Early warning signs preceded deadly collapse of Krakatau volcano

The 2018 collapse of the Anak Krakatau volcano, which triggered deadly tsunamis, was preceded by several early warning signs, according to a new study.

In December, the volcano's cone collapsed and slid into the Sunda Strait. The resulting tsunami affected nearly 200 miles of coastline on the Indonesian islands of Sumatra and Java, killing 437 people and injuring thousands more.


The latest survey of ground-based measurements, as well as data collected by drones and satellites, revealed increased temperatures and ground movement along the volcano's southwestern flank a few months before the catastrophe.

Scientists also found evidence of a pattern of low-frequency sound waves produced by a smaller earthquake two minutes prior to the deadly collapse. The seismic data helped scientists analyze the cone's collapse and descent into the ocean.

Researchers hope their analysis, published this week in the journal Nature Communications, can help forecasters predict the next collapse.

"At Krakatau, we were able to observe for the first time how the erosion of such a volcanic flank took place and which signals announced it," Thomas Walter, a volcanologist at the German Research Centre for Geosciences GFZ in Potsdam, said in a news release.

An analysis of satellite data showed the slow formation, over several months, of a slide-like structure flanking toward the sea. The collapse, which lasted two minutes, was picked up by seismographs and infrasound networks before the tsunamis reached shore. Had a monitoring system picked up on the signals, early warnings to seek shelter along the Indonesian coast might have saved lives.


"We used an exceptionally broad range of methods: From satellite observation to ground-based seismic data, from infrasound to drone data, from temperature measurements to chemical analysis of eruption products," Walter said. "Today's almost unrestricted access to worldwide data was critical in this. In the days following the tsunami, it allowed us to analyze this event at different locations in different countries at the same time."

The research team suggests their analysis can be used to build tsunami warning systems anywhere a volcano or landslide is at risk of plunging into the ocean. Traditional monitoring systems would be unlikely to recognize the significance of a similar event. Unlike earthquakes, the seismic signature of the flank collapse produced very few high frequencies. Instead, the collapse generated sound waves in the range of low frequencies.

"This property was the reason why the event was not detected in any routine evaluation," said GFZ seismologist Frederik Tilmann.


To update monitoring systems, researchers suggest officials enhance existing measurement methods with added sensors and update algorithms to look for the signatures of volcanic collapses.

"We are confident that our findings will lead to the development of improved monitoring systems," said Walter.

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