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The Earth's Crust And The Sun

Although the problems of this book may lead far afield, they ultimately

bring us back to the earth and to the present. Several times in the

preceding pages there has been mention of the fact that periods of

extreme climatic fluctuations are closely associated with great

movements of the earth's crust whereby mountains are uplifted and

continents upheaved. In attempting to explain this association the

general tendency h
s been to look largely at the past instead of the

present. Hence it has been almost impossible to choose among three

possibilities, all beset with difficulties. First, the movements of the

crust may have caused the climatic fluctuations; second, climatic

changes may cause crustal movements; and third, variations in solar

activity or in some other outside agency may give rise to both types of

terrestrial phenomena.

The idea that movements of the earth's crust are the main cause of

geological changes of climate is becoming increasingly untenable as the

complexity and rapidity of climatic changes become more clear,

especially during post-glacial times. It implies that the earth's

surface moves up and down with a speed and facility which appear to be

out of the question. If volcanic activity be invoked the problem becomes

no clearer. Even if volcanic dust should fill the air frequently and

completely, neither its presence nor absence would produce such peculiar

features as the localization of glaciers, the distribution of loess, and

the mild climate of most parts of geological time. Nevertheless, because

of the great difficulties presented by the other two possibilities many

geologists still hold that directly or indirectly the greater climatic

changes have been mainly due to movements of the earth's crust and to

the reaction of the crustal movements on the atmosphere.

The possibility that climatic changes are in themselves a cause of

movements of the earth's crust seems so improbable that no one appears

to have investigated it with any seriousness. Nevertheless, it is worth

while to raise the question whether climatic extremes may cooeperate with

other agencies in setting the time when the earth's crust shall be


As to the third possibility, it is perfectly logical to ascribe both

climatic changes and crustal deformation to some outside agency, solar

or otherwise, but hitherto there has been so little evidence on this

point that such an ascription has merely begged the question. If

heavenly bodies should approach the earth closely enough so that their

gravitational stresses caused crustal deformation, all life would

presumably be destroyed. As to the sun, there has hitherto been no

conclusive evidence that it is related to crustal movements, although

various writers have made suggestions along this line. In this chapter

we shall carry these suggestions further and shall see that they are at

least worthy of study.

As a preliminary to this study it may be well to note that the

coincidence between movements of the earth's crust and climatic changes

is not so absolute as is sometimes supposed. For example, the profound

crustal changes at the end of the Mesozoic were not accompanied by

widespread glaciation so far as is yet known, although the temperature

appears to have been lowered. Nor was the violent volcanic and

diastrophic activity in the Miocene associated with extreme climates.

Indeed, there appears to have been little contrast from zone to zone,

for figs, bread fruit trees, tree ferns, and other plants of low

latitudes grew in Greenland. Nevertheless, both at the end of the

Mesozoic and in the Miocene the climate may possibly have been severe

for a time, although the record is lost. On the other hand, Kirk's

recent discovery of glacial till in Alaska between beds carrying an

undoubted Middle Silurian fauna indicates glaciation at a time when

there was little movement of the crust so far as yet appears.[125] Thus

we conclude that while climatic changes and crustal movements usually

occur together, they may occur separately.

According to the solar-cyclonic hypothesis such a condition is to be

expected. If the sun were especially active when the terrestrial

conditions prohibited glaciation, changes of climate would still occur,

but they would be milder than under other circumstances, and would leave

little record in the rocks. Or there might be glaciation in high

latitudes, such as that of southern Alaska in the Middle Silurian, and

none elsewhere. On the other hand, when the sun was so inactive that no

great storminess occurred, the upheaval of continents and the building

of mountains might go on without the formation of ice sheets, as

apparently happened at the end of the Mesozoic. The lack of absolute

coincidence between glaciation and periods of widespread emergence of

the lands is evident even today, for there is no reason to suppose that

the lands are notably lower or less extensive now than they were during

the Pleistocene glaciation. In fact, there is much evidence that many

areas have risen since that time. Yet glaciation is now far less

extensive than in the Pleistocene. Any attempt to explain this

difference on the basis of terrestrial changes is extremely difficult,

for the shape and altitude of continents and mountains have not changed

much in twenty or thirty thousand years. Yet the present moderately mild

epoch, like the puzzling inter-glacial epochs of earlier times, is

easily explicable on the assumption that the sun's atmosphere may

sometimes vary in harmony with crustal activity, but does not

necessarily do so at all times.

