3 October 2018
High resolution imagery of the Palu landslides
DigitalGlobe have now released high-resolution imagery of two of the Palu landslides, triggered by the 2018 Sulawesi earthquake. This is available as a “before and after” slider on the CNN website (and elsewhere). The level of detail is remarkable, and we are now starting to get an idea of the mechanisms and motion of the landslides.
As I noted yesterday, the landslide, which moved from east towards the west, appears to be a planar flowslide of some type, with a very strong lobate form away from the source. Note that the landslide debris itself is an extraordinary combination of soil and debris, and houses and buildings, all compressed together. The total loss of homes is very high, but it is hard to know as to the likely level of human casualties. There are multiple tension cracks in the headscarp area, and this will be a very challenging site in the next heavy rainfall event.
meanwhile, the smaller landslide at Balaroa, the site closest to the city centre, is also terrible. On this occasion the landslide moved from west to east, and again a large, mostly evacuated, source area can be seen. There is a raft of mostly intact debris and vegetation downslope, and then an extraordinary compression zone of destroyed buildings. The slide is about 1 km long. The level of loss is once again high, but the number of fatalities is difficult to determine.
It is worth noting that for the survivors this type of event is worse than the more normal building collapse. In an earthquake-induced building collapse people may lose their property and most of their possessions, but at least sometimes still have their land. In the case of a landslide they typically lose property, possessions and the land that they inhabited or owned, as the image above shows. This is devastating in the long term.
InSAR data of the area affected by the earthquake is now becoming available, which shows the line of the fault. As expected, this data suggests that these big landslides are close to the fault trace. Unexpectedly, the data also suggest that the fault that ruptured extends into the bay north of Palu, but then steps onshore to run through the isthmus rather than offshore. This step may explain the tsunami, rather than it being a submarine landslide (the jury remains out on that one). Rupture appears to have been strongly directional, propagating south from the epicentre at the northern end of the earthquake zone. The upshot is that Palu suffered a bullseye hit from a large earthquake, with a strong directional component. It is no wonder that the level destruction is so high. In that context the number of human casualties currently reported seems surprisingly low to me.