Fig. 13. Wandering of the pole from 1890 to 1898. (After Moulton.)
If the foregoing reasoning is correct, the great and especially the sudden departures from the smooth gyroscopic circle described by the pole in the Eulerian motion would be expected to occur at about the same time as unusual earthquake activity. This brings us to an interesting inquiry carried out by Milne[141] and amplified by Knott.[142] Taking Albrecht's representation of the irregular spiral-like motion of the pole, as given in Fig. 13, they show that there is a preponderance of severe earthquakes at times when the direction of motion of the earth in reference to its axis departs from the smooth Eulerian curve. A summary of their results is given in Table 9. The table indicates that during the period from 1892 to 1905 there were nine different times when the curve of Fig. 13 changed its direction or was deflected by less than 10° during a tenth of a year. In other words, during those periods it did not curve as much as it ought according to the Eulerian movement. At such times there were 179 world-shaking earthquakes, or an average of about 19.9 per tenth of a year. According to the other lines of Table 9, in thirty-two cases the deflection during a tenth of a year was between 10° and 25°, while in fifty-six cases it was from 25° to 40°. During these periods the curve remained close to the Eulerian path and the world-shaking earthquakes averaged only 8.2 and 12.9. Then, when the deflection was high, that is, when meteorological conditions threw the earth far out of its Eulerian course, the earthquakes were again numerous, the number rising to 23.4 when the deflection amounted to more than 55°.
| [TABLE 9] | |||
|---|---|---|---|
| DEFLECTION OF PATH OF POLE COMPARED WITH EARTHQUAKES | |||
| Deflection | No. of Deflections | No. of Earthquakes | Average No. of Earthquakes |
| 0-10° | 9 | 179 | 19.9 |
| 10-25° | 32 | 263 | 8.2 |
| 25-40° | 56 | 722 | 12.9 |
| 40-55° | 19 | 366 | 19.3 |
| over 55° | 7 | 164 | 23.4 |
In order to test this conclusion in another way we have followed a suggestion of Professor Schlesinger. Under his advice the Eulerian motion has been eliminated and a new series of earthquake records has been compared with the remaining motions of the poles which presumably arise largely from meteorological causes. For this purpose use has been made of the very full records of earthquakes published under the auspices of the International Seismological Commission for the years 1903 to 1908, the only years for which they are available. These include every known shock of every description which was either recorded by seismographs or by direct observation in any part of the world. Each shock is given the same weight, no matter what its violence or how closely it follows another. The angle of deflection has been measured as Milne measured it, but since the Eulerian motion is eliminated, our zero is approximately the normal condition which would prevail if there were no meteorological complications. Dividing the deflections into six equal groups according to the size of the angle, we get the result shown in Table 10.
| [TABLE 10] | |
|---|---|
| EARTHQUAKES IN 1903-1908 COMPAREDWITH DEPARTURES OF THE PROJECTEDCURVE OF THE EARTH'SAXIS FROM THE EULERIANPOSITION | |
| Average angle of deflection (10 periods of 1/10 year each) | Average daily number of earthquakes |
| -10.5° | 8.31 |
| 11.5° | 8.35 |
| 25.8° | 8.23 |
| 40.2° | 8.14 |
| 54.7° | 8.86 |
| 90.3° | 11.81 |
Here where some twenty thousand earthquakes are employed the result agrees closely with that of Milne for a different series of years and for a much smaller number of earthquakes. So long as the path of the pole departs less than about 45° from the smooth gyroscopic Eulerian path, the number of earthquakes is almost constant, about eight and a quarter per day. When the angle becomes large, however, the number increases by nearly 50 per cent. Thus the work of Milne, Knott, and Jeffreys is confirmed by a new investigation. Apparently earthquakes and crustal movements are somehow related to sudden changes in the load imposed on the earth's crust by meteorological conditions.
This conclusion is quite as surprising to the authors as to the reader—perhaps more so. At the beginning of this investigation we had no faith whatever in any important
relation between climate and earthquakes. At its end we are inclined to believe that the relation is close and important.
It must not be supposed, however, that meteorological conditions are the cause of earthquakes and of movements of the earth's crust. Even though the load that the climatic agencies can impose upon the earth's crust runs into millions of tons per square mile, it is a trifle compared with what the crust is able to support. There is, however, a great difference between the cause and the occasion of a phenomenon. Suppose that a thick sheet of glass is placed under an increasing strain. If the strain is applied slowly enough, even so rigid a material as glass will ultimately bend rather than break. But suppose that while the tension is high the glass is tapped. A gentle tap may be followed by a tiny crack. A series of little taps may be the signal for small cracks to spread in every direction. A few slightly harder taps may cause the whole sheet to break suddenly into many pieces. Yet even the hardest tap may be the merest trifle compared with the strong force which is keeping the glass in a state of strain and which would ultimately bend it if given time.