30 May, 2006

Earthquake in Java

When they first heard news of Friday's earthquake in Java, many people's first reaction was undoubtedly to wonder if this latest event is in any way related to the Boxing Day 2004 quake. Here's some information on Friday's earthquake, courtesy of the National Earthquake Information Center (more specifically, here):

This image shows both the earthquake's location and a beachball-like symbol, known as a focal mechanism, which indicates the type of fault generating the earthquake. More in this in a minute, but first lets look at the same information for the Boxing Day Earthquake (full summary here).

You should ignore the magnitude, which was actually 9.0 - the automatic system which generates this information estimates magnitudes from the amplitude of seismic waves of a particular type and frequency, which are saturated (can’t get much bigger) for very large earthquakes and hence lead to an under-estimate.

The Boxing Day tsunami was caused by a rupture of the subduction megathrust at the boundary between two tectonic plates, just off the Indonesian coast. This is obvious from the fact that the hypocentre (point of first rupture) is very close to the Sunda trench, which is where the megathrust breaks the surface. It is also obvious from the focal mechanism, which is an idealised plan view of the movement associated with the earthquake, divided into compressional (solid areas of the beachball) or dilational (white) zones. This distribution is constructed by combining the directions of first motion, compared to the direction of the source, received by the global seismometer network; this will vary depending on the relative position of the station. The dividing lines between the solid and white zones represent the two possible fault planes along which this motion occurred - movement along either of these planes can produce the same focal mechanism (for slightly more detail, see here or here). The focal mechanism for the Boxing Day 2004 earthquake is a thrust fault - there is a compressive zone in the centre of the beachball which shows that shortening of the crust is occurring along either a very steep, or very shallow, thrust plane. Because we know it was at a subduction zone, it's probably the latter (the megathrust is normally shallowly dipping - see my previous post on subduction zones).

However, looking back at the information for Friday's earthquake, you can see that it is located much further away from the trench, in the forearc region of the overriding plate. Also, the focal mechanism is completely different; the pattern of compression and dilation implies horizontal strike-slip along a vertical fault plane. This indicates that this earthquake did not occur at the megathrust on the plate boundary.

However, things start to get interesting if we look at the relative motion across this boundary, which, as the figure below indicates, is generally oblique to the trend of the Sunda Trench. In contrast, the motion at the subduction megathrust (as indicated by the Boxing Day focal mechanism, and the focal mechanism of the 28th March earthquake in the adjacent section of the subduction zone) is almost exactly perpendicular to that trend. It seems that motion along the megathrust cannot account for all of the relative plate motion. However, the 'left-over' motion (see inset) can be accommodated by events like Friday's earthquake, if we assume a NW-SE-trending fault.

This phenomenon - different parts of the plate motion being taken up by separate sets of faults - is known as strain partitioning, and is very common in regions of oblique subduction. Thus the two events can be viewed as complementary - the strike slip fault in the forearc region is taking up the component of plate motion that the subduction thrust does not. The Boxing Day earthquake did not directly cause Friday's earthquake, but a study of its focal mechanism clearly requires events like it.

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