A tsunami is quite horrible and tragic – the 2011 Japanese and 2004 Indonesian tsunamis being great examples.
However, they’re not as scary as megatsunamis. Whereas a regular tsunami starts out at sea after an earthquake, a megatsunami is created nearer the shore – or even in a lake – by the force or an impact, usually a giant landslide, although in the ancient past they were also caused by asteroids.
Whereas a tsunami can take hours to reach land and become a wave, a megatsunami wave is created instantaneously, frequently much larger than a regular tsunami wave, with a much greater surge – meaning hills & mountains offer little protection. And you don’t even need to be anywhere near the ocean to be affected – two of the 3 megatsunamis last century took place in mountain lakes/reservoirs, a long way from the ocean.
THE LARGEST WAVE IN MODERN HISTORY (LITUYA BAY, ALASKA, USA – 1958)
Prior to the 1958 earthquake that put megatsunamis in the public consciousness, Lituya Bay – first explored (by Europeans) in 1786 – had a known history of megatsunamis. None, however, topped the 1958 event, which produced a wave 524 metres (1720 feet) high. From Wikipedia:
- Reports by early explorers of the loss of all trees and vegetation along the shore, and cut tree-lines. One example is the log of Jean Francois de Galaup who discovered the bay in 1786.
- “At least one and possibly two waves” between 1854 and 1916, based on photographic evidence.
- A further event that erased the above evidence and uprooted trees over 150 meters (492 feet) up the sides of the bay, in 1936.
- The 1958 event.
The quake and tsunami itself:
Lituya Bay is a fjord located on the Fairweather Fault in the northeastern part of the Gulf of Alaska. It is a T-shaped bay with a width of 2 miles (3 km) and a length of 7 miles (11 km). Lituya Bay is an ice-scoured tidal inlet with a maximum depth of 722 feet (220 m). The narrow entrance of the bay has a depth of only 33 feet (10 m). The two arms that create the top of the T-shape of the bay are the Gilbert and Crillon inlets and are a part of a trench on the Fairweather Fault. In the past 150 years Lituya Bay has had three other tsunamis of over 100 ft: 1854 (395 ft or 120 m), 1899 (200 ft or 61 m), and 1936 (490 ft or 150 m).
Near the crest of the Fairweather Mountains sit the Lituya and the North Crillon glaciers. They are each about 12 miles (19 km) long and 1 mile (1.6 km) wide with an elevation of 4,000 feet (1,200 m). The retreats of these glaciers form the present “T” shape of the bay, the Gilbert and Crillon inlets.
The major earthquake that struck on the Fairweather Fault had a moment magnitude of 7.8 and a maximum perceived intensity of XI (Extreme) on the Mercalli intensity scale. The epicenter of the quake was at latitude 58.37° N, longitude 136.67° W near the Fairweather Range, 7.5 miles (12.1 km) east of the surface trace of the Fairweather fault, and 13 miles (21 km) southeast of Lituya Bay. This earthquake had been the strongest in over 50 years for this region: the Cape Yakataga earthquake, with an estimated magnitude of 8.2 on the Richter scale, occurred on September 4, 1899. The shock was felt in southeastern Alaskan cities over an area of 400,000 square miles (1,000,000 km2), as far south as Seattle, Washington, and as far east as Whitehorse, Yukon, Canada.
The earthquake caused a subaerial rockfall in the Gilbert Inlet. Over 30 million cubic meters of rock fell from a height of several hundred meters into the bay, creating the megatsunami. Two people from a fishing boat died as a result of having been caught by a wave in the bay. In Yakutat, the only permanent outpost close to the epicenter at the time, infrastructure such as bridges, docks, and oil lines all sustained damage. A water tower collapsed, and a cabin was damaged beyond repair. Sand boils and fissures occurred near the coast southeast of there, and underwater cables that supported the Alaska Communication System were cut. Lighter damage was also reported in Pelican and Sitka.
After the earthquake it was observed that a subglacial lake, located northwest of the bend in the Lituya Glacier at the head of Lituya Bay, had dropped 100 ft (30 m). This proposed another possible cause to the production of the 100 ft (30 m) wave which caused destruction as high as 1,720 ft (524 m) above the surface of the bay as its momentum carried it upslope. It is possible that a good amount of water drained from the glacial lake through a glacial tunnel flowing directly in front of the glacier, though neither the rate of drainage nor the volume of water drained could produce a wave of such magnitude. Even if a large enough drainage were to take place in front of the Gilbert Glacier, the run-off would have been projected to be on the opposite side in Crillon Inlet. After these considerations it was determined that glacial drainage was not the mechanism that caused the giant wave.
