Earthquakes are geophysical phenomena, and have no known connection with the weather. None the less, in bygone times there were persistent theories that they were often associated with certain meteorological conditions. Aristotle, for example, states authoritatively that "earthquakes are sometimes preceded during the day, or after sundown in clear weather, by a thin layer of cloud that spreads out into space."
Charles Darwin was more circumspect. He devotes a considerable amount of space to such theories in The Voyage of the Beagle, his personal account of the events which led to his adoption of the theory of evolution, and gives a qualified endorsement to a theory current at the time. "There appears much probability in the view that when the barometer is low, and when rain might naturally be expected to occur, the diminished pressure of the atmosphere over a wide extent of country might well determine the precise day on which the Earth, already stretched to the utmost by subterranean forces, should yield, crack, and consequently tremble."
This concept, that atmospheric conditions may sometimes be a seismic catalyst, and that changes in atmospheric pressure may trigger the beginning of a tremor, has been resurrected in Japan in recent years. The Japanese archipelago is in one of the most active seismic regions in the world. It lies at the interface between the great "Eurasian" and "Pacific" plates. And looking back over centuries of records, a seismologist called Masakazu Outake has noticed that all 13 major Japanese earthquakes between AD 684 and 1946 have occurred during the autumn and winter - a seasonal bias against whose happening by chance, it seems, the odds are something like 1000 to one.
Outake, however, has also noted that the average monthly atmospheric pressure in the region is about 10 millibars higher between August and February than it is in the other half of the year: he believes this may have something to do with the frequency of serious tremors in the winter months.
He points out that in the vicinity of Japan the Pacific plate is sliding like a wedge westwards and downwards beneath the Eurasian plate adjacent to it. The Japanese islands are situated on this latter upper plate, and the extra 10 millibars of pressure on them during the winter is the equivalent of placing a 15 stone weight on each square yard of territory. Variations in the atmospheric pressure, moreover, affect the force exerted by the upper on the lower plate, and may at times, the theory goes, be sufficient to trigger a sudden relative movement of the two - in other words, an earthquake.