Anak Krakatau's Tsunami Had A Frighteningly Small Trigger, But At Least Gave Some Warning

Anak Krakatau in 2017, before its partial collapse. The Wild eyed/Shutterstock.com

In December last year part of the Anak Krakatau volcano collapsed into the ocean. The resulting tsunami killed 430 people and destroyed the homes of tens of thousands. If scientists had seen it coming, most of the lives could have been saved. Two studies released over the last month seek to learn for future such disasters. One presents bad news, suggesting even worse versions may be more common than we thought, but the other offers hope we may get better at identifying future events before they occur.

The damage from the 2018 events was tragic enough, but Anak Krakatau troubles volcanologists for its potential. The 1883 eruption by the original Krakatau led to 36,000 deaths and changed the planet’s weather for more than a year. Anak Krakatau formed from the remains.

The disturbing conclusion from a paper in Geology is that even modest events could have more serious consequences. Using satellite radar images that reveal the island through its smoke before and after the flank collapse, Dr Rebecca Williams from Hull University calculated only 0.1 cubic kilometers (3.5 billion cubic feet) slid into the ocean in the initial, tsunami-causing collapse, a third of what she expected. Previous estimates had included the collapse of the volcano’s crown and crater, but Williams showed these were lost over several subsequent days, rather than in the single dramatic event that triggered the tsunami.

If such a modest amount of rock could cause such a devastating wave, how much worse would things have been if it had all gone at once? "I would challenge that the models are currently underestimating the ability of these landslides to make these larger tsunamis," Williams said to BBC News. With Krakatau located between Indonesia’s two most populous islands, the danger is enormous.

Preventing such disasters is almost certainly impossible, but predicting them might be another matter. Dr Thomas Walter of the German Research Center for Geosciences led a team seeking overlooked hints the volcano gave of the impending disaster.

In Nature Communications they report: “Prior to its collapse, the volcano exhibited an elevated state of activity, including precursory thermal anomalies, an increase in the island’s surface area, and a gradual seaward motion of its southwestern flank.”

Some of the warnings came too late to be much use, such as the small earthquake two minutes before the collapse. However, for six months prior to the collapse thermal sensors had indicated 100 times the normal heat emissions and faster movement of the island’s flanks. Various sensors around the volcano picked up signs of movement and degassing shortly before the collapse that individually were not enough to sound an alert, but looked at collectively might have provided the necessary warning.

Finding a way to integrate this data is certainly an issue we need to get to grips with. As Walter and colleagues note, 130 similar events have been recorded since 1600, and there were almost certainly more in areas with small populations at the time.

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