Turning now to the main problem of how climatic changes may be connected

with movements of the earth's crust, let us follow our usual method and

examine what is happening today. Let us first inquire whether

earthquakes, which are one of the chief evidences that crustal movements

are actually taking place in our own times, show any connection with

sunspots. In order to test this, we have compared Milne's Catalogue of

Destructive Earthquakes from 1800 to 1899, with Wolf's sunspot numbers

for the same period month by month. The earthquake catalogue, as its

compiler describes it, "is an attempt to give a list of earthquakes

which have announced changes of geological importance in the earth's

crust; movements which have probably resulted in the creation or the

extension of a line of fault, the vibrations accompanying which could,

with proper instruments, have been recorded over a continent or the

whole surface of our world. Small earthquakes have been excluded, while

the number of large earthquakes both for ancient and modern times has

been extended. As an illustration of exclusion, I may mention that

between 1800 and 1808, which are years taken at random, I find in

Mallet's catalogue 407 entries. Only thirty-seven of these, which were

accompanied by structural damage, have been retained. Other catalogues

such as those of Perry and Fuchs have been treated similarly."[126]

If the earthquakes in such a carefully selected list bear a distinct

relation to sunspots, it is at least possible and perhaps probable that

a similar relation may exist between solar activity and geological

changes in the earth's crust. The result of the comparison of

earthquakes and sunspots is shown in Table 7. The first column gives the

sunspot numbers; the second, the number of months that had the

respective spot numbers during the century from 1800 to 1899. Column C

shows the total number of earthquakes during the months having any

particular degree of spottedness; while D, which is the significant

column, gives the average number of destructive earthquakes per month

under each of the six conditions of solar spottedness. The regularity of

column D is so great as to make it almost certain that we are here

dealing with a real relationship. Column F, which shows the average

number of earthquakes in the month succeeding any given condition of the

sun, is still more regular except for the last entry.



A: Sunspot numbers

B: Number of months per Wolf's Table

C: Number of earthquakes

D: Average number of earthquakes per month

E: Number of earthquakes in succeeding month

F: Average number of earthquakes in succeeding month


0-15 344 522 1.52 512 1.49

15-30 194 306 1.58 310 1.60

30-50 237 433 1.83 439 1.85

50-70 195 402 2.06 390 2.00

70-100 135 286 2.12 310 2.30

over 100 95 218 2.30 175 1.84

The chance that six numbers taken at random will arrange themselves in

any given order is one in 720. In other words, there is one chance in

720 that the regularity of column D is accidental. But column F is as

regular as column D except for the last entry. If columns D and E were

independent there would be one chance in about 500,000 that the six

numbers in both columns would fall in the same order, and one chance in

14,400 that five numbers in each would fall in the same order. But the

two columns are somewhat related, for although the after-shocks of a

great earthquake are never included in Milne's table, a world-shaking

earthquake in one region during a given month probably creates

conditions that favor similar earthquakes elsewhere during the next

month. Hence the probability that we are dealing with a purely

accidental arrangement in Table 7 is less than one in 14,400 and greater

than one in 500,000. It may be one in 20,000 or 100,000. In any event it

is so slight that there is high probability that directly or indirectly

sunspots and earthquakes are somehow connected.

In ascertaining the relation between sunspots and earthquakes it would

be well if we could employ the strict method of correlation

coefficients. This, however, is impossible for the entire century, for

the record is by no means homogeneous. The earlier decades are

represented by only about one-fourth as many earthquakes as the later

ones, a condition which is presumably due to lack of information. This

makes no difference with the method employed in Table 7, since years

with many and few sunspots are distributed almost equally throughout the

entire nineteenth century, but it renders the method of correlation

coefficients inapplicable. During the period from 1850 onward the record

is much more nearly homogeneous, though not completely so. Even in these

later decades, however, allowance must be made for the fact that there

are more earthquakes in winter than in summer, the average number per

month for the fifty years being as follows:

Jan. 2.8 May 2.4 Sept. 2.5

Feb. 2.4 June 2.3 Oct. 2.6

Mar. 2.5 July 2.4 Nov. 2.7

Apr. 2.4 Aug. 2.4 Dec. 2.8

The correlation coefficient between the departures from these monthly

averages and the corresponding departures from the monthly averages of

the sunspots for the same period, 1850-1899, are as follows:

Sunspots and earthquakes of same month: +0.042, or 1.5 times the

probable error.

