Part 17 (1/2)
These coefficients are all small, but the number of individual cases, 600 months, is so large that the probable error is greatly reduced, being only 0.027 or 0.028. Moreover, the nature of our data is such that even if there is a strong connection between solar changes and earth movements, we should not expect a large correlation coefficient.
In the first place, as already mentioned, the earthquake data are not strictly h.o.m.ogeneous. Second, an average of about two and one-half strong earthquakes per month is at best only a most imperfect indication of the actual movement of the earth's crust. Third, the sunspots are only a partial and imperfect measure of the activity of the sun's atmosphere. Fourth, the relation between solar activity and earthquakes is almost certainly indirect. In view of all these conditions, the regularity of Table 7 and the fact that the most important correlation coefficient rises to more than four times the probable error makes it almost certain that the solar and terrestrial phenomena are really connected.
We are now confronted by the perplexing question of how this connection can take place. Thus far only three possibilities present themselves, and each is open to objections. The chief agencies concerned in these three possibilities are heat, electricity, and atmospheric pressure.
Heat may be dismissed very briefly. We have seen that the earth's surface becomes relatively cool when the sun is active. Theoretically even the slightest change in the temperature of the earth's surface must influence the thermal gradient far into the interior and hence cause a change of volume which might cause movements of the crust. Practically the heat of the surface ceases to be of appreciable importance at a depth of perhaps twenty feet, and even at that depth it does not act quickly enough to cause the relatively prompt response which seems to be characteristic of earthquakes in respect to the sun.
The second possibility is based on the relations.h.i.+p between solar and terrestrial electricity. When the sun is active the earth's atmospheric electrical potential is subject to slight variations. It is well known that when two opposing points of an ionized solution are oppositely charged electrically, a current pa.s.ses through the liquid and sets up electrolysis whereby there is a segregation of materials, and a consequent change in the volume of the parts near the respective electrical poles. The same process takes place, although less freely, in a hot ma.s.s such as forms the interior of the earth. The question arises whether internal electrical currents may not pa.s.s between the two oppositely charged poles of the earth, or even between the great continental ma.s.ses and the regions of heavier rock which underlie the oceans. Could this lead to electrolysis, hence to differentiation in volume, and thus to movements of the earth's crust? Could the results vary in harmony with the sun? Bowie[127] has shown that numerous measurements of the strength and direction of the earth's gravitative pull are explicable only on the a.s.sumption that the upheaval of a continent or a mountain range is due in part not merely to pressure, or even to flowage of the rocks beneath the crust, but also to an actual change in volume whereby the rocks beneath the continent attain relatively great volume and those under the oceans a small volume in proportion to their weight. The query arises whether this change of volume may be related to electrical currents at some depth below the earth's surface.
The objections to this hypothesis are numerous. First, there is little evidence of electrolytic differentiation in the rocks. Second, the outer part of the earth's crust is a very poor conductor so that it is doubtful whether even a high degree of electrification of the surface would have much effect on the interior. Third, electrolysis due to any such mild causes as we have here postulated must be an extremely slow process, too slow, presumably, to have any appreciable result within a month or two. Other objections join with these three in making it seem improbable that the sun's electrical activity has any direct effect upon movements of the earth's crust.
The third, or meteorological hypothesis, which makes barometric pressure the main intermediary between solar activity and earthquakes, seems at first sight almost as improbable as the thermal and electrical hypotheses. Nevertheless, it has a certain degree of observational support of a kind which is wholly lacking in the other two cases. Among the extensive writings on the periodicity of earthquakes one main fact stands out with great distinctness: earthquakes vary in number according to the season. This fact has already been shown incidentally in the table of earthquake frequency by months. If allowance is made for the fact that February is a short month, there is a regular decrease in the frequency of severe earthquakes from December and January to June. Since most of Milne's earthquakes occurred in the northern hemisphere, this means that severe earthquakes occur in winter about 20 per cent oftener than in summer.
