19 July, 2006

Latest earthquake in Java

Monday saw another tsunami hit Indonesia, triggered by an earthquake offshore. But what more can we say about this quake? We can immediately use our knowledge about earthquakes and plate tectonics to make some basic predictions:

The earthquake will probably be due to movement along a thrust fault, probably associated with the Sunda trench. Tsunamis are caused when vertical (up and down) motions of the ocean bottom associated with an earthquake displaces the water above, creating a large wave. Vertical motions occur on contractional (thrust) and extensional (normal) faults. Java is close to a convergent plate boundary, so the earthquake is most likely to be on a thrust fault associated with this subduction zone.

The epicentre will be shallow. In order to cause a tsunami, the fault rupture must have propogated up to the ocean bottom, suggesting that it initiated at a fairly shallow depth.

Here's the USGS moment tensor solution:

Lo and behold, a shallow thrust (6 km may not seem particularly shallow, but at a subduction zone you can get earthquakes down to several hundred km) close to the Sunda Trench. The beachball like focal mechanism shows a thrust, and is very similar to that for the Boxing Day 2004 Earthquake (see also my previous post on seismology in this region).

Thus we have simulataneously demonstrated both the power and the impotence of plate tectonics at present. The fulfilment of my predictions shows that this earthquake fits in well with our overall tectonic picture of the region, but it is only a retrospective understanding; on Sunday, there was no way of predicting that an earthquake of that particular size would occur in that particular location.

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