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## A Universal Law May Govern Time Elapsed Between Earthquakes

03.03.2004

Surprisingly, the probability that an earthquake should reoccur in any part of the world is smaller, the longer the time since the last quake took place. This is one of the conclusions reached by the physicist Álvaro Corral, researcher at the Universitat Autònoma de Barcelona (UAB). Corral has been the first to observe that there is a relation between consecutive quake-to-quake time intervals that follows a universal distribution of probability. This in turn suggests the existence of a simple physical mechanism that regulates the process of earthquake generation. The research is to be published shortly in the prestigious journal Physical Review Letters.

If you throw a loaded dice, no-one can know the result beforehand, as this will be random; but if the dice is thrown a great number of times, one given result will come up more times than any other. Although the process is random, there is a distribution of probability that favours one result over the others.

A physicist at the UAB, Álvaro Corral, has discovered that earthquake behaviour follows a similar logic. Nobody knows the time interval between one quake and the next - this, too, is a random process - but Corral has found out that the process is “loaded”, that is, there is a distribution of probabilities that favours earthquakes being grouped together over time. As the researcher indicates, this tendency towards grouping shows itself in the very long term, and so goes far beyond the grouping of the successive replicas that occur in the immediate aftermath of an earthquake.

A surprising consequence of all this, although it may seem to run against the expected, is that the probability of an earthquake repeating is smaller, the longer the time since the last quake occurred. So, “the longer we’ve been waiting for the big one, the longer it’ll take for it to come”, says Corral.

The relation discovered by Álvaro Corral is found hidden under the register data for earthquakes that have taken place around the world since the 1970s. Corral has analysed this data from a perspective that is somewhat different to the traditional approach. Whilst it is true that, up to now, scientists have studied the intervals between quakes with a reductionist approach, differentiating between the main quake and its replicas, the UAB researcher has placed both quake types in the same basket. This approach is in accordance with the view taken by the philosophy of complexity, aiming to find descriptions for phenomena on a global scale. The approach is inspired by the late Danish physicist, Per Bak.

As the article published in Physical Review Letters points out, Corral divides the surface of the earth into different regions and takes into consideration the intervals of time between all the consecutive earthquakes catalogued for each of these zones. In analysing the distribution of probability for these time intervals, the UAB physicist discovered that, in spite of their randomness, the quakes are consistent with a universal law that tends to group them together.

This grouping tendency has appeared in all the statistical analyses carried out on the data catalogued, independently of the minimum intensities considered (running from 2 to 6.5 on the Richter Scale), or of the regions studied (running from those having 400 km2 to the entire Earth). In this way, the fact that all quakes obey a universal law of probability suggests the existence of a simple physical mechanism that regulates the process of earthquake generation on a global scale.

Octavi López Coronado | alfa
Further information:
http://www.uab.es

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