Experts interviewed by the BBC think that a massive landslide on a volcanic ocean island is the most likely future cause of a megatsunami. 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.
Some geologists consider that 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, could collapse into the lake, generating a megatsunami that might destroy the town of Harrison Hot Springs (located at its south end).
Geologists S. Day and S. Ward consider that a megatsunami could be generated during a future eruption involving the Cumbre Vieja on the volcanic ocean island of La Palma, in the Canary Islands.
In 1949, the Cumbre Vieja volcano erupted at its Duraznero, Hoyo Negro and San Juan vents. During this eruption, an earthquake with an epicentre near the village of Jedy occurred. The following day Rubio Bonelli, a local geologist, visited the summit area and discovered that a fissure about 2.5 km long had opened on the eastern side of the summit. As a result, the western half of the Cumbre Vieja (which is the volcanically active arm of a triple-armed rift) had slipped about 2 m downwards and 1 m westwards towards the Atlantic Ocean.
The Cumbre Vieja volcano is currently in a dormant stage, but will almost certainly erupt again in the future. Day and Ward hypothesize that if such an eruption causes the western flank to fail, a megatsunami will be generated.
Geologists and volcanologists disagree about whether an eruption on the Cumbre Vieja would cause a single large gravitational landslide or a series of smaller landslides.
Prehistoric sedimentary deposits on the Kohala Volcano, Lanai and Molokai controversially indicates that landslides from the flank of the Kilauea and Mauna Loa volcanoes in Hawaiimay have triggered past megatsunamis, most recently at 120,000 BP. A future tsunami event is also possible, with the tsunami potentially reaching up to about a kilometer in height.
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