3 January 2019
Anak Krakatau: Planet Labs imagery of the aftermath of the landslide
Planet Labs has now captured a high-resolution SkySat image of the aftermath of the Anak Krakatau landslide, which generated the deadly tsunami shortly before Christmas. These images are featured on the BBC website too, with an excellent commentary from Jonathan Amos. The images are spectacular, as usual.
So, here is an image collected on 17th December 2018 showing the volcano shortly before the landslide. Note the smoking volcanic cone:-
I can see no signs of any precursory deformation features on the volcano, which is perhaps a little surprising. The second image shows the volcano after the landslide, in an image captured on 30th December 2018:-
This image shows the bowl-shaped landslide scar, and the new, small, bay formed by the collapse. There is a huge amount of sediment in the water around Anak Krakatau. The final image was captured using the SkySat high resolution instrument, providing the best image yet of the aftermath of the landslide:-
So what can we say about the landslide on Anak Krakatau? To me the scar appears to have a bowl shape, suggesting a complex (multi-phase?), possibly rotational (?) slip. 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. Whilst 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 (i.e. occurred above sea level). No remains of the landslide are seen – it left almost nothing behind as far as I can see – so the entire volume of 150-170 million cubic metres 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 (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?
Comments please from those who know volcanoes well.