Tsunami in Indonesia: the slide on the side of the volcano may have caused giant waves | World

It is clear that everyone in the area knew Anak Krakatau, the volcano that appeared on the seaway less than 100 years ago. However, experts describe the volcano eruptions as being of low magnitude or semi-continuous.

• See photos of the tsunami in Indonesia

Volcanoes have the ability to generate large waves. The mechanism that allows this phenomenon is, as always, a large displacement of water.

However, unlike a tsunami caused by an earthquake, in which the seabed moves up and down, which triggered the event in Indonesia was apparently a kind of landslide during the eruption of Krakatau.

It is not clear if part of the volcano's flank has collapsed and if the materials have fallen into the sea, pushing the water towards the front, or if the movement on the flank caused a rapid fall of sediments already under water.

The second option seems to be the most likely, according to experts heard by the BBC. But the effect is the same in both cases: the water column moves and the waves spread to the coast.

Tide level gauges in the Sunda Strait indicated a high tide about half an hour after the last eruptive activity of Anak Krakatau, around 9 pm Saturday local time.

Professor Dan Parson of the University of Hull in the United Kingdom said the flanks of volcanoes, flanks, are unstable and may collapse. "The flanks are notoriously unstable, it seems to me that in this case, the slide or already at the bottom of the sea has generated a significant tsunami," he told the BBC.

"The original Krakatoa exploded and was destroyed in 1883 and since then it has slowly recovered.As volcanoes form, their flanks may become unstable and collapse even without activity volcanic, "he says.

After the Saturday phenomenon, scientists began to study the possible causes of the tsunami. Preliminary results suggest that something has happened on the southern flank of Krakatau. In the coming days, this area will be subject to a thorough review by experts.

Sliding or landslides can cause gigantic tsunamis, as was the case in Indonesia. In geological history, they have been responsible for major disasters.

• "I tried to hang on to everything to survive"

In 2017, for example, a rocky slide caused giant waves that affected western Greenland. It is also suspected that a tsunami that occurred last September on the island of Sulawesi, also in Indonesia, was reinforced, at least in part, by the mbadive movement of sediment, either by entering the shoreline waters or by sliding on submerged slopes. in Palu Bay.

Witnesses testified that on Saturday the waves that struck the Sunda Strait were about five meters high. Even large, these waves can dissipate quickly once they move away from their source.

One of the most desperate things in such occasions is watching videos in which people are trapped in total ignorance of what will happen. In other words, in the case of Indonesia, people had no way of knowing that a tsunami would occur because they had not been warned by an earlier earthquake. At other times, earthquakes announce that a giant wave can hit the shore, which often leaves time for evacuation.

But although there was seismicity (movement of the earth) reported by instruments, it was not serious enough to change people's behavior – hence the surprise when the wave has arrived.

"The signal buoys are positioned to warn tsunamis caused by an earthquake on the boundaries of the underwater tectonic plates.Even if there was a buoy near Anak Krakatau, it is so near the coast as the time of evacuation would have been minimal.the tsunami speeds, "said Professor Dave Rothery of the UK Open University.

According to experts, what an event like this (and the Palu Bay, which also attracted the unprepared population) teaches us, is that it takes a lot more research into the dangers that far exceed what we normally expect.

• VIDEO: the giant wave reaches the stage of the show

After the 2004 tsunami, when more than 200,000 people died in the Indian Ocean region, important research efforts have have been deployed to understand what is called subduction earthquakes and tsunamis. , zone of convergence between the tectonic plates. Science must now study in depth the wider problems of the region, as in the case of volcanoes.

This alert was made public at the United States Geophysical Union meeting this month, the largest annual meeting of scientists in the world.

"The center of attention is always where the light is," said Professor Hermann Fritz of the Georgia Institute of Technology in the United States. "The focus has been on Sumatra and Java in the major subduction trenches." The alert centers have also focused on this topic because we had big events like Japan in 2011, Chile in 2010 and Sumatra in 2004. These are all the clbadic events in the subduction zone, so everything was for them, "he said.