The dramatic images from Indonesia are unforgettable: on December 22 of last year, a tsunami devastated the coasts of the islands of Sumatra and Java. A cell phone video showed how water masses flashed over a concert of the Korean band Seventeen. The bassist and a manager were killed. In total, 430 people were killed, 14,000 injured and over 30,000 lost to their homes.
The deadly floods came without warning. With around 300 measuring stations and various GPS units, Indonesia has one of the most advanced tsunami early warning systems available. Warnings can be issued just minutes after a quake.
But this time, the cause was not a seaquake. Rather, a huge landslide was responsible, as flanks of the island volcano Anak Krakatau collapsed. More than a million cubic meters of earth slipped into the sea, they set the fatal tidal waves in motion. But the system did not sound an alarm.
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Researchers led by Thomas Walter from the German Geo Research Center in Potsdam (GFZ) have reconstructed the course of the disaster in a new study. "The volcano has been almost permanently active since the 1940s," he says.
According to the investigation, the collapse was preceded by warning signals. The researchers recognized this, in the evaluation of data from earth satellites and drones as well as from control rooms on the ground, they far back to the collapse. The results have now been published in the journal "Nature Communications".
Often volcanic islands are not very stable. Therefore, a collapse of volcanic flanks occurs again and again, but so far missing detailed measurements of such events. "At the Krakatau we were able to observe for the first time exactly how the demolition of such a volcanic flank took place and which signals it announced," says volcanologist Walter.
GFZ
Drone image of the volcano: The mountain has lost more than 300 meters in altitude
For example, months before the disaster measurements of a Nasa satellite showed much more heat at the southwestern volcanic flank than usual. "Probably the area of the leaked lava had increased significantly and radiated the increased heat," said Walter. Radar data also prove that there were always ground movements - also measured from space.
On December 22, the situation got worse. The registered seismometer also quasi from the first row, the next station was only 65 kilometers from the volcano. Their needles hit 115 seconds before the collapse - probably caused by a small earthquake. Apparently it was the very last event before the unstable mountain flank slipped off and possibly also the trigger, write the authors.
Even 1150 kilometers away, an Australian infrasound detector from the Nuclear Test Ban Test Network (CTBT) registered the event. But to identify the approximately two-minute slide itself in the measurements, the researchers had to look very closely, because it revealed in an unfamiliar seismic pattern. The seismometers measured very slow frequencies of about 0.03 Hertz, so only one oscillation per 33 seconds. In the routine evaluation, the signal had initially been overlooked, says GFZ expert Frederik Tilmann.
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The fatal tsunami hit the coastal cities between 31 and 57 minutes after the slide. However, since the seismic waves are much faster, a warning would have been possible in principle. For this, existing measurement methods would have to be supplemented by additional sensors and new algorithms, according to the researchers.
Next, they want to transfer their results to other risk volcanoes to improve early detection. Tsunami warning systems should also consider events caused by landslides, Walter said. However, it is confident that improved monitoring systems can be developed with the new findings, according to a statement by the GFZ.
The Anak Krakatau is located in one of the most geologically active areas of the world. Sometimes it shakes many times a day and is considered a powder keg, even among the region's 130 active volcanoes. Even before the outbreak in 2018, geologists had noticed that the volcano had grown because it had thrown much rock to the surface.
The Anak Krakatau stands in the same place as once the Krakatau. Its eruption in 1883 is considered one of the greatest volcanic catastrophes of modern times. It killed more than 36,000 people. The vast majority in the triggered tsunami.
