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The area of influence of earthquakes could be larger than is currently thought

November 22, 2006

Dr Álvaro Corral, a Ramón y Cajal researcher for the UAB Department of Physics, studies the relationships between the time and place of earthquake occurrences (ie, the jumps between an initial earthquake and another earthquake at a later time in another place) using statistical physics methods. By analysing data on the distance between consecutive earthquakes, Dr Corral has concluded that the area of influence of seismic activity could be larger than was thought until now. The result of his work has been published in Physical Review Letters.

According to Corral, this work could lead to support for the idea of long-range earthquake triggering. It has always been thought that the influence of an earthquake was restricted to the rupture zone created by the earthquake at a geological fault, but the researchers now suspect that an earthquake may produce "aftershocks" much further afield, even on the opposite side of a tectonic plate to a main shock.

The diffusion of earthquake occurrences could be like a drop of ink in water. When the ink drop is added (the type of problem usually studied in statistical physics), an ink molecule collides with the water molecules at certain moments and in certain positions; similarly, a series of earthquakes are said to appear in time and in space. However, the reality is that the characteristics of these two cases are very different.

The expansion of the ink molecules occurs on a characteristic scale: that of the ink molecules colliding with water molecules (ie, they always collide after moving a relatively set distance in a relatively set amount of time). Yet earthquakes do not spread in such a normal, regular way. The distance between one earthquake and the subsequent earthquake can be larger or smaller than in previous cases, and the variation seems to be completely arbitrary. There is no characteristic scale.

The data observed seem to imply that the boundary for the influence of earthquakes could be much further away from the epicentre than was previously thought. It is difficult to calculate this boundary, since beyond a distance of 200 kilometres, the influence of an earthquake is hard to distinguish from "background seismicity", that is, the occurrence of other, unrelated earthquakes. Dr Corral believes that more sophisticated analysis techniques could be used to overcome this problem.

Scale models

The researcher has also observed that the earthquake occurrences in a certain region, such as California, could be extrapolated to the whole planet. In other words, the spatiotemporal occurrence of earthquakes in California is a scale model of what happens in the whole world. By observing this region, therefore, we are seeing a smaller version of the whole world. This shows the strange, fractal nature of seismicity, that is, that it maintains its form irrespective of its scale.

The results of this research also show that the diffusion of earthquakes does not depend on their size: small and large earthquakes spread in the same way. Therefore, small earthquakes, which are much more frequent, are the best model to use for the occurrence of larger earthquakes. This magnitude independence is anti-intuitive, and the researcher cannot yet offer any explanation for the phenomenon.

Universitat Autonoma de Barcelona