Sunspots of a given month and earthquakes of that month and the

next: +0.084, or 3.1 times the probable error.

Sunspots of three consecutive months and earthquakes of three

consecutive months allowing a lag of one month, i.e., sunspots of

January, February, and March compared with earthquakes of February,

March, and April; sunspots of February, March, and April with

earthquakes of March, April, and May, etc.; +0.112, or 4.1 times the

probable error.

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 homogeneous. 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


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 relationship 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 passes 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 mass such as forms the interior of the earth. The question arises

whether internal electrical currents may not pass between the two

oppositely charged poles of the earth, or even between the great

continental masses 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 assumption 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 relationship is indicated,

while the ratio falls below three only in Japan 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.




A: Region

B: Limiting Dates

C: Number of Shocks

D: Maximum Month

E: Amplitude

F: Expected Amplitude

G: Ratio of Actual to Expected Amplitude


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


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

Japan 1878-1881 246 Dec. 0.460 0.113 4.1

Japan 1872-1880 367 Dec.-Jan. 0.256 0.093 2.8

Japan 1876-1891 1104 Feb. 0.190 0.053 3.6

Japan 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 relationship 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 Japan 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 passage 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 Japan, 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 Japan, 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.

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 latitudes 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 latitudes, 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 latitude 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 latitude 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

fashion illustrated 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 assumes 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 shift the earth several feet from its normal position in

respect to the axis.

(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 deg.

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 deg. and 25 deg., while in fifty-six cases it was from 25 deg. to 40 deg.. 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 deg..



No. of No. of Average No.

Deflection Deflections Earthquakes of Earthquakes

0-10 deg. 9 179 19.9

10-25 deg. 32 263 8.2

25-40 deg. 56 722 12.9

40-55 deg. 19 366 19.3

over 55 deg. 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.




Average angle of deflection Average daily number

(10 periods of 1/10 year each) of earthquakes

-10.5 deg. 8.31

11.5 deg. 8.35

25.8 deg. 8.23

40.2 deg. 8.14

54.7 deg. 8.86

90.3 deg. 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 deg. 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.

The earth as a whole appears to stand between steel and glass in

rigidity. It is a matter of common observation that rocks stand high in

this respect and in the consequent difficulty with which they can be

bent without breaking. Because of the earth's contraction the crust

endures a constant strain, which must gradually become enormous. This

strain is increased by the fact that sediment is transferred from the

lands to the borders of the sea and there forms areas of thick

accumulation. From this has arisen the doctrine of isostasy, or of the

equalization of crustal pressure. An important illustration of this is

the oceanward and equatorial creep which has been described in Chapter

XI. There we saw that when the lands have once been raised to high

levels or when a shortening of the earth's axis by contraction has

increased the oceanic bulge at the equator, or when the reverse has

happened because of tidal retardation, the outer part of the earth

appears to creep slowly back toward a position of perfect isostatic

adjustment. If the sun had no influence upon the earth, either direct or

indirect, isostasy and other terrestrial processes might flex the

earth's crust so gradually that changes in the form and height of the

lands would always take place slowly, even from the geological point of

view. Thus erosion would usually be able to remove the rocks as rapidly

as they were domed above the general level. If this happened, mountains

would be rare or unknown, and hence climatic contrasts would be far less

marked than is actually the case on our earth where crustal movements

have repeatedly been rapid enough to produce mountains.

