Friday, July 9, 2010

52 years ago today: the biggest tsunami in recorded history


At about 10:16 p.m. on July 9th, 1958, the largest tsunami ever recorded clobbered Lituya Bay in Alaska. Classified as a 'megatsunami' because of its monstrous size, the wave was measured at a colossal 1,720 feet (524 meters) above sea level. That's 250 feet taller than the Empire State Building! In case you're wondering how it was measured; no, it wasn't someone riding it on a surfboard - although someone did ride it in a boat, and, incredibly, lived to tell the tale! It was calculated by measuring the height above sea level below which trees and other vegetation were stripped from the shoreline around Lituya Bay.

The cause of the megatsunami was an earthquake measuring 8.3 on the Richter scale, with its epicenter 45 miles south of Lituya Bay. It ran along the Fairweather Fault (known as the Queen Charlotte Fault in Canada), which passes directly beneath the inmost part of Lituya Bay. The water there is over 800 feet deep, as the mountains plunge directly to the sea at a very steep angle, and continue down underwater at the same angle for a great distance. The earthquake caused a landslide on an unnamed peak at the inner edge of the bay, right next to a small inlet known as the Gilbert Inlet. Something like 30 million cubic yards of ice, rock and soil, weighing approximately 90 million tons, were dislodged from the face of the mountain (shown below after the rockslide) and plunged straight down into the deep water of the Inlet.




The wave instantly rose to its greatest height as the rockslide displaced hundreds of millions of tons of water. It reached 1,720 feet up the far side of the Gilbert Inlet, as can be seen in the photograph below. Below that height every tree and plant was stripped from the earth as if it had been sandpapered away.




As the monster wave rolled out of the Gilbert Inlet, it entered the long, narrow expanse of Lituya Bay proper. It had seven miles to run until it reached the sea. Near the head of the bay, it rose to 600 feet above normal water level, as the photograph below shows, clearing all vegetation from its path. (The picture was taken a couple of years after the wave had passed, which is why you can see new growth over the previously cleared area.)




Further down the bay, as the wave spread out, it dropped to a lower height, although still well into megatsunami territory. The diagram below shows the heights it reached down the sides of the bay as it approached the sea. Note that even after spreading out and traveling seven miles, it was still over 100 feet high as it came to the mouth of the bay.




Unfortunately for those on board, three fishing vessels had taken shelter inside the mouth of the bay. The survivors later described what they experienced. In a 2008 article, the Sitka News reported:

There were two commonly used anchorages near the mouth of Lituya Bay on that July night in 1958. Ulrich and his eight-year-old son "Sonny" chose to anchor about half a mile in from the mouth on the south side of the bay in [a] place called, appropriately enough "Anchorage Cove."

Two other trolling boats, the Sunmore (with Orville and Mickey Wagner on board) and the Badger (operated by Bill and Vivian Swanson) anchored up on the opposite side of the bay, just behind the nearly mile long spit that extended most of the way across the mouth of the bay. The opening itself was less than 300 yards across and most of that water was too shallow for boats to cross. Some fishermen estimate there may less than 50 feet of open water at times.

. . .

With the first jolt [of the earthquake], Bill Swanson tumbled out of his bunk on the Badger. He later told the Alaska Sportsman that the he could see even the highest mountains shaking. Then he saw the Lituya Glacier appear to literally rise up into the air.

"I know you can't ordinarily see that glacier from where I was anchored," he was quoted in the October, 1958 issue. "People shake their head when I tell them I saw it that night. I can't help it if they don't believe me but I know what I saw that night."

From his vantage point, he could also see the mountainside slide away and crash towards the water. From his viewpoint on the other side of the bay, Ulrich could see the wave rise up and devastate the forested hillside.

All three boat crews had been woken up by the earthquake. But while the captains of the Badger and Edrie were transfixed by the spectacle at the head of the bay, the Wagners on the Sunmore jumped to immediate action and got their boat headed out of the bay.

It proved to be a fatal decision. Swanson reported seeing the Sunmore just about to turn out into the entrance when it was caught by the wave and flung over Harbor Point across the opening from the end of the spit. All that was found later was an oil slick marking the spot where the boat went down in deep water.

Ulrich told Alaska Sportsman that he was "petrified" by the landslide and the wave and didn't begin to act until he saw the wave, then still some 300 feet high engulf Cenotaph Island some two miles away from his boat.

"I began to move then," he said. "And I moved fast, cursing myself for not moving sooner."

He got a life jacket on his son and started the engine. He started to pull the anchor and found it unmovable.

"The wave (then estimated at more than 75 feet high) was almost upon us and we were fastened to the bottom with a heavy chain," Ulrich said. He surmised that the quake had caused the anchor to get tangled on the bottom. He let out all his extra chain, in hopes that it would be enough to allow the boat to ride up and over the wave. Then he turned the boat into the wave and held on.

"As the Edrie began her almost perpendicular ascent to the crest of the wave, the chain snapped," Ulrich said, adding that he wanted to get a message out so his wife in Pelican would eventually find out her husband and son had died. "There seemed to be no hope for survival. I grabbed the handset of my radiophone and yelled into it: 'Mayday! Mayday! This is the Edrie in in Lituya Bay. All hell has broken loose in here. I think we've had it. Goodbye.' "

But the luck that had abandoned the Wagners was still on Ulrich's side. At the top of wave, he regained enough control of his boat to hold on and steer around the debris being carried by the wave. A few seconds later, the worst had passed.