At 22:15 hours PST on July 9, 1958, which was still daylight at that time of year, an earthquake with a magnitude of 7.9 struck the Lituya Bay area. The tide was ebbing at about plus 1.5 m and the weather was clear. Anchored in a cove near the west side of the entrance of the bay, Bill and Vivian Swanson were on their boat fishing when the earthquake hit:
With the first jolt, I tumbled out of the bunk and looked toward the head of the bay where all the noise was coming from. The mountains were shaking something awful, with slide of rock and snow, but what I noticed mostly was the glacier, the north glacier, the one they call Lituya Glacier. I know you can’t ordinarily see that glacier from where I was anchored. People shake their heads when I tell them I saw it that night. I can’t help it if they don’t believe me. I know the glacier is hidden by the point when you’re in Anchorage Cove, but I know what I saw that night, too. The glacier had risen in the air and moved forward so it was in sight. It must have risen several hundred feet. I don’t mean it was just hanging in the air. It seems to be solid, but it was jumping and shaking like crazy. Big chunks of ice were falling off the face of it and down into the water. That was six miles away and they still looked like big chunks. They came off the glacier like a big load of rocks spilling out of a dump truck. That went on for a little while—it’s hard to tell just how long—and then suddenly the glacier dropped back out of sight and there was a big wall of water going over the point. The wave started for us right after that and I was too busy to tell what else was happening up there.
The wave caused damage to the vegetation up the headlands around the area where the rockfall occurred, up to a height of 520 metres (1,710 ft), as well as along the shoreline of the bay.
An analysis of the conditions that caused it:
I don’t know about you, but a 524 metre wave is pretty f***ing scary! (Please excuse my French.)
1963 VAJONT DAM MEGATSUNAMI
Yes, the second of the three 20th-century was created by a DAM! The largest dam in the world at the time, the Vajont Dam in northern Italy, 262 metres tall. Swamped by a wave 250 metres. That is, the wave rose 250 metres ABOVE THE DAM. Destroyed several villages. Killed almost 2000 people.
If you were standing in the spot the above picture was taken from on that fateful day – 9 October 1963 – then the top half of the frame, between the two mountains, would be filled with water, the sky (in the above frame) almost completely blocked by the wave.
Early signs of disaster
On 22 March 1959, during construction of the Vajont dam, a landslide at the nearby Pontesei dam created a 20-meter (66 ft) high wave that killed one person.
Throughout the summer of 1960, minor landslides and earth movements were noticed. However, instead of heeding these warning signs, the Italian government chose to sue the handful of journalists reporting the problems for “undermining the social order”.
On 4 November 1960, with the water level in the reservoir at about 190 metres (620 ft) of the planned 262 metres (860 ft), a landslide of about 800,000 cubic metres (1,000,000 cu yd) collapsed into the lake. SADE stopped the filling, lowered the water level by about 50 metres (160 ft), and started to build an artificial gallery in the basin in front of Monte Toc to keep the basin usable even if additional landslides (which were expected) divided it into two parts.
In October 1961, after the completion of the gallery, SADE resumed filling the narrow reservoir under controlled monitoring. In April and May 1962, with the basin water level at 215 metres (705 ft), the people of Erto and Casso reported five “grade five” Mercalli intensity scale earthquakes. SADE downplayed the importance of these quakes. SADE was then authorized to fill the reservoir to the maximum level.
In July 1962, SADE’s own engineers reported the results of model-based experiments on the effects of further landslides from Monte Toc into the lake. The tests indicated that a wave generated by a landslide could top the crest of the dam if the water level was 20 metres (66 ft) or less from the dam crest. It was therefore decided that a level 25 metres (82 ft) below the crest would prevent any displacement wave from over-topping the dam. However, a decision was made to fill the basin beyond that, because the engineers thought they could control the rate of the landslide by controlling the level of water in the reservoir.
In March 1963, the dam was transferred to the newly constituted government service for electricity, ENEL. During the following summer, with the basin almost completely filled, slides, shakes, and movements of the ground were continuously reported by the alarmed population. On 15 September, the entire side of the mountain slid down by 22 centimetres (8.7 in). On 26 September, ENEL decided to slowly empty the basin to 240 metres (790 ft), but in early October the collapse of the mountain’s south side looked unavoidable: one day it moved almost 1 metre (3.3 ft).
Landslide and wave
On 9 October 1963, engineers saw trees falling and rocks rolling down into the lake where the predicted landslide would take place. Before this, the alarming rate of movement of the landslide had not slowed as a result of lowering the water, although the water had been lowered to what SADE believed was a safe level to contain the displacement wave should a catastrophic landslide occur. With a major landslide now imminent, engineers gathered on top of the dam that evening to witness the tsunami.
At 10:39 pm, a massive landslide of about 260,000,000 cubic metres (340,000,000 cu yd) of forest, earth, and rock fell into the reservoir at up to 110 kilometres per hour (68 mph), completely filling the narrow reservoir behind the dam. The landslide was complete in just 45 seconds, much faster than predicted, and the resulting displacement of water caused 50,000,000 cubic metres (65,000,000 cu yd) of water to overtop the dam in a 250-metre (820 ft) high wave.
The flooding in the Piave valley from the huge wave destroyed the villages of Longarone, Pirago, Rivalta, Villanova and Faè, killing around 2,000 people and turning the land below the dam into a flat plain of mud with an impact crater 60 metres (200 ft) deep and 80 metres (260 ft) wide. Many small villages near the landslide along the lakefront also suffered damage from a giant displacement wave. Villages in the territory of Erto e Casso and the village of Codissago, near Castellavazzo, were largely wrecked.