The most thorough investigation of this subject seems to have been that of Davisson.[128] His results have been worked over and amplified by Knott,[129] who has tested them by Schuster's exact mathematical methods. His results are given in Table 8.[130] Here the northern hemisphere is placed first; then come the East Indies and the Malay Archipelago lying close to the equator; and finally the southern hemisphere. In the northern hemisphere practically all the maxima come in the winter, for the month of December appears in fifteen cases out of the twenty-five in column D, while January, February, or November appears in six others. It is also noticeable that in sixteen cases out of twenty-five the ratio of the actual to the expected amplitude in column G is four or more, so that a real relations.h.i.+p is indicated, while the ratio falls below three only in j.a.pan and Zante. The equatorial data, unlike those of the northern hemisphere, are indefinite, for in the East Indies no month shows a marked maximum and the expected amplitude exceeds the actual amplitude. Even in the Malay Archipelago, which shows a maximum in May, the ratio of actual to expected amplitude is only 2.6. Turning to the southern hemisphere, the winter months of that hemisphere are as strongly marked by a maximum as are the winter months of the northern hemisphere. July or August appears in five out of six cases. Here the ratio between the actual and expected amplitudes is not so great as in the northern hemisphere. Nevertheless, it is practically four in Chile, and exceeds five in Peru and Bolivia, and in the data for the entire southern hemisphere.
TABLE 8
SEASONAL MARCH OF EARTHQUAKES
AFTER DAVISSON AND KNOTT
A: _Region_ B: _Limiting Dates_ C: _Number of Shocks_ D: _Maximum Month_ E: _Amplitude_ F: _Expected Amplitude_ G: _Ratio of Actual to Expected Amplitude_
A B C D E F G
Northern Hemisphere 223-1850 5879 Dec. 0.110 0.023 4.8 Northern Hemisphere 1865-1884 8133 Dec. 0.290 0.020 14.5 Europe 1865-1884 5499 Dec. 0.350 0.024 14.6 Europe 306-1843 1961 Dec. 0.220 0.040 5.5 Southeast Europe 1859-1887 3470 Dec. 0.210 0.030 7.0 Vesuvius District 1865-1883 513 Dec. 0.250 0.078 3.2 Italy: Old Tromometre 1872-1887 61732 Dec. 0.490 0.007 70.0 Old Tromometre 1876-1887 38546 Dec. 0.460 0.009 49.5 Normal Tromometre 1876-1887 38546 Dec. 0.490 0.009 52.8 Balkan, etc. 1865-1884 624 Dec. 0.270 0.071 3.8 Hungary, etc. 1865-1884 384 Dec. 0.310 0.090 3.4 Italy 1865-1883 2350 Dec.(Sept.)0.140 0.037 3.8 Grecian Archip. 1859-1881 3578 Dec.-Jan. 0.164 0.030 5.5 Austria 1865-1884 461 Jan. 0.370 0.083 4.4 Switzerland, etc. 1865-1883 524 Jan. 0.560 0.077 7.3 Asia 1865-1884 458 Feb. 0.330 0.083 4.0 North America 1865-1884 552 Nov. 0.350 0.075 4.7 California 1850-1886 949 Oct. 0.300 0.058 5.2 j.a.pan 1878-1881 246 Dec. 0.460 0.113 4.1 j.a.pan 1872-1880 367 Dec.-Jan. 0.256 0.093 2.8 j.a.pan 1876-1891 1104 Feb. 0.190 0.053 3.6 j.a.pan 1885-1889 2997 Oct. 0.080 0.032 2.5 Zante 1825-1863 1326 Aug. 0.100 0.049 2.0 Italy, North 1865-1883 1513 Sept.(Nov.) 0.210 0.046 4.6 of Naples East Indies 1873-1881 515 Aug., Oct., 0.071? 0.078 0.9 or Dec.?
