The Banjarnegara landslide

Late on Friday night a large and apparently very rapid landslide was triggered by heavy rainfall at Banjarnegara in Central Java, Indonesia.  Latest reports suggest that 32 bodies have been recovered to date, with a further 76 people thought to be missing.  The likelihood of their having survived the landslide is low; one can only hope that, as is often the case, it turns out that at least some of those reported missing were elsewhere at the time of the landslide.  A further 15 people were injured, 11 of those seriously.

This image, from the Straits Times, shows the landslide site from close to the toe:

Banjarnegara landslide, via the Straits Times

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As well as being a tragedy, this is an interesting landslide for two reasons.  First, the slide appears to have been a very mobile earthflow – in fact there appears to be two lobes with an untouched area between.  Second, the landslide appears to be surprisingly deep-seated in the source area judging by the size of the lateral scarp on the left side of the lower part of the source area.  It seems that this deep-seated nature created a very large mobile mass that overran the village.

The source area of the landslide is quite well illustrated in this AP image via CTV:

Banjarnegara landslide (AP via CTV)

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As far as I can see the slide has occurred in deeply weathered residual soil, with no evidence of bedrock, or a bedrock – regolith contact, in any of the images that I’ve seen. I wonder if this was a static liquefaction type process to generate the exceptional mobility.  An alternative might be an undrained loading process generated by an initial failure near to the crown of the landslide, which would also explain the mobility, although the depth of the failure surface might be harder to explain.

Whatever the cause, this is a landslide that deserves detailed analysis.  Java is a global hotspot for landslide deaths because of the toxic combination of steep slopes, volcanic soils, heavy rainfall and lots of people. There is an urgent need to understand better the mechanics of these large and immensely damaging landslides there.