Editorial: Prediction gets tougher

Published in The Hindu on October 24, 2009

Despite years of unrelenting efforts, predicting earthquakes — the holy grail of seismology — has only got tougher. A series of high magnitude earthquakes that struck different parts of the globe in recent years has made scientists realise that rocks respond to strain and seismic waves in a far more dynamic manner than was originally assumed. They are now compelled to revisit the well-accepted process used for explaining earthquake occurrences. According to the elastic rebound theory, proposed in 1910 by geologist Henry Fielding Reid of Johns Hopkins University, rocks along the faults are like twigs that bend under stress but snap when stress crosses the threshold. The rocks along faults tend to accumulate strain as plates collide against one another and finally give way when it becomes unbearable. Sudden snapping of the rocks and release of strain causes earthquakes. Since the theory is based on strain accumulation, high magnitude earthquakes should occur only after a long interval. But the occurrence of many high magnitude quakes before strain can accumulate and cross the threshold shows that the elastic rebound theory alone cannot explain the triggering mechanism.

In fact, many high magnitude quakes have occurred within a relatively short period of time. For instance, the three quakes of more than 7 magnitude that struck Vanuatu, an island nation in the South Pacific Ocean, on October 7 is a classic example of clustering. Sumatra has also been witnessing similar clustering in the last few years. A series of quakes of magnitude greater than 8 have occurred off the coast of Sumatra following the 9.1 magnitude quake of December 26, 2004. In fact, it may be wrong to assume that fault systems lying thousands of kilometres away are behaving independently. While it has been known that high magnitude earthquakes tend to set off tremors by altering the strength of faults lying several hundreds of kilometres away, scientists studying the San Andreas Fault at Parkfield, California, have for the first time been able to monitor this recently. Parkfield, one of the closely monitored earthquake zones in the world, provided the much-needed evidence that the 1992 Landers earthquake in California and 2004 Sumatra earthquake altered its fault strength and triggered tremors. While the 2008 Uniform California Earthquake Rupture Forecast 2 had found that the association of earthquake cycle with the elastic rebound theory can be highly irregular, the latest evidence from San Andreas Fault will further dilute the universality of the elastic rebound theory in explaining earthquake occurrence.