Tuesday, December 20, 2011

"The 1,700-Foot Tsunami That Struck Alaska in 1958"

"The 1,700-Foot Tsunami That Struck Alaska in 1958"
by Casey Kazan

"On the night of July 7th, 1958, the world’s largest wave in recorded history engorged Alaska's Lituya bay, located about 250 miles west of Juneau in the Gulf of Alaska. It was 1,700 feet, or 520 meters -almost twice the height of the Eiffel Tower. The tsunami was triggered by a magnitude 8.3 earthquake, which caused an enormous landslide along the Fairweather Fault. The resulting crash of rock into water caused the largest wall of water in human history. The deadly wave hurtled at jet speeds and wiped out everything within a four mile radius. Fortunately Lituya Bay was virtually uninhabited, otherwise it would have caused unprecedented destruction, far greater than the tsunami that struck Thailand in 2004.
U.S.G.S. Aerial photo of Lituya Bay taken after July 9, 1958 event.
Note the extent of the non-forested areas of land lining the shore of the bay,
which marks the approximate reach of the tsunami's runup.

At the time of the colossal wave there were three fishing boats anchored at the mouth of Lituya Bay. There were human witnesses to the catastrophic event. The Science Channel and Dr George PC quoted one of the survivors Howard G. Ulrich in a recent article about the wave impact: Ulrich heard the sound of the enormous wave ripping through the land and obscuring the sky, he reportedly said to his 8-year-old child “Son…start praying.” The Ulrich's story:


Unfortunately, one of the boats was close to shore and the huge waves overtook it, killing the two people on board. Amazingly, the other two boats "rode" the tidal waves as they washed from the source of the landslide and resonated around the bay, like water sloshing in a wash basin. The boaters watched in horror as the first enormous wave engulfed the small fishing boat and wiped everything in its path off the land. If there had been a town or city on the shores of the bay everyone in it would have been killed. Fortunately, because it was an unpopulated area, the loss of life was minimal (although, the family of the victims hardly think that it was good fortune).

Can a similar tsunami strike the west coast of the U.S. again? Geological evidence makes it almost a certainty -the region is the heart of the world's most active sesmic zone: the Pacific Rim of Fire. On January the 26th, sometime around 9 p.m. local time, the Juan de Fuca segment of the planet shifted -suddenly. It slipped some 60 feet eastward beneath the North American plate, and caused a monster magnitude 9 quake. It set in motion tsunamis that struck the coast of North America and traveled to the shores of Japan. Seismic research shows that these mega quakes occur every 400 to 500 years.

Kim Olsen of San Diego State University (SDSU) and his team created a supercomputer-powered “virtual earthquake” program that allowed them to recreate such an event. This program encompassed the work of scientists from SDSU, San Diego Supercomputer Center at University of California/San Diego, and the U.S. Geological Survey. To ensure that the entire representation of what could happen is accurate, William Stephenson of the U.S. Geological Survey worked with Olsen and Andreas Geisselmeyer from Ulm University in Germany to create an accurate representation of the earth’s subsurface layering that area. This “velocity model” – the first of its kind – expressed how the structure will bend, reflect, and change in size and direction.

Their scenario depicted a rupture beginning in the north and propagating toward the south along the 600-mile long Cascadia Subduction Zone (an area where two tectonic plates move towards one another, forcing one to slide beneath the other). In their scenario, the ground moved about 1.5 feet per second in Seattle, nearly 6 inches per second in Tacoma, Olympia and Vancouver, and 3 inches in Portland, Oregon. “We also found that these high ground velocities were accompanied by significant low-frequency shaking, like what you feel in a roller coaster, that lasted as long as five minutes – and that’s a long time,” said Olsen. “One thing these studies will hopefully do is to raise awareness of the possibility of mega-thrust earthquakes happening at any given time in the Pacific Northwest,” Olsen added. “Because these events will tend to occur several hundred kilometers from major cities, the study also implies that the region could benefit from an early warning system that can allow time for protective actions before the brunt of the shaking starts.”

This is bad news for the North West for two reasons: one, because the combined long-duration shaking and high ground velocities raise the possibility that such an earthquake could inflict major damage on downtown Seattle; and two, areas like Seattle, Tacoma and Olympia sit on top of sediment filled geological basins, thus, amplifying the waves generated by major earthquakes. “The information from these simulations can also play a role in research into the hazards posed by large tsunamis," added Olsen, "which can originate from such mega-thrust earthquakes like the ones generated in the 2004 Sumatra-Andeman earthquake in Indonesia."

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