Unpredictability adds to the danger of volcanic tsunamis

In 1883, after weeks of rumbling and spectacular fireworks, the Krakatoa volcano between the Indonesian islands of Java and Sumatra erupted with devastating force.

A series of explosions sent ash tens of kilometres into the sky and resulted in more than 36,000 deaths.

Many victims died as a result of the rock fragments and gases that were blasted out, but others perished because of the terrifying 40-metre tsunamis created by the volcano's collapse.

More than four decades later, in 1927, the Anak Krakatoa (Child of Krakatoa) island volcano emerged above the sea from the remnants of the old volcano.

Last Saturday, history repeated itself on a smaller but still devastating scale when the apparent movement of sediment beneath the surface of the sea triggered tsunamis that have killed hundreds on Java and Sumatra and left 1,000 injured. Concerns remain high that there could be further activity creating follow-up tsunamis.

Although the 1883 eruption and its appalling consequences are well known, when it comes to tsunamis created by volcanoes,“there haven't been many significant examples” in more recent times, says Dr Stephen Hicks, a seismologist at the University of Southampton in the United Kingdom.

“It's very unusual in terms of living memory,” he said. “You cannot predict when it's going to happen; the warning time will be non-existent or minimal.”

Movements of the seafloor caused by earthquakes are a more common cause of tsunamis, including the 2004 earthquake with an epicentre off northern Sumatra that killed more than 225,000 people in more than a dozen countries.

Volcano-generated tsunamis, while less frequent, are likewise caused by the rapid displacement of large amounts of water.

“In general, movement of magma beneath the volcano surface pushes up some of the volcano flank and causes it to be gravitationally unstable and to collapse,” said Dr Hicks.

“The question is whether it gradually causes a slide over a few days or a catastrophic collapse within tens of seconds. That can cause material to go into the sea – hundreds of tonnes of material within seconds.”

What one volcanologist, Professor Simon Karn of Michigan Technological University, described on Twitter as “fairly rapid recent expansion” this year of the southern part of the Anak Krakatoa island could have made the flank unstable – and liable to collapse.

The shifting of material on the flank may have caused the movement of underwater sediment, which disturbed the surrounding waters.

Prof Karn also said that the remnants of the old volcanic crater may have affected how the tsunami waters dispersed and, in some directions, magnified their effects. A full moon causing high seas is another possible contributory factor.

Dangerous region

The wider Indonesian region is prone to seismic activity, sitting as it does on the Pacific Ocean's Ring of Fire, a horseshoe-shaped area covering tens of thousands of kilometres that contains 452 dormant and active volcanoes, with 127 active volcanoes in Indonesia alone.

The area's instability is caused by the movement of plates of the Earth's crust and upper mantle, a gradual process over millions of years that can, nevertheless, have explosive results.

The way in which volcanoes generate tsunamis is, says Dr Hicks, especially difficult for scientists to get to grips with.

“It's such a complex process; you cannot model or simulate such an event,” he said, contrasting this with the better understanding and prediction possible with tsunamis caused by earthquakes.

“Because we know the exact location of the earthquake, we can work out its depth; is it close to the sea floor; is it likely to cause displacement? If the fault has moved horizontally it won't displace water; if it's vertical it can.”

Another volcano that has caused concerns is the distinctively named Kick 'em Jenny, which sits underwater about 8 kilometres from the Caribbean island of Grenada. Earlier this year ships were warned against getting too close to the volcano after it became more active, although reports indicated that concerns were centred on the risk posed by volcanic gases, rather than the possibility of a tsunami.

In a powerful demonstration of the effects of landslides on water, in mid-2017 the plunging of a huge quantity of rock into a fjord on Greenland's west coast created a 100-metre tsunami, one of the biggest ever recorded. The remnants of the wall of water devastated a fishing village, Nuugaatsiaq, on an island in the fjord, killing several people.

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