Geoscience Reference
In-Depth Information
never be possible to predict earthquakes with certainty, predictions in terms of probability
are getting better and better; the closer to the time of the quake, the better the accuracy of
the forecast.
Traditional signs
Long before the age of scientific instruments, people had been looking for early warnings
of an impending quake. The Chinese in particular have become quite adept at noticing
strange animal behaviour, sudden changes of water level and gas content in wells, and other
signs that could prelude a quake. Using such indicators, the City of Haicheng was evacu-
ated in 1975, hours before a devastating earthquake, saving hundreds of thousands of lives.
But, a year later 240,000 people died in Tengshan, where no warning had been given. Other
clues may include tiny flashes of light and electricity, possibly produced as mineral crystals
are squeezed, in the same way that squeezing a piezo-electric gas lighter produces a spark.
There is serious research into how animals are able to sense an imminent quake; for ex-
ample, in Japan to see if catfish may behave abnormally due to electrical disturbances. But
what constitutes abnormal behaviour in a catfish? And how many householders will mon-
itor one? There's also some evidence that a big quake may be preceded by very low-fre-
quency electromagnetic waves. But the best indicators seem to be in the pattern of seismic
waves running through the ground.
Playing the odds
Most big earthquakes are preceded by foreshocks. The trouble is, it's hard to say whether a
minor earth tremor is an isolated event or the prelude to a major quake. But it can change
the odds. From historical records, it may be possible to say that a big quake is likely some-
time in the next 100 years. But that puts the chance of an earthquake happening tomorrow
at one in 36,500. There maybe ten minor tremors a year, any of which could be the fore-
shock heralding a big quake. So the detection of a minor tremor increases the probability
of the quake in the next 24 hours to one in 1,000. By understanding where all the faults are,
when they last cracked, and by having instruments in all the right places, it is sometimes
possible to increase the accuracy of the prediction to one in 20. But that is still the same as
saying that there is a 95% chance that there
won't
be a quake tomorrow - hardly a statistic
to announce over the radio and evacuate a city. It may, however, be sufficient to alert emer-
gency services and to stop the transport of hazardous chemicals.
Major earthquakes
