This blog provides commentary on interesting geological events occurring around the world in the context of my own work. This work is, broadly, geological fluid dynamics. The events that I highlight here are those that resonate with my professional life and ideas, and my goal is to interpret them in the context of ideas I've developed in my research. The blog does not represent any particular research agenda. It is written on a personal basis and does not seek to represent the University of Illinois, where I am a professor of geology and physics. Enjoy Geology in Motion! I would be glad to be alerted to geologic events of interest to post here! I hope that this blog can provide current event materials that will make geology come alive.

Banner image is by Ludie Cochrane..

Susan Kieffer can be contacted at s1kieffer at gmail.com

Wednesday, April 6, 2011

"Largest tsunami" ever recorded: Lituya Bay, Alaska--was it really a tsunami?

Landsat image of Lituya Bay
as presented in the Geology.com article referenced in the text
Often referred to as "the largest tsunami" ever recorded**, a wall of water 1720 feet high surged over a spur of land at the head of Lituya Bay, Alaska, following a rockslide at the head of Lituya Bay, Alaska.  This rockslide was triggered by the magnitude 7.7 Alaska earthquake on the July 9, 1958. An excellent summary and collection of photographs can be found at Geology.com. This report is based on the U.S.G.S. Professional Paper 354-C, "Giant waves in Lituya Bay, Alaska, 1960" by Don J. Miller. Miller had been working in the area documenting evidence for at least four large waves previously, estimated to have been in 1936, 1899, 1874 and 1853-54. The discoverer of Lituya Bay, the French explorer, LaPerouse, noted the lack of trees and vegetation on the sides of the bay in his ship log, commenting that it looked "as though everything had been cut cleanly like with a razor blade."(Reference from here, which also contains an excellent discussion of the event and of possible mechanisms of origin of the wave).

Lituya Bay and the elevations of the wave
from Geology.com based on Miller's USGS PP (1960)
As illustrated in the second figure here, the Fairweather Fault trends nw-se across the head of the Bay, giving it a T-shape.  The weaker fault material has been scoured by glaciers to produce the Fairweather Trench along the fault zone. The earthquake was centered on this fault zone.  A rock slide at the head of the bay (red zone in the second figure) fell from an elevation of about 3000 feet (914 meters); its volume was about 40 million cubic yards (30.6 million cubic meters). (Assuming no resistance from either air or rock that it was sliding along, this highest part of the rockfall would have hit the ground at nearly 300 miles per hour, or 133 meters/second. The center of mass was at about 2000 feet so the average velocity about 240 miles per hour.)

The impact from this rockfall on the water generated a huge splash wave within Gilbert Inlet. The impact of this rock mass disturbed not only the water, but also the sediments under it, and also tore off part of the toe of Lituya Glacier, causing drainage of a subglacial lake.  The "tsunami height" of 1720 m is taken from the height that "this impact splash" reached on the ridge at the southwest side of Gilbert Inlet, in close proximity to the point of impact.  As the splash traveled out into Lituya Bay, it quickly decayed to less than 200 feet high, and maintained a height on the order of 100 feet throughout much of its passage down the Bay.

Remembering that the word "tsunami" means "harbor wave," this wave in Lituya Bay qualifies as a "harbor wave."  It is may be the best studied example of the near-field dynamics of a big splash. Astrogeologists would also call it "impact ejecta."  Further technical references are given at the end of this article.

**An example this wave being called a tsunami is the BBC Nature program Mega Tsunami-Alaskan Super Wave--Amazing Survival."

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