He sent out another message saying he thought they'd made it through. Immediately, other boats outside Lituya began radioing back. But, Ulrich noted, the silence from the Badger and the Sunmore was ominous.

Just about the time that Bill Swanson saw the wave engulf the Sunmore as it tried to escape, the wave - estimated to be approximately 80 feet high - hit his boat and carried it over the La Chaussee Spit and dumped it stern first into the open ocean.

"We went way over the trees and I looked down on rocks as big as an ordinary house as we crossed the spit," Swanson told the Alaska Sportsman. "We were way up above them. It felt like we were in a tin can and somebody was shaking it."

After the crash landing, the boat immediately began to sink. They were surrounded by acres of wood debris - including a large tree that smashed through the pilot house and broke several of Swanson's ribs - but managed to get into [an] eight foot skiff with only their underclothes on, according to Fradkin's book. At nearly midnight, they were found by the crew of the vessel Lumen which was picking its way through the miles of debris looking for signs of survivors in the pitch dark.


There's more at the link.

The spits of land enclosing the mouth of Lituya Bay, over which the Badger was carried, can be seen in the photograph below, which also shows the land on either side of the bay stripped clean of vegetation by the tsunami. (The land spits were much wider and higher prior to the wave, of course - they were eroded almost out of existence by the rush of water.)




Here's a video clip from the BBC in which Ulrich and his son describe their experiences.







For those who'd like to learn more, a good analysis of the geological and hydrological mechanisms that gave rise to the Lituya Bay megatsunami may be found here. The color photograph and diagram above came from that source. Interestingly, the author postulates that the Lituya Bay event may be useful in predicting the effects of a tsunami caused by an asteroid striking the Earth. He notes:

... based on the measured parameters of inundation, speed, and water particle velocities of the giant 1958 Lituya Bay waves, coefficients of friction can be derived empirically. These coefficients can be used to estimate more realistically wave attenuation over a land mass, of an asteroid-generated tsunami as it travels chaotically past the sea-land boundary.

... with proper scale corrections, analogies could be drawn between the impulsive impact of the Lituya Bay rockfall to asteroid impact on ocean floor sediments and on such impulsive wave generation. Although, the trajectory angle, terminal velocity and total mass and density of material of an asteroid would be significantly different than those of the Lituya Bay rockfall, ... these could be scaled and adjusted for the purpose of validating a model of asteroid impact. For example, an asteroid would be expected to approach the earth at a much lower angle of perhaps only 15 degrees from horizontal and would impact the ocean with a terminal speed which could be 20 km/second or more. Although there would be of differences in mass, trajectory angle, and speed at impact, the effects on the ocean floor could be markedly different, but these too could be scaled.




For example, even a small asteroid of perhaps the same dimensions and mass would be expected to disturb the ocean sediments to a far greater extent than the gravity-driven rockfall of Lituya Bay. A small asteroid of only 1/3 mile in diameter falling in the ocean at 20 km/second at a low angle of entry, would be expected to carve a path of at least twelve miles on the ocean floor and to create a much larger cavity which would be cylindrical rather than radial, as in Lituya Bay. Horizontal and vertical accelerations of seismic waves from asteroid impact would be expected to be much greater. However, because of the lower trajectory angle of entry, wave generation and splashing action to a nearby shoreline may or may not be as great as that caused by the Lituya Bay rockfall. Also, it was possible that an asteroid's impact on a hard basalt ocean bottom with a thin layer of sediment would not not cause the same effect as the Lituya rockfall on softer and thicker sediment layers. Yet, in spite of potential differences, such analogies could be drawn.


A series of photographs taken in Lituya Bay by the US Geological Survey, after the megatsunami, may be viewed here. Click on each thumbnail image to expand it, then click the bigger image for a still larger view. The three monochrome photographs in this article came from that source.

No higher wave has been recorded in human history. Let's hope none of us ever have to face anything even remotely as large!





Peter

3 comments:

reflectoscope said...

The mind boggles indeed!

Jim

Kansas Scout said...

Yet another awesome posting by Sir Peter. Thanks!

raven said...

We used to anchor in Lituya Bay, during storms- When fishing the Fairweather grounds in the Gulf of Alsaka, it is the only secure anchorage in a storm for a long way to the north and south.

This was thirty years ago , but one could clearly see the high water mark made by the wave. -and at the edge, past the new growth (at that time 25 years old) an enormous wall of debris made of trees one had to climb over to gain access to the old growth interior forest.
Lituya has othe surprises, as the French explorer LaPeruse found in the mid 1700's- the huge deep bay empties through a very narrow shallow entrance channel. At the turn of the tide,from flood to ebb, a huge standing set of waves develops in minutes, in a series stacked one after the other out to sea for half a mile or so, with a very short distance between crests. This means you can only enter the bay on a flood or slack tide, otherwise you just have to jog in place for six hours till the tide turns. We left one day a minute or so late and stuffed the bow of our 47 foot troller completely under green water, coming down the wave face.

The ebb speed is terrific, and LaPeruse, after anchoring in the bay and issuing detailed orders to his sounding party to avoid the mouth, lost 21 men in a few minutes. They approached the mouth of the bay during an ebb tide, and with all hands furiously rowing were unable to gain safety and were sucked out through the breakers. The island in the center of Lituya was named Cenotaph Is. by him in comemoration of the loss of his sailors. How quickly the sea can claim us, he had come all the way from France without losing a man.

When I was there years ago it was a wild and lovely place, filled with shrimp, crab, and clams-we would feast while waiting out the storms, often take the dingy to shore and with rifle in hand explore the area. (bears were abundant)