Science Alert: On Dec. 22 at 9:03 pm local time, a 64-hectare (158-acre) chunk of Anak Krakatau volcano, in Indonesia, slid into the ocean following an eruption. This landslide created a tsunami that struck coastal regions in Java and Sumatra, killing at least 426 people and injuring 7,202.
Satellite data and helicopter footage taken on Dec. 23 confirmed that part of the southwest sector of the volcano had collapsed into the sea. In a report on Dec. 29, Indonesia’s Center of Volcanology and Geological Hazard Mitigation said that the height of Anak Krakatau went from 338 meters (1,108 feet) above sea level to 110 meters (360 feet).
My colleagues and I published a study in 2012 looking at the hazards this site posed and found that, although it was very difficult to forecast if and when Anak Krakatau would partially collapse, the characteristics of the waves produced by such event were not totally unpredictable.
Landslide-triggered: Although most tsunamis have a seismic origin (for example, the Sumatra, Indonesia one in 2004 and at Tohoku, Japan in 2011), they may also be triggered by phenomena related to large volcanic eruptions.
Tsunamis caused by volcanoes can be triggered by submarine explosions or by large pyroclastic flows – a hot mix of volcanic gases, ash and blocks travelling at tens of miles per hour – if they enter in a body of water.
Another cause is when a large crater forms due to the collapse of the roof of a magma chamber – a large reservoir of partially molten rock beneath the surface of the Earth – following an eruption.
At Anak Krakatau, a large, rapidly sliding mass that struck the water led to the tsunami.
These types of events are usually difficult to predict as most of the sliding mass is below water level.
These volcanic landslides can lead to major tsunamis. Landslide-triggered tsunamis similar to what happened at Anak Krakatau occurred in December 2002 when 17 millions cubic meters (600 millions cubic feet) of volcanic material from Stromboli volcano, in Italy, triggered a 8-meter-high wave.
More recently in June 2017, a 100-meter-high wave was triggered by a 45-million-cubic-meter (1.6-billion cubic-feet) landslide in Karrat Fjord, in Greenland, causing a sudden surge of seawater that wreaked havoc and killed four people in the fishing village of Nuugaatsiaq located about 20 km (12.5 miles) away from the collapse.
These two tsunamis had few fatalities as they occurred either in relatively isolated locations (Karrat Fjord) or during a period of no tourist activity (Stromboli). This was obviously not the case at Anak Krakatau on Dec. 22.
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