Malay Archip. 1865-1884 598 May 0.190 0.072 2.6 New Zealand 1869-1879 585 Aug.-Sept. 0.203 0.073 2.8 Chile 1873-1881 212 July 0.480 0.122 3.9 Southern Hemisphere 1865-1884 751 July 0.370 0.065 5.7 New Zealand 1868-1890 641 March, May 0.050 0.070 0.7 Chile 1865-1883? 316 July, Dec. 0.270 0.100 2.7 Peru, Bolivia 1865-1884 350 July 0.480 0.095 5.1
The whole relations.h.i.+p between earthquakes and the seasons in the northern and southern hemispheres is summed up in Fig. 12 taken from Knott. The northern hemisphere shows a regular diminution in earthquake frequency from December until June, and an increase the rest of the year. In the southern hemisphere the course of events is the same so far as summer and winter are concerned, for August with its maximum comes in winter, while February with its minimum comes in summer. In the southern hemisphere the winter month of greatest seismic activity has over 100 per cent more earthquakes than the summer month of least activity. In the northern hemisphere this difference is about 80 per cent, but this smaller figure occurs partly because the northern data include certain interesting and significant regions like j.a.pan and China where the usual conditions are reversed.[131] If equatorial regions were included in Fig. 12, they would give an almost straight line.
The connection between earthquakes and the seasons is so strong that almost no students of seismology question it, although they do not agree as to its cause. A meteorological hypothesis seems to be the only logical explanation.[132] Wherever sufficient data are available, earthquakes appear to be most numerous when climatic conditions cause the earth's surface to be most heavily loaded or to change its load most rapidly. The main factor in the loading is apparently atmospheric pressure. This acts in two ways. First, when the continents become cold in winter the pressure increases. On an average the air at sea level presses upon the earth's surface at the rate of 14.7 pounds per square inch, or over a ton per square foot, and only a little short of thirty million tons per square mile. An average difference of one inch between the atmospheric pressure of summer and winter over ten million square miles of the continent of Asia, for example, means that the continent's load in winter is about ten million million tons heavier than in summer.
Second, the changes in atmospheric pressure due to the pa.s.sage of storms are relatively sharp and sudden. Hence they are probably more effective than the variations in the load from season to season. This is suggested by the rapidity with which the terrestrial response seems to follow the supposed solar cause of earthquakes. It is also suggested by the fact that violent storms are frequently followed by violent earthquakes.
”Earthquake weather,” as Dr. Schlesinger suggests, is a common phrase in the typhoon region of j.a.pan, China, and the East Indies. During tropical hurricanes a change of pressure amounting to half an inch in two hours is common. On September 22, 1885, at False Point Lighthouse on the Bay of Bengal, the barometer fell about an inch in six hours, then nearly an inch and a half in not much over two hours, and finally rose fully two inches inside of two hours. A drop of two inches in barometric pressure means that a load of about two million tons is removed from each square mile of land; the corresponding rise of pressure means the addition of a similar load. Such a storm, and to a less degree every other storm, strikes a blow upon the earth's surface, first by removing millions of tons of pressure and then by putting them on again.[133] Such storms, as we have seen, are much more frequent and severe when sunspots are numerous than at other times. Moreover, as Veeder[134] long ago showed, one of the most noteworthy evidences of a connection between sunspots and the weather is a sudden increase of pressure in certain widely separated high pressure areas. In most parts of the world winter is not only the season of highest pressure and of most frequent changes of Veeder's type, but also of severest storms. Hence a meteorological hypothesis would lead to the expectation that earthquakes would occur more frequently in winter than in summer. On the Chinese coast, however, and also on the oceanic side of j.a.pan, as well as in some more tropical regions, the chief storms come in summer in the form of typhoons. These are the places where earthquakes also are most abundant in summer. Thus, wherever we turn, storms and the related barometric changes seem to be most frequent and severe at the very times when earthquakes are also most frequent.
