Landslide Tsunamis: Are They Dangerous?
What's a Tsunami
A tsunami is a Japanese word that simply means “harbor wave”. Also known as a tidal wave, these walls of water are most often created, when an earthquake occurs near the sea floor of an ocean or other large body of water. If conditions are right, ensuing shock waves are transferred from the sea floor to the water, creating a series of waves that can be quite destructive when they hit land.
What Causes a Tsunami?
It is important to note that Tsunamis are not generated with every underwater quake. In general, there has to be up and down movement along the fault line, as slip/slide faults do not usually generate tidal waves. In contrast, a thrust fault earthquake, where one slab of earth slides on top of another, can and do generate tsunamis, though not every time. Even though the seismic activity may occur on a sea floor thousands of feet from the ocean surface, enough energy can be generated so that surface waves travel hundreds or even thousand of miles across open water before hitting land and causing havoc with much loss of life and destruction.
A classic example of this phenomena occurred on April Fool's Day 1946, when a 8.6 quake in the Aleutian Islands, created a 40 foot tidal wave that hit Hilo, Hawaii and killed over a hundred people. The Alaska earthquake was created from thrust fault action.
A Change in Scientific Insight
In 1958, events at Lituya Bay on the coast of Alaska changed the way that the scientific community viewed tsunamis. After studying this event, earth scientists initially became aware that landslides can generate a tsunami with waves many times higher than ever thought possible. At Lituya Bay, it is estimated that landslides from the nearby glacier generated waves that exceeded 100 feet in height. And then, when these waves hit land, they were able to strip trees along the shoreline up to a height of 1700 feet.
Granted these observations were made in a nearly enclosed glacial inlet, but nonetheless, the results shed new light to what can happen on the open water, when tidal waves are created from cascading debris from glaciers and volcanoes.
Megatsunami is a relatively new scientific term, as the realization that landslides can cause grandiose tidal waves, is only a few decades old. In essence, a megatsunami is an extremely large wave (often over a 100 feet high) that is created by rock and earth debris falling down a steep slope. These landslides can can occur both on land and underwater, but the terrestrial landslides must reach a large body of water, so they can produce a large wave.
The "Child of Krakatoa" Destroys Itself
Recent events (December 2018), at the Anak Krakatoa volcanic island of Indonesia has caused more concern among vulcanologists and other earth scientists about the destructive power of a landslide tsunami. The tsunami in the Sunda Strait was caused by an underwater landslide on the southwest flank of the volcanic cone. Shock waves fanned out from epicenter, striking both the island of Sumatra and Java, while killing over four hundred people. The worst part of this disaster was that there was no advance warning, mainly because the tsunami was not generated by seismic activity.
Furthermore, if the landslide had started on the terrestrial slopes of the volcano and then slid into the sea, the tsunami waves could have been much larger and more destructive.
Lituya Bay, Alaska
On July 10, 1958 an earthquake hit coastal Alaska, causing much physical damage, but little loss of life. Only five people died in the quake, but all deaths were the results of the sea reacting violently to a 7.8 quake than ran for 125 miles along the Fairweather fault. Shocks from the event were felt as far away as Seattle.
Though no lives were loss in Lituya Bay, two fishing boats got tossed around like they were toys in a bathtub. Survivors reported riding out huge waves that might have been at least a hundred feet tall. The first geologist to investigate the aftermath, observed that the mountainside slopes had been stripped bare up to a height of 1,700 feet. Not surprisingly nobody believed him, but subsequent scientific analysis confirmed the geologist's data. Indeed there had been a wave in Lituya Bay that reached ridiculous heights. All of this cause by rock and ice debris tumbling down a mountain and sheering off the edge of a glacier.
In July 2017, similar events in a remote region of Greenland, confirms what scientists discovered in Lituya Bay, some 60 years earlier. The destructive power and size of landslide generated tidal waves is enormous.
In Greenland, a landslide on the walls of the Karrat Fjord, fell over 3,000 feet before landing in the icy waters below. The resulting tsunami hit the remote town of Nuugaatsiaq, where it destroyed homes and washed four people out to sea. Near the landslide, water from the wave left its highest mark 300 feet up a rock wall. However, across the bay, the high water mark was only a 150 feet above sea level.
Krakatoa from a Drone
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© 2018 Harry Nielsen