Nature's methods rarely allow so gradual an adjustment to the forces of

isostasy. While the crust is under a strain, not only because of

contraction, but because of changes in its load through the transference

of sediments and the slow increase or decrease in the bulge at the

equator, the atmosphere more or less persistently carries on the tapping

process. The violence of that process varies greatly, and the variations

depend largely on the severity of the climatic contrasts. If the main

outlines of the cyclonic hypothesis are reliable, one of the first

effects of a disturbance of the sun's atmosphere is increased storminess

upon the earth. This is accompanied by increased intensity in almost

every meteorological process. The most important effect, however, so far

as the earth's crust is concerned would apparently be the rapid and

intense changes of atmospheric pressure which would arise from the swift

passage of one severe storm after another. Each storm would be a little

tap on the tensely strained crust. Any single tap might be of little

consequence, even though it involved a change of a billion tons in the

pressure on an area no larger than the state of Rhode Island. Yet a

rapid and irregular succession of such taps might possibly cause the

crust to crack, and finally to collapse in response to stresses arising

from the shrinkage of the earth.

Another and perhaps more important effect of variations in storminess

and especially in the location of the stormy areas would be an

acceleration of erosion in some places and a retardation elsewhere. A

great increase in rainfall may almost denude the slopes of soil, while a

diminution to the point where much of the vegetation dies off has a

similar effect. If such changes should take place rapidly, great

thicknesses of sediment might be concentrated in certain areas in a

short time, thus disturbing the isostatic adjustment of the earth's

crust. This might set up a state of strain which would ultimately have

to be relieved, thus perhaps initiating profound crustal movements.

Changes in the load of the earth's crust due to erosion and the

deposition of sediment, no matter how rapid they may be from the

geological standpoint, are slow compared with those due to changes in

barometric pressure. A drop of an inch in barometric pressure is

equivalent to the removal of about five inches of solid rock. Even under

the most favorable circumstances, the removal of an average depth of

five inches of rock or its equivalent in soil over millions of square

miles would probably take several hundred years, while the removal of a

similar load of air might occur in half a day or even a few hours. Thus

the erosion and deposition due to climatic variations presumably play

their part in crustal deformation chiefly by producing crustal stresses,

while the storms, as it were, strike sharp, sudden blows.

Suppose now that a prolonged period of world-wide mild climate, such as

is described in Chapter X, should permit an enormous accumulation of

stresses due to contraction and tidal retardation. Suppose that then a

sudden change of climate should produce a rapid shifting of the deep

soil that had accumulated on the lands, with a corresponding

localization and increase in strains. Suppose also that frequent and

severe storms play their part, whether great or small, by producing an

intensive tapping of the crust. In such a case the ultimate collapse

would be correspondingly great, as would be evident in the succeeding

geological epoch. The sea floor might sink lower, the continents might

be elevated, and mountain ranges might be shoved up along lines of

special weakness. This is the story of the geological period as known to

historical geology. The force that causes such movements would be the

pull of gravity upon the crust surrounding the earth's shrinking

interior. Nevertheless climatic changes might occasionally set the date

when the gravitative pull would finally overcome inertia, and thus usher

in the crustal movements that close old geologic periods and inaugurate

new ones. This, however, could occur only if the crust were under

sufficient strain. As Lawson[139] says in his discussion of the "elastic

rebound theory," the sudden shifts of the crust which seem to be the

underlying cause of earthquakes "can occur only after the accumulation

of strain to a limit and ... this accumulation involves a slow creep of

the region affected. In the long periods between great earthquakes the

energy necessary for such shocks is being stored up in the rocks as

elastic compression."

If a period of intense storminess should occur when the earth as a whole

was in such a state of strain, the sudden release of the strains might

lead to terrestrial changes which would alter the climate still further,

making it more extreme, and perhaps permitting the storminess due to the

solar disturbances to bring about glaciation. At the same time if

volcanic activity should increase it would add its quota to the tendency

toward glaciation. Nevertheless, it might easily happen that a very

considerable amount of crustal movement would take place without causing

a continental ice sheet or even a marked alpine ice sheet. Or again, if

the strains in the earth's crust had already been largely released

through other agencies before the stormy period began, the climate might

become severe enough to cause glaciation in high latitudes without

leading to any very marked movements of the earth's crust, as apparently

happened in the Mid-Silurian period.


[Footnote 125: E. Kirk: Paleozoic Glaciation in Alaska; Am. Jour. Sci.,

1918, p. 511.]

[Footnote 126: J. Milne: Catalogue of Destructive Earthquakes; Rep.

Brit. Asso. Adv. Sci., 1911.]

[Footnote 127: Wm. Bowie: Lecture before the Geological Club of Yale

University. See Am. Jour. Sci., 1921.]

