On 22 December back in 2018, Indonesia’s Anak Krakatau volcano erupted. The collapse of its flank triggered a tsunami that killed more than 400 and injured at least 30,000 people. 

To understand how the events of that day unfolded with such catastrophic effect, an international research team led by Research Associate Anna Perttu from the Earth Observatory of Singapore (EOS) turned to the data collected from monitoring stations from all around the region, official reports, as well as satellite and visual observations.

Eruptions produce a wide range of signals. Humans can see and hear some of them only when close enough to the eruption. Monitoring instruments help fill the gap. Satellites can be used to image volcanic plumes that are too high or obstructed from an observer. And seismic and infrasound sensors are able to detect very small ground movements (e.g. earthquakes) and pressure waves (e.g. volcanic explosions).  

These waves can travel over very long distances and be recorded by stations hundreds of kilometres (km) away from their source. The signals associated with the Krakatau eruption even reached Singapore 840 km away, where EOS scientists maintain an infrasound network, as well as Cocos Island over 1,000 km away.

Seismic waves travel quickly underground and are recorded almost instantaneously by regional stations. Because infrasound waves travel in the air at much slower speeds and are affected by wind conditions, they arrive later at the stations. An infrasound signal from Anak Krakatau’s eruption, for example, took approximately one hour to reach Singapore.

These are just some of the factors the scientists had to consider when comparing their datasets against visual observations in order to reconstruct the eruption at Anak Krakatau. Ms Perttu acknowledged that “bringing all that data together into a coherent timeline was one of the most challenging parts” of the project.

After careful analysis of the data, the scientists are now able to take us on a journey that reveals the dramatic turn of events at Anak Krakatau on that fateful day. Bringing us back to approximately six months before the deadly eruption, Anak Krakatau was already actively erupting. These eruptions were relatively small, producing a few lava flows, plumes not exceeding 5-km in height, and the occasional seismic and infrasound signals.

But things changed drastically at around 12.30pm (local time), when the eruptions grew more intense and the detection of infrasound signals became continuous.

At about 9pm, a large signal was recorded on regional seismic and infrasound stations. This signal (typical of mass movements) represents the collapse of the volcano’s flank into the sea. This collapse triggered the tsunami that devastated the coastline. The recorded seismic data tells us that the tsunami had repeatedly struck the shore around the Sunda Strait over a period of 40 minutes. The local fishermen recall seeing a series of waves, with the last one being the largest.

In the few hours that followed, the volcano was obscured by a dark cloud. But satellites and an eyewitness were able to observe three distinct volcanic plumes. The eruption continued unabated till 28 December 2018, with a distinct signature in its monitoring data. Strong activity in January 2019 caused more eruptions to occur and the deposits from those began to rebuild the volcano. Eventually, the activity slowed down to its current level which consists of occasional eruptions.

“Monitoring data is a powerful storyteller, but eyewitness reports are very helpful with filling in some of the gaps”, said Ms Perttu. Survivor stories, in particular, can provide valuable insight. In the video below, the story of a fisherman who was in the waters close to Anak Krakatau at the time is retold using animation.

According to Ms Perttu, “Several signals for this eruption are still not completely understood. EOS is part of a collaboration to install more local stations to help us better understand future volcanic activity.”

For more information on this research project, the published paper can be read here.