New satellite image interpretations released by Planet Labs show the scale of the collapse of the Anak Krakatau volcano on 22 December which triggered a fatal tsunami.
It is now thought that 430 people died along the coastlines of Java and Sumatra when the collapse of the volcano triggered an underwater landslide and subsequent tsunami.
Planet Labs’ Dove and Sky Sat platforms used radar to pierce the cloud cover which had prevented other satellite imagery from giving a clear view of the sequence of events.
The images show that the 340m high volcano almost completely collapsed to leave just a small cove. Indonesia’s disaster agency has said that up to 170M.m3 of material is missing from the volcano and the resulting landslide triggered the tsunami.
Source: Planet Labs
Landslide specialist Dave Petley, who is also Sheffield University pro vice chancellor (research and innovation), has reviewed the images to learn about the landslide failure mechanism. “To me the scar appears to have a bowl shape, suggesting a complex – maybe multi-phase, possibly rotational slip,” he said. “However, the toe of the landslide was extremely broad - look at the alignment of the rear scarp on the far side of the scar - meaning that the slide would have been able to displace water across a very large front.
“While early reports of the landslide were that it was probably submarine, the images are clear that a very large part of it - possibly the largest part - was subaerial. No remains of the landslide are seen – it left almost nothing behind as far as I can see – so the entire volume of 150M.m3 to 170M.m3 entered the water. It is unsurprising therefore that it generated a very significant tsunami in the local area.
“An interesting aspect is the peninsular left on the southern (nearest) part of the bay, seen in the second image of Anak Krakatau. This is an unusual feature for a landslide scar. Compare the before and after images – is the peninsular composed of harder volcanic rock, maybe lava flow, rather than the looser cone deposits? So, in this area did the landslide occur primarily in the looser deposits? Or did this peninsular form in the volcanic eruptions immediately after the landslide? Some of the radar imagery might suggest that this is the case?”