Following on from the devastating earthquake in the Chinese province of Sichuan, scientists writing in Nature have predicted the possibility of significant aftershocks, following on from the initial impact.
Prof George Helffrich, Professor of Seismology at the University of Bristol, said:
“Earthquakes are intrinsically unpredictable, and tools to forecast them rare. But after a tremble or the release of built-up stresses at the epicenter changes the regional stresses in the crust. In areas of increased stress, one may reasonably expect the risk of future earthquakes to be higher. Parsons and co-workers use this idea to highlight areas of potentially increased risk in the area around western Sichuan, China, where the damaging earthquake of early May occurred. The area, on the eastern margin of a mountainous plateau, is riddled by many faults. Those trending away to the northwest from the recent earthquake are the most strongly loaded. Given the few-month lag between major earthquakes linked by stress transfer, the study provides a tool for future disaster mitigation in those areas.”
Prof John McCloskey, Professor of Geophysics at the University of Ulster, said:
“We can’t predict earthquakes and many scientists, including me, think we may never be able to but we can do a lot better than nothing. Some of us are becoming more convinced that even though we do not know the whole answer we know a lot of the features of the answer and these features could be of great importance to people charged with preparing populations for earthquakes or other disasters.
Currently, the areas of high hazard – the red areas – on global earthquake hazard maps are too big. It is one reasonable objective of contemporary earthquake physics to identify areas of particularly high risk – sort of super red areas. If this could be done then even in the absence of prediction we can do a lot to mitigate the risk by focusing scarce resource, including scientific resource, on the really dangerous locations.
Several groups in Europe, America and Japan are developing techniques to pinpoint these super red areas and one of the most promising techniques uses the stresses caused by one earthquake to help us locate where others might be occur. The technique goes like this:
1.,Large earthquakes, such as the recent earthquake in China, distort the surrounding rocks and perturb the stress field in them.
2.,Rapid methods for estimating the details of the slip distribution in the mainshock – where and by how much the fault moved during the earthquake – mean that the size of these stress changes can be calculated quickly.
3.,Knowledge of the geological structure around the earthquake allows the identification of at-risk faults which are capable of producing further large damaging earthquakes.
4.,The stress changes can be resolved onto these structures pushing some closer to failure – usually plotted in shades of red on hazard maps – and relaxing some, making future events less likely on them. The red structures are the ones most likely to fail in the near future.
Parsons and his team have illustrated how this can be done rapidly following an earthquake and they have shown clearly that several large faults which have experienced increases in stress big enough to trigger other large earthquakes. Some of their calculated stresses are many times bigger than those known to be responsible for triggering great earthquakes in the past.
Their work not only identifies faults and locations where future earthquakes are more likely but also tells us which structures are in the greatest need of scientific study so we can estimate how long it has been since their last big earthquakes and therefore how close they might be to failure. These future studies can take years but must be started urgently.
As well as having practical consequences this work also lays down a marker with reference to where we are in this type of earthquake forecasting. Parsons and his co-workers are saying unambiguously where current science indicates the highest risk. By back analysing these forecasts, made without the benefit of hindsight, we should be able see which areas of the contemporary earthquake physics are doing a good job and which need more attention. Such prospective forecasts are essential if we are to constructively criticise the state of our work.
It is important to stress that this is not an earthquake prediction. These stressing studies sometimes identify areas which fail rapidly, within weeks or months. In other cases larger stresses have not produced triggered earthquakes years after the initial event. We currently understand many deficiencies in the technique, we can only estimate the change in stress, for example, we cannot measure its absolute value. Neither do we know the strength of the target fault and in many cases, such as here, we are uncertain of how far the target faults are from failure or even when they failed last. Earthquake triggering is very like the straw that breaks the camel’s back. We can measure precisely the weigh of the straw but we don’t know how much more the camel can take. This science is absolutely work in progress. The Parsons paper is another brick in a very big wall.”