That the rise of the tide is accompanied by a greater percolation of water to volcanic foci, which, in consequence, assume a greater state of activity, is a theory which was advanced many years ago. To determine how far tides may directly be connected with earthquakes, the necessary records have yet to be examined.
Variations in atmospheric pressure.—When we consider the immense load which, by a sudden rise of the barometer, is placed upon the area over which this rise takes place, it is not difficult to imagine that this rise may occasionally be the final cause which makes the crust of the earth to give way. A barometric rise of an inch is equivalent to a load of about seventy-two pounds being put upon every square foot of area over which this rise takes place. On the other hand, a fall in the barometric column indicates that a load has been removed, and whatever elastic effort may be exerted by subterranean forces in endeavouring to escape, being met by less resistance, they may burst these bonds, and an earthquake will result. For reasons such as these the final cause of earthquakes has often been attributed to variations in atmospheric pressure. In Japan there are practically as many earthquakes with a high barometer as with a low one.
The extent to which barometric fluctuations have acted as final causes in the production of earthquakes may be judged of by a comparison of the times of barometric variation and the times at which earthquakes have occurred.
Three important laws of barometric variation are the following:—
1. In the world generally the average barometric pressure is highest in winter. (Exceptions occur near Iceland and in the North Pacific.)
2. The summer and winter monthly mean barometer differs least near the equator and over the great oceans. They differ most over the great continents and generally with increasing latitude.
3. The greatest number of barometrical fluctuations usually take place in winter.
Inasmuch as there are generally more earthquakes in winter than in summer, the first of these laws would indicate that this might be due to the greater load which acts upon the crust of the earth at that season. The second law would indicate that the distinction between the winter and summer earthquakes ought to be most marked in high latitudes, which, if we refer to the table on p. 257, we observe to be borne out by the results of observation. The countries where there are as many earthquakes in winter as in summer are chiefly those in low latitudes. The number of these countries from which we have records are, however, few.
Facts opposed to the idea that earthquakes may be caused by an increase of barometric pressure are the results of observations like those of Schmidt and Rossi, which show that earthquakes chiefly occur with a low barometer.
Assuming that these latter observations will be found by future investigators to be generally true, we must conclude that the relief of atmospheric pressure has an influence upon the occurrence of earthquakes. Such a conclusion would partially accord with the third barometrical law, or the fact that there are more occasions on which we get a low barometer during the winter months.