26 December 2018
The Anak Krakatau landslide and tsunami
The disastrous and tragic landslide and tsunami from Anak Krakatau (Anak Krakatoa) has rightly dominated the news over the Christmas break. At least 429 people are known to have been killed, making this one of the worst landslide-induced disasters of the year, with a further 154 people thought to be missing. Severe weather, and in particular heavy rainfall, is making both the assessment of events to date and in the assessment of what might come next extremely challenging. Further landslides cannot be ruled out.
We can say with some certainty that the tsunami was triggered by a landslide from Anak Krakatau. There are two strong pieces of evidence for this. First, whilst it has not been possible to obtain visual satellite imagery of the island because of the weather, radar data (which is not obscured by cloud) has been available. The ESA Sentinel-1 satellite collected a decent image on 22nd December:-
This is a remarkable image, showing amongst other things the volcanic plume being blown to the east and the radial waves from the island being generated by the ongoing eruptions. But most importantly it shows what appears to be a large scarp to the west of the volcano cone, presumably marking a large-scale deformation (landslide) of the volcanic edifice of Anak Krakatau. There would have been a large submarine element to this, which presumably generated the tsunami.
Secondly, a strong seismic signal was generated by the landslide, which was detected on a number of seismometers. Jamie Gurney (@UKEQ_Bulletin) has done a great job of collating that information via Twitter. The seismic signal has a very strong low frequency component, characteristic of a landslide-induced, rather than a tectonic, event.
The challenge now is to interpret what might be happening on the volcano, and what might happen next. The latest Sentinel-1 image does not help:-
I am no expert on radar imagery, and others may be able to provide more information about what this shows. It seems to me that a large part of the island appears to be generating very little useful data – is this because of a drape of new deposits from the eruptions of the last few days? The upshot is, I think, that we cannot really interpret whether the volcano is deforming (or has deformed) further.
Quite rightly, the authorities are concerned about further landslides, but the only advice at this point is to stay away from the coast and to be vigilant.
Volcanic flank collapses are a well-known hazard, and the potential for the collapse of Anak Krakatau has been highlighted previously. These landslides have huge destructive potential but are inadequately researched when compared to many other hazards.