Estimates of the dead range from 1,900 to 2,500 people, and about 350 families lost all members. Most of the survivors had lost relatives and friends along with their homes and belongings.
The dam was largely undamaged. The top 1 metre (3.3 ft) or so of masonry was washed away, but the basic structure remained intact and still exists today.
Causes and responsibility
Immediately after the disaster, the government (which at the time owned the dam), politicians and public authorities insisted on attributing the tragedy to an unexpected and unavoidable natural event.
The debate in the newspapers was heavily influenced by politics. The paper l’Unità, the mouthpiece of the Partito Comunista Italiano (PCI), was the first to denounce the actions of the management and government, as it had previously carried a number of articles by Tina Merlin addressing the behaviour of the SADE management in the Vajont project and elsewhere. Indro Montanelli, then the most influential Italian journalist and a vocal anti-communist, attacked l’Unità and denied any human responsibility; l’Unità and the PCI were dubbed “jackals, speculating on pain and on the dead” in many articles by the Domenica del Corriere and a national campaign poster paid for by Democrazia Cristiana (DC). The catastrophe was attributed only to natural causes and God’s will.
The campaign accused the PCI of sending agitprops into the refugee communities, as relief personnel; most of them were partisans from Emilia Romagna who fought on Mount Toc in the Second World War and often had friends in the stricken area.
Democrazia Cristiana, the party of prime minister Giovanni Leone, accused the Communist Party of ‘political profiteering’ from the tragedy. Leone promised to bring justice to the people killed in the disaster. A few months after he lost the premiership, he became the head of SADE’s team of lawyers, who significantly reduced the amount of compensation for the survivors and ruled out payment for at least 600 victims.
Apart from journalistic attacks and the attempted cover-up from news sources aligned with the government, there had been proven flaws in the geological assessments, and disregard of warnings about the likelihood of a disaster by SADE, ENEL and the government.
The trial was moved to L’Aquila, in Abruzzo, by the judges who heard the preliminary trial, thus preventing public participation, and resulted in lenient sentencing for a handful of the SADE and ENEL engineers. One SADE engineer (Mario Pancini) committed suicide in 1968. The government never sued SADE for damage compensation.
Subsequent engineering analysis has focused on the cause of the landslide, and there is ongoing debate about the contribution of rainfall, dam level changes and earthquakes as triggers of the landslide, as well as differing views about whether it was an old landslide that slipped further or a completely new one.
There were a number of problems with the choice of site for the dam and reservoir: the canyon was steep sided, the river had undercut its banks, and the limestone and clay-stone rocks that made up the walls of the canyon were inter-bedded with the slippery clay-like Lias and Dogger Jurassic-period horizons and the Cretaceous-period Malm horizon, all of which were inclined towards the axis of the canyon. In addition, the limestone layers contained many solution caverns that became only more saturated because of rains in September.
Prior to the landslide that caused the overtopping flood, the creep of the regolith had been 1.01 centimetres (0.40 in) per week. In September, this creep reached 25.4 centimetres (10.0 in) per day until finally, the day before the landslide, the creep was measured at 1 metre (3.3 ft).
1980 MOUNT ST HELENS ERUPTION
Yes, the most famous eruption in North America also created the world’s last known megatsunami – or should I say most recent, because literally only God can know when one will strike. It struck Spirit Lake.
Mount St. Helens eruption
During the 1980 eruption of Mount St. Helens, Spirit Lake received the full impact of the lateral blast from the volcano. The blast and the debris avalanche associated with this eruption temporarily displaced much of the lake from its bed and forced lake waters as a wave as much as 850 ft (260 m) above lake level on the mountain slopes along the north shoreline of the lake. The debris avalanche deposited about 430,000,000 cubic metres (350,000 acre⋅ft) of pyrolized trees, other plant material, volcanic ash, and volcanic debris of various origins into Spirit Lake. The deposition of this volcanic material decreased the lake volume by approximately 56,000,000 cubic metres (45,000 acre⋅ft). Lahar and pyroclastic flow deposits from the eruption blocked its natural pre-eruption outlet to the North Fork Toutle River valley at its outlet, raising the surface elevation of the lake by between 197 ft (60 m) and 206 ft (63 m). The surface area of the lake was increased from 1,300 acres to about 2,200 acres and its maximum depth decreased from 190 ft (58 m) to 110 ft (34 m). The eruption tore thousands of trees from the surrounding hillsides and swept them into Spirit Lake. These thousands of shattered trees formed a floating log raft on the lake surface that covered about 40% of the lake’s surface after the eruption.
After the eruption, Spirit Lake contained highly toxic water with volcanic gases seeping up from the lake bed. A month after the eruption, the bacteria-carrying water was devoid of oxygen. Scientists predicted that the lake would not recover quickly, but the reemergence of phytoplankton starting in 1983 began to restore oxygen levels. Amphibians such as frogs and salamanders recolonized the lake, and fish (reintroduced by fishermen) thrived.
Oh, the joys of nature.