[Footnote 128: Chas. Davisson: On the Annual and Semi-annual Seismic

Periods; Roy. Soc. of London, Philosophical Transactions, Vol. 184,

1893, 1107 ff.]

[Footnote 129: C. G. Knott: The Physics of Earthquake Phenomena, Oxford,


[Footnote 130: In Table 8 the first column indicates the region; the

second, the dates; and the third, the number of shocks. The fourth

column gives the month in which the annual maximum occurs when the crude

figures are smoothed by the use of overlapping six-monthly means. In

other words, the average for each successive six months has been placed

in the middle of the period. Thus the average of January to June,

inclusive, is placed between March and April, that for February to July

between April and May, and so on. This method eliminates the minor

fluctuations and also all periodicities having a duration of less than a

year. If there were no annual periodicity the smoothing would result in

practically the same figure for each month. The column marked

"Amplitude" gives the range from the highest month to the lowest divided

by the number of earthquakes and then corrected according to Schuster's

method which is well known to mathematicians, but which is so confusing

to the layman that it will not be described. Next, in the column marked

"Expected Amplitude," we have the amplitude that would be expected if a

series of numbers corresponding to the earthquake numbers and having a

similar range were arranged in accidental order throughout the year.

This also is calculated by Schuster's method in which the expected

amplitude is equal to the square root of "pi" divided by the number of

shocks. When the actual amplitude is four or more times the expected

amplitude, the probability that there is a real periodicity in the

observed phenomena becomes so great that we may regard it as practically

certain. If there is no periodicity the two are equal. The last column

gives the number of times by which the actual exceeds the expected

amplitude, and thus is a measure of the probability that earthquakes

vary systematically in a period of a year.]

[Footnote 131: N. F. Drake: Destructive Earthquakes in China; Bull.

Seism. Soc. Am., Vol. 2, 1912, pp. 40-91, 124-133.]

[Footnote 132: The only other explanation that seems to have any

standing is the psychological hypothesis of Montessus de Ballore as

given in Les Tremblements de Terre. He attributes the apparent seasonal

variation in earthquakes to the fact that in winter people are within

doors, and hence notice movements of the earth much more than in summer

when they are out of doors. There is a similar difference between

people's habits in high latitudes and low. Undoubtedly this does have a

marked effect upon the degree to which minor earthquake shocks are

noticed. Nevertheless, de Ballore's contention, as well as any other

psychological explanation, is completely upset by two facts: First,

instrumental records show the same seasonal distribution as do records

based on direct observation, and instruments certainly are not

influenced by the seasons. Second, in some places, notably China, as

Drake has shown, the summer rather than the winter is very decidedly the

time when earthquakes are most frequent.]

[Footnote 133: A comparison of tropical hurricanes with earthquakes is

interesting. Taking all the hurricanes recorded in August, September,

and October, from 1880 to 1899, and the corresponding earthquakes in

Milne's catalogue, the correlation coefficient between hurricanes and

earthquakes is +0.236, with a probable error of +-0.082, the month being

used as the unit. This is not a large correlation, yet when it is

remembered that the hurricanes represent only a small part of the

atmospheric disturbances in any given month, it suggests that with

fuller data the correlation might be large.]

[Footnote 134: Ellsworth Huntington: The Geographic Work of Dr. M. A.

Veeder; Geog. Rev., Vol. 3, March and April, 1917, Nos. 3 and 4.]

[Footnote 135: Frank Schlesinger: Variations of Latitude; Their Bearing

upon Our Knowledge of the Interior of the Earth; Proc. Am. Phil. Soc.,

Vol. 54, 1915, pp. 351-358. Also Smithsonian Report for 1916, pp.


[Footnote 136: Harold Jeffreys: Causes Contributory to the Annual

Variations of Latitude; Monthly Notices, Royal Astronomical Soc., Vol.

76, 1916, pp. 499-525.]

[Footnote 137: John Milne: British Association Reports for 1903 and


[Footnote 138: C. G. Knott: The Physics of Earthquake Phenomena, Oxford,


[Footnote 139: A. C. Lawson: The Mobility of the Coast Ranges of

California; Univ. of Calif. Pub., Geology, Vol. 12, No. 7, pp. 431-473.]