theEdrie the wave front appeared very steep, and 50 to75 feet high. No lowering or other disturbance of thewater around the boat, other than vibration due to theearthquake, was noticed before the wave arrived. Theanchor chain snapped as the boat rose with the wave.The boat was carried toward and probably over thesouth shore, and then, in the backwash, toward thecenter of the bay. The wave crest seemed to be only 25to 50 feet wide, and the back slope less steep than thefront. After the giant wave passed the water surfacereturned to about normal level, but was very turbulent, with much sloshing back and forth from shore to shoreand with steep, sharp waves up to 20 feet high. Thesewaves, however, did not show any definite movementeither toward the head or the mouth of the bay. After25 to 30 minutes the bay became calm, although floatinglogs covered the water near the shores and weremoving out toward the center and the entrance. Afterthe first giant wave passed Ulrich managed to keep theboat under control, and went out the entrance at 11 :00p.m. on what seemed to be a normal ebb flow. Account of William A. Swanson Mr. and Mrs. Swanson on the Badger entered LituyaBay about 9:00 p.m., first going in as far as CenotaphIsland and then returning to Anchorage Cove on thenorth shore near the entrance, to anchor in about 4fathoms of water. Mr.Swanson was wakened by violent vibration of the boat,and noted the time on the clock in the pilot house. Alittle more than a minute after the shaking was firstfelt, but probably before the end of the earthquake,Swanson looked toward the head of the bay, past thenorth end of Cenotaph Island and saw what he thoughtto be the Lituya Glacier, which had "risen in the airand moved forward so it was in sight. * * * It seemedto be solid, but was jumping and shaking * * * Bigcakes of ice were falling off the face of it and down intothe water." After a little while "the glacier droppedback out of sight and there was a big wall of watergoing over the point" (the spur southwest of GilbertInlet). Swanson next noticed the wave climb up onthe south shore near Mudslide Creek. As the wavepassed Cenotaph Island it seemed to be about 50 feethigh near the center of the bay and to slope up towardthe sides. It passed the island about 2 1/2 minutes afterit was first sighted, and reached the Badger about 11/2minutes later. No lowering or other disturbance ofthe water around the boat was noticed before the wavearrived. The Badger, still at anchor, was lifted up by the waveand carried across La Chaussee Spit, riding stern firstjust below the crest of the wave, like a surfboard.Swanson looked down on the trees growing on the spit,and believes that he was about 2 boat lengths (morethan 80 feet) above their tops. The wave crest brokejust outside the spit and the boat hit bottom and founderedsome distance from the shore. Looking back 3 to 4minutes after the boat hit bottom Swanson saw waterpouring over the spit, carrying logs and other debris.He does not know whether this was a continuationof the wave that carried the boat over the spit or asecond wave. Mr. and Mrs. Swanson abandoned theirboat in a small skiff, and were picked up by anotherfishing boat about 2 hours later. Conclusions A third boat was in Lituya Bay at the time of the Tsunami.  It was anchored near the mouth of the bay and was sunk by the big wave.  There are no known survivors from this boat and it was believed that there were two people on board. Prior to the July, 1958 tsunami Don J. Miller of the United States Geological Survey had been studying evidence for the occurrence of large waves in Lituya Bay.  He had documented evidence for at least four previous large waves with estimated dates of 1936, 1899, 1874 and 1853 (or 1854).  All of these waves were significant in size but shoreline evidence for all of them was removed by the 1958 wave.Mr. Miller was in Alaska when the July 1958 wave occurred and flew to Lituya Bay the following day. He took the photographs shown above in July and August and documented the older waves in United States Geological Survey Professional Paper 354-C, Giant Waves in Lituya Bay, Alaska, 1960. With such a history of large waves Lituya Bay should be considered as a dangerous body of water prone to a few large waves every century.  When will the next one occurSource URL  http://geology.com/records/biggest-tsunami.shtml  ?

Tsunami Geology - What Causes a Tsunami?

Tsunami Geology

Subduction Zones are Potential Tsunami Locations
Most tsunamis are caused by earthquakes generated in a subduction zone, an area where an oceanic plate is being forced down into the mantle by plate tectonic forces. The friction between the subducting plate and the overriding plate is enormous. This friction prevents a slow and steady rate of subduction and instead the two plates become "stuck".                     

Accumulated Seismic Energy

As the stuck plate continues to descend into the mantle the motion causes a slow distortion of the overriding plage. The result is an accumulation of energy very similar to the energy stored in a compressed spring. Energy can accumulate in the overriding plate over a long period of time - decades or even centuries.

Earthquake Causes Tsunami

Energy accumulates in the overriding plate until it exceeds the frictional forces between the two stuck plates. When this happens, the overriding plate snaps back into an unrestrained position. This sudden motion is the cause of the tsunami - because it gives an enormous shove to the overlying water. At the same time, inland areas of the overriding plate are suddenly lowered.

Tsunami Races Away From the Epicenter

The moving wave begins travelling out from where the earthquake has occurred. Some of the water travels out and across the ocean basin, and, at the same time, water rushes landward to flood the recently lowered shoreline.

Tsunamis Travel Rapidly Across Ocean Basis

Tsunamis travel swiftly across the open ocean. The map below shows how a tsunami produced by an earthquake along the coast of Chile in 1960 traveled across the Pacific Ocean, reaching Hawaii in about 15 hours and Japan in less than 24 hours.

Tsunami "Wave Train"

Many people have the mistaken belief that tsunamis are single waves. They are not. Instead tsunamis are "wave trains" consisting of multiple waves. The chart below is a tidal gauge record from Onagawa, Japan beginning at the time of the 1960 Chile earthquake. Time is plotted along the horizontal axis and water level is plotted on the vertical axis. Note the normal rise and fall of the ocean surface, caused by tides, during the early part of this record. Then recorded are a few waves a little larger than normal followed by several much larger waves. In many tsunami events the shoreline is pounded by repeated large waves

source URL  http://geology.com/articles/tsunami-geology.shtml:.