On Saturday at around 21:30 local time (15:30 CET), a tsunami struck Indonesia’s Sunda Strait, which is located between the islands of Java and Sumatra. Coastal towns in the region were badly affected, with government officials reporting that over 280 people have been killed and more than 1000 injured. The tsunami may have been caused by undersea landslides following the eruption of the Anak Krakatau volcano. European Geosciences Union (EGU) experts comment on the events.

What caused the tsunami? Initial reports indicate that it was caused by undersea landslides after the Anak Krakatau volcano erupted. What caused the landslide?

Mike Burton, President of the EGU Division on Geochemistry, Mineralogy, Petrology & Volcanology says

“We don’t know yet but initial satellite pictures suggest a significant flank collapse.”

Raphaël Paris, winner of the EGU Plinius Medal in 2011, says

“What we know so far is that Krakatau volcano was in a phase of reinforced activity. The Sentinel [satellite] images confirmed that a 64 ha piece of its western flank collapsed to the sea and this collapse is most probably the cause of the tsunami that struck the coasts of Java (after about 30 minutes of travel time in the shallow water Sunda Strait) and Sumatra (I got less info from Sumatra). There’s no tsunami warning system available for Krakatau. In 2012, we have published a study on a possible flank collapse of Anak Krakatau and simulated numerically the related tsunami. The volume of collapse simulated is larger than what occurred Saturday (fortunately) and our scenario can thus be considered as a worst-case scenario. However, there is a big uncertainty on the stability of the volcanic cone now and the probability for future collapses and tsunamis is perhaps non negligible.”

How can volcanic eruptions trigger tsunamis and how common is it for this to happen?

Mike Burton, President of the EGU Division on Geochemistry, Mineralogy, Petrology & Volcanology says

“There are several processes by which a volcanic eruption can trigger a tsunami. Large volume pyroclastic flows entering the sea, submarine explosions and flank collapse. Volcanoes can grow rapidly through lava flows, and this growth may create instabilities, such that a small eruption can trigger a flank collapse / landslide and the landslide displaces water creating a tsunami. A small subaerial landslide can generate a larger submarine landslide. This was the case in the 2002 eruption of Stromboli, Italy, for example, which created tsunami damage on the island itself and nearby land. Day et al 2015 report 4 flank-collapse induced tsunamis in the 20^th century. The last one before Anak Krakatau (assuming this is the mechanism for Anak, as seems likely but has not been categorically demonstrated yet) was Stromboli 2002, which was probably 10 times smaller. Every 100 years or so there is a larger event, such as the 1888 collapse of Ritter Island where 4.2 km³ of material produced a major tsunami, with waves up to 4.5 m in Rabaul, 540 km away. The 1883 eruption of Krakatau created a much larger tsunami than the December 2018 tsunami, probably due to a combination of pyroclastic flow entry into the sea and phreatomagmatism, where hot rock flashes water to steam generating explosions.

Important to note that Giachetti et al. 2012 identified the high risk of a flank collapse at Anak Kraktau and modelled a tsunami generated by such a collapse which appears to be very similar to that which occurred on 22 December. The hazard scenario was therefore understood, but the management of such a hazard obviously remains a major challenge.”

Ira Didenkulova, Deputy President of the EGU Division on Natural Hazards says

“Volcanic eruptions are known to cause tsunami. It is not the most frequent types of tsunami origin, but rather known. I would put it after the earthquake, landslides and meteotsunamis for the frequency. There are at least two mechanisms involved depending on the type of volcano and the type of eruption. It may be similar to the landslide if we have piroclastic flow going into the water. If eruption is so strong that it goes into explosion, then we can talk about explosive mechanism.

There may also be a real submarine landslide involved and the eruption of Anak Krakatau could just trigger it. In this case it would be a regular landslide tsunami, which was triggered by the volcanic eruption.”

Why was the tsunami so devastating? Did “high tides at full moon” play a part as some Indonesia officials have mentioned?

“The damage is high because of the proximity of people and infrastructure to the coastline, and its likely that a high tide would produce a larger impact from the tsunami compared with a low tide.”, says Burton.

How long has Anak Krakatau been erupting for? How and when did Anak Krakatau form and when did it first erupt?

“Anak means child. So, it is a child of big Krakatau, which exploded in 1883. Anak Krakatau was mostly growing after 1883 event, but even while growing it was active.” says Didenkulova.

“Anak is growing by eruptions, so it first erupted in 1927 from the remnants of the crater left after the major 1883 eruption of Krakatau.” says Burton. “[It] has been continuously subaerial since 1930.”

How common are tsunamis and volcanic eruptions in Indonesia?

“There is plenty of evidence for major tsunami impacts in Indonesia resulting primarily from earthquake-induced tsunami but also eruption induced. There are many active volcanoes in Indonesia, with 40 eruptions from different volcanoes since 2000 https://volcano.si.edu/,” says Burton.

Didenkulova says, commenting on tsunamis associated with eruptions of Krakatau: “The largest occurred in 1883 and it is still among the strongest global tsunamis propagating worldwide. Tsunami waves from that eruption were recorded in all Indian Ocean, but also in Europe and USA. At that time the process of Krakatau eruptions lasted for several months. It started with a series of normal eruptions with piroclastic flows, which caused several 1-2 m tsunamis, then there was a stronger tsunami of 10 m, and all this ended with explosive eruption of Krakatau, when 2/3 of the entire volcano-island was destroyed (the wave height was up to 30 m).”

Is there still a threat of further volcanic-caused tsunamis at present given the ongoing activity at Anak Krakatau?

“While there is still significant material in the edifice of Anak there is a possibility for further collapse, but we don’t have constraints on the probability for this,” says Burton.

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Mike Burton is Professor of Volcanology at the University of Manchester, UK, Ira Didenkulova is Senior Research Scientist at Tallinn University of Technology, Estonia, and Raphaël Paris is a researcher at the Laboratoire Magmas & Volcans, Aubière, France.

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