Anak Krakatau: How does a volcanic eruption trigger a deadly tsunami?

Anak Krakatau (Anak Krakatoa), meaning the ‘child of Krakatau’, is a cinder cone volcano formed inside the original Krakatau caldera. It is the fourth island to have formed following the historic 1883 explosive eruption and caldera collapse of Krakatau, and it first broke the waters in 1930.

Since the 1950s, Anak Krakatau grew at an average rate of 13cm per week, or 6.8m on average per year following almost continuous Strombolian style eruptions. Since 2008 activity has been more closely monitored as a lava dome grew. In 2012, Giachetti, Paris, Kelfoun and Ontowirjo wrote a paper titled ‘Tsunami hazard related to a flank collapse of Anak Krakatau volcano’ (see useful links below) where they warned that a tsunami triggered by a flank collapse of the volcano was very likely, since the volcano had built up on a steep incline along the edge of the old 1883 caldera and as such was likely to become unstable.

From June 2018 a new eruptive phase began, with a combination of strong Strombolian or weak Vulcanian eruptions including lava bombs, hot gases, lava flows and ash plumes.

On 22^nd December 2018 the eruption escalated, including Surtseyan behaviour. The volcano which had grown to over 340m high became unstable (as had been predicted), and the flank collapse and undersea landslides following the eruption (where a 158acre size section of the original cone detached) triggered a 3m high tsunami in the Sunda Strait that swept across coastal towns on the islands of Sumatra and Java which has led to the deaths of at least 437 people, with over 14’000 injured and 16’000 people displaced.

Satellite imagery such as here https://www.bbc.co.uk/news/science-environment-46743362 and https://www.volcanodiscovery.com/krakatau-satImages.html show that the volcano has lost over 2/3 of its original elevation and volume, and it is currently just 110m tall. The crater has now broken open to form a small bay, as shown in these before and after images:

There has been widespread criticism of the Indonesian authorities about the lack of tsunami warning, which has largely been non-existent since 2012 due to a lack of funds, vandalism of monitoring buoys, and technical problems. However, a volcano collapse triggering a tsunami is different to an earthquake-induced tsunami and gives less warning. For example, the classic seawater receding before a tsunami does not take place when a volcano collapse triggers the tsunami, and so even had warning buoys been in place these may not have detected changes in ocean waves until too late.

Useful links:

http://sp.lyellcollection.org/content/361/1/79 (research paper by Giachetti et al)

https://www.bbc.co.uk/news/science-environment-46666633

https://www.bbc.co.uk/news/world-asia-46674490

https://www.volcanodiscovery.com/krakatau.html (volcanic activity advisory)

https://www.nationalgeographic.com/science/2018/12/indonesia-volcano-tsunami-surprise-explained-anak-krakatau/

Did you know?

Edvard Munch’s famous painting ‘The Scream’ may well have been inspired by the original 1883 Krakatau eruption.