[Ill.u.s.tration: _Fig. 12. Seasonal distribution of earthquakes. (After Davisson and Knott.)_
solid line ---- Northern Hemisphere.
dashed line .... Southern Hemisphere.]
Other meteorological factors, such as rain, snow, winds, and currents, probably have some effect on earthquakes through their ability to load the earth's crust. The coming of vegetation may also help. These agencies, however, appear to be of small importance compared with the storms. In high lat.i.tudes and in regions of abundant storminess most of these factors generally combine with barometric pressure to produce frequent changes in the load of the earth's crust, especially in winter.
In low lat.i.tudes, on the other hand, there are few severe storms, and relatively little contrast in pressure and vegetation from season to season; there is no snow; and the amount of ground water changes little.
With this goes the twofold fact that there is no marked seasonal distribution of earthquakes, and that except in certain local volcanic areas, earthquakes appear to be rare. In proportion to the areas concerned, for example, there is little evidence of earthquakes in equatorial Africa and South America.
The question of the reality of the connection between meteorological conditions and crustal movements is so important that every possible test should be applied. At the suggestion of Professor Schlesinger we have looked up a very ingenious line of inquiry. During the last decades of the nineteenth century, a long series of extremely accurate observations of lat.i.tude disclosed a fact which had previously been suspected but not demonstrated, namely, that the earth wabbles a little about its axis. The axis itself always points in the same direction, and since the earth slides irregularly around it the lat.i.tude of all parts of the earth keeps changing. Chandler has shown that the wabbling thus induced consists of two parts. The first is a movement in a circle with a radius of about fifteen feet which is described in approximately 430 days. This so-called Eulerian movement is a normal gyroscopic motion like the slow gyration of a spinning top. This depends on purely astronomical causes, and no terrestrial cause can stop it or eliminate it. The period appears to be constant, but there are certain puzzling irregularities. The usual amplitude of this movement, as Schlesinger[135] puts it, ”is about 0”.27, but twice in recent years it has jumped to 0”.40. Such a change could be accounted for by supposing that the earth had received a severe blow or a series of milder blows tending in the same direction.” These blows, which were originally suggested by Helmert are most interesting in view of our suggestion as to the blows struck by storms.
The second movement of the pole has a period of a year, and is roughly an ellipse whose longest radius is fourteen feet and the shortest, four feet; or, to put it technically, there is an annual term with a maximum amplitude of about 0”.20. This, however, varies irregularly. The result is that the pole seems to wander over the earth's surface in the spiral fas.h.i.+on ill.u.s.trated in Fig. 13. It was early suggested that this peculiar wandering of the pole in an annual period must be due to meteorological causes. Jeffreys[136] has investigated the matter exhaustively. He a.s.sumes certain reasonable values for the weight of air added or subtracted from different parts of the earth's surface according to the seasons. He also considers the effect of precipitation, vegetation, and polar ice, and of variations of temperature and atmospheric pressure in their relation to movements of the ocean. Then he proceeds to compare all these with the actual wandering of the pole from 1907 to 1913. While it is as yet too early to say that any special movement of the pole was due to the specific meteorological conditions of any particular year, Jeffreys' work makes it clear that meteorological causes, especially atmospheric pressure, are sufficient to cause the observed irregular wanderings. Slight wanderings may arise from various other sources such as movements of the rocks when geological faults occur or the rush of a great wave due to a submarine earthquake. So far as known, however, all these other agencies cause insignificant displacements compared with those arising from movements of the air. This fact coupled with the mathematical certainty that meteorological phenomena must produce some wandering of the pole, has caused most astronomers to accept Jeffreys' conclusion. If we follow their example we are led to conclude that changes in atmospheric pressure and in the other meteorological conditions strike blows which sometimes s.h.i.+ft the earth several feet from its normal position in respect to the axis.
[Ill.u.s.tration: _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[137] and amplified by Knott.[138] 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.