Why Didn’t the Foreshocks Predict the Great Japan Earthquake?

Alan Kafka
Weston Observatory
Department of Earth and Environmental Sciences
Boston College

In the aftermath of the devastating magnitude 9.0 earthquake in Japan, many people are asking whether we “should have known” that the smaller earthquakes that preceded it were warnings of worse to come. But while it is clear in retrospect that there were foreshocks during the few days preceding the main shock, including a magnitude 7.3 foreshock and two other magnitude 6.0 foreshocks, before the fact there was no way to know that they were indeed foreshocks.

Let me explain.

Some large earthquakes have foreshocks, and some don’t. How to (in advance) distinguish a foreshock from any other earthquake remains an enigma. There are, for example, more than 100 magnitude 6 to 7 earthquakes occurring each year, the vast majority of which are not foreshocks of huge earthquakes like the Japan quake. It would, therefore, not be practical to issue an earthquake prediction and call for evacuation every few days when these inevitable magnitude 6 to 7 earthquakes occur, on the slight chance that they might be foreshocks of a huge quake.

So unfortunately, in spite of the now clearly identified foreshocks of the Japan earthquake, we still had no way of predicting the magnitude 9.0 earthquake. It is, however, very clear that a huge earthquake was inevitable along the coast of Japan, even though nobody knew when that earthquake was going to occur. It might not have occurred for 10 years, 100 years, 1,000 years, or even 10,000 years. It is, of course, wise to prepare for the inevitability of major earthquakes in seismically active regions like Japan, but that doesn’t mean that we are even close to being able to predict when those earthquakes will occur. The sober reality is that there is no reliable, scientifically proven method of predicting where and when the next big earthquake will occur.

As Yogi Berra said, “It’s tough to make predictions, especially about the future.”

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