(fw) Whether, when and where earthquakes will occur in the near future is something that the people who live in high-risk areas would dearly like to know. Up until now, however, it has only been possible to forecast general risk, using special maps. A group of researchers from the Swiss Seismological Service (SED) at ETH Zurich (1) and the US Geological Survey (USGS) (2) present a method in the latest issue of "Nature" that goes a step further (3). It enables scientists to continuously assess the hazard situation after a major earthquake in California and predict the risk of further quakes.

Combination of natural laws

Until now forecasting models have been based on two natural laws, which earthquakes follow. First, major earthquakes are considerably less likely to happen than minor ones; and, second, earthquakes occur most often in certain areas. The Swiss-US research team has now added a third element to these criteria, namely the observation that aftershocks often follow a major earthquake. Their force generally declines over time, but, in certain cases it can be stronger than in the initial quake. If, for any given region, one has enough recorded data of earlier occurrences, it is possible, using these three indices, to assess the probability of further quakes occurring in the next 24 hours.

For the populations concerned, however, it is not only important to know where an earthquake will take place but also how badly the surrounding ground will be shaken by it. The maps calculated with the model do not therefore show where the earthquake could happen, but where the ground motion is likely to at least damage buildings. These maps can be consulted on the internet (4).

A much better instrument

Thanks to the wealth of data available in California researchers have been able to calibrate and test the method comprehensively. "Our tests have shown that the new instrument for forecasting is far more reliable than the models we've had up until now," says Stefan Wiemer from the Swiss Seismological Service. For the time being the new method is being used by scientists and for teaching purposes. It is (still) not robust enough to put into operational use. "The highest level only indicates, that the probability of a quake occurring within the next 24 hours is higher than 10 percent," says Wiemer. "That would hardly be enough time to evacuate a region or warn the rescue services." At USGS they had not been very keen initially on the use of the colour red to indicate the highest level of danger, because this colour is generally used to signal acute danger.

The new method will only be used in California for the time being. Wiemer says that he can well imagine that it could be used in other earthquake regions in future. "If it is suitably adapted, the model could be used anywhere in principle, even in weakly active regions such as Switzerland. Of course, it's not likely that the hazard index would change very much here from day to day."