Earthquake diagnosis.

The earthquake in Java is a tragedy. The loss of so much life it terrible, but it’s not so much of a natural disaster as a man-made one. Here’s why I think that…

Java is in a region where there are quite a few earthquakes, many of them with a magnitude of around 6 (which is moderately strong but very common, at least a couple a week globally). Admittedly, most of the earthquakes in the region where this one took place are quite a bit deeper than this one which, at 10km, is pretty shallow.

If you look at the USGS’s summary page on the earthquake you can get some idea of the background. If you can also understand the moment tensor solution for the earthquake you will see that it’s almost a completely vertical fault plane with a horizontal slip, which is unusual for the area but would mean that the ground motion would be almost purely horizontal.

Anyway, normally, in an area with many earthquakes it would be considered that the degree of damage caused by such a moderate earthquake would be relatively small. After all, the buildings should be built with such quakes in mind. But this is obviously not the case here. From what I’ve seen, the area is built on reasonably stable ash deposits so it’s unlikely that ground motion amplification due to sediment basin resonance and liquifaction would be a key factor. No, it’s probably down to poor building design.

Reports from the area say that when driving through the more affluent areas in the region you can drive for miles without seeing much damage at all and then you’ll hit a group of houses which have totally failed and that the majority of the casualties have happened in the outlying villages.

Surely, in such an earthquake rich area the local authorities should have would have made sure that the buildings were at least resistant in their design? It doesn’t really mean that the buildings would have to be more expensive to build even.. just make sure that there’s cross-bracing even in shacks.

Maybe it’s just the mind set of the local population which is the problem. There have been quotes from people from that area after volcanic eruptions where large numbers of people have been killed asking them why they have re-populated the area of known high risk where they say that it’s not a problem as that’s the future and it my not happen in their lifetime. It sounds like they’d make excellent politicians.

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