"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."

"Inland tsunami" in Toowoomba, Queensland, Australia; floods in Brazil; Sri Lanka

AP photo from here

More than 375 people are reported dead in floods and landslides in Brazil (note added on 1/17/11: over 600 deaths now reported). A million are reported homeless due to floods in Sri Lanka. Major floods have been occurring in Australia, building up for several months and culminating last night with the flooding of Brisbane, Australia's third-largest city. In spite of individual stories of courage and heroism, there have been at least 34 deaths there. The National Post in Canada reports "Horrific picture emerges of Australia flash floods", and that Brisbane resembled a "war zone" as the flood hit 30,000 properties.

Here and here are two excellent and scary videos of the Toowoomba flood:

The water rose so quickly in Toowoomba that the event has been described as an "inland tsumani".     Perhaps this poem--one verse from a nineteenth century Queensland droving ballad, "The Overlander"  about northern Queenslan--summarizes the setting:

"I come from the northern plains

Where girls and grass are scanty;

Where the creeks run dry or ten feet high,

And it's either drought or plenty."

The whole ballad is sung on this video.

A discussion of the role of La Nina and the North Atlantic Oscillation in these weather events is here. Although there is much speculation on the WWW about the role or implications of these events and climate change, there are also lessons to be learned by looking back 150 years at the history of this part of Queensland.  It is tragically like many, many other places in the world where humans have altered their landscapes. I excerpt the following history from this reference.

For more than a century, the Queensland government had a vision, and determination, to introduce farming into this region. Australian history in the 1800's is complex.  Queensland became an independent state in 1850, with a complex history of replacing pastoralism with farming and dairying--dairying being needed to sustain the farmers until crops were sold, or in case they failed.  In the 1887 a Department of Agriculture was established in Queensland, and by World War One, the region around Toowoomba was exporting grain.

In order to meet the demand for fencing, an enormous number of trees were felled, depleting the forest resource.  Reserves and parks were established in the headwaters, but tree-clearing and increasing stock numbers caused erosion on ridges and compaction of vegetation on floodplains. In the 1870's and 180's, residents of Toowoomba changed the forest into orchards, which caused the water table to rise and swamps to developed.  Drainage became a problem, and mixing of raw sewage and groundwater wells resulted in a series of typhoid epidemics in the 1870's and 1880's. This was corrected by civil engineering and introduction of sewage systems.

However, as has been observed around the world, the replacement of vegetation with urban roads and buildings increased the rate of runoff to the main river channels. Flash flooding became a common experience in the main towns. Thus, more than a century of history preceded the tragedy of this week.

More on flash flood dynamics in a later post.