Megatsunami

The term "megatsunami" has been defined by media and has no precise definition, although it is commonly taken to refer to tsunamis over 100 metres (330 ft) high.

[3][4] By contrast, megatsunamis are caused by landslides and other impact events that displace large volumes of water, resulting in waves that may exceed the height of an ordinary tsunami by tens or even hundreds of metres.

[13] Recent findings show that the nature of a tsunami is dependent upon volume, velocity, initial acceleration, length and thickness of the contributing landslide.

In most cases the submarine landslides are noticeably subcritical, that is the Froude number (the ratio of slide speed to wave propagation) is significantly less than one.

Conversely tsunamis triggered by earthquakes are more critical when the seabed displacement occurs in the deep ocean as the first wave (which is less affected by depth) has a shorter wavelength and is enlarged when travelling from deeper to shallower waters.

[16] Before the 1950s, scientists had theorized that tsunamis orders of magnitude larger than those observed with earthquakes could have occurred as a result of ancient geological processes, but no concrete evidence of the existence of these "monster waves" had yet been gathered.

The block fell almost vertically and hit the water with sufficient force to create a wave that surged up the opposite side of the head of the bay to a height of 520 metres (1,710 feet), and was still many tens of metres high further down the bay when it carried eyewitnesses Howard Ulrich and his son Howard Jr. over the trees in their fishing boat.

Neither water drainage from a lake, nor a landslide, nor the force of the earthquake itself were sufficient to create a megatsunami of the size observed, although all of these may have been contributing factors.

[18] The rockfall also caused air to be "dragged along" due to viscosity effects, which added to the volume of displacement, and further impacted the sediment on the floor of the bay, creating a large crater.

The large monolithic mass of rock struck the sediments at bottom of Gilbert Inlet at the head of the bay with great force.

The impact created a large crater and displaced and folded recent and Tertiary deposits and sedimentary layers to an unknown depth.

Also, the impact and the sediment displacement by the rockfall resulted in an air bubble and in water splashing action that reached the 1,720-foot (524 m) elevation on the other side of the head of Gilbert Inlet.

[66] The massive explosion of Krakatoa created pyroclastic flows which generated megatsunamis when they hit the waters of the Sunda Strait on 27 August 1883.

The steamship Berouw, a colonial gunboat, was flung over a mile (1.6 km) inland on Sumatra by the wave, killing its entire crew.

[69] The eruption also generated what is often called the loudest sound in history, which was heard 4,800 kilometres (3,000 mi; 2,600 nmi) away on Rodrigues in the Indian Ocean.

The waves destroyed the villages of Bødal and Nesdal near the southern end of the lake, killing 61 people – half their combined population – and 261 farm animals and destroying 60 houses, all the local boathouses, and 70 to 80 boats, one of which – the tourist boat Lodalen – was thrown 300 metres (1,000 ft) inland by the last wave and wrecked.

When it entered the water, it generated a megatsunami which broke tree branches 34 metres (110 ft) above ground level 0.8 kilometres (0.5 mi) away.

At a distance of 24 kilometres (15 mi) from the slide, observers at Russell Fjord reported a series of large waves that caused the water level to rise and fall 5 to 6 metres (15 to 20 ft) for a half-hour.

[73] On 9 July 1958, a giant landslide at the head of Lituya Bay in Alaska, caused by an earthquake, generated a wave that washed out trees to a maximum elevation of 520 metres (1,710 ft) at the entrance of Gilbert Inlet.

Refracted energy from the tsunami created a wave that destroyed boats at the closest populated village, Saqqaq, on the southwestern coast of the Nuussuaq Peninsula 40 kilometres (25 mi) from the landslide.

[80] Occurring in an uninhabited area, the event was unwitnessed, and several hours passed before the signature of the landslide was noticed on seismographs at Columbia University in New York City.

Eleven buildings were swept into the sea, four people died, and 170 residents of Nuugaatsiaq and Illorsuit were evacuated because of a danger of additional landslides and waves.

[88][89][90][91][92] On 28 November 2020, unseasonably heavy rainfall triggered a landslide of 18,000,000 m3 (24,000,000 cu yd) into a glacial lake at the head of Elliot Creek.

The sudden displacement of water generated a 100 m (330 ft) high megatsunami that cascaded down Elliot Creek and the Southgate River to the head of Bute Inlet, covering a total distance of over 60 km (37 mi).

[95] In a BBC television documentary broadcast in 2000, experts said that they thought that a landslide on a volcanic ocean island is the most likely future cause of a megatsunami.

[96] The size and power of a wave generated by such means could produce devastating effects, travelling across oceans and inundating up to 25 kilometres (16 mi) inland from the coast.

[100] Some geologists consider an unstable rock face at Mount Breakenridge, above the north end of the giant fresh-water fjord of Harrison Lake in the Fraser Valley of southwestern British Columbia, Canada, to be unstable enough to collapse into the lake, generating a megatsunami that might destroy the town of Harrison Hot Springs (located at its south end).

[108][109] A 2008 study examined this scenario and concluded that while it could cause a megatsunami, it would be local to the Canary Islands and would diminish in height, becoming a smaller tsunami by the time it reached the continents as the waves interfered and spread across the oceans.

[109] In 2018, shortly after the beginning of the 2018 lower Puna eruption, a National Geographic article responded to such claims with "Will a monstrous landslide off the side of Kilauea trigger a monster tsunami bound for California?

In all likelihood, it wouldn't even impact the other Hawaiian islands.Another volcanologist, Janine Krippner, added:[100] People are worried about the catastrophic crashing of the volcano into the ocean.