EARTH PULSATIONS.
Definition of an earth pulsation—Indications of
pendulums—Indications of levels—Other phenomena indicating
the existence of earth pulsations—Disturbances in lakes and
oceans—Phenomena resultant on earth pulsations—Cause of earth
pulsations.
The object of the present chapter is to show that from time to time it
is very probable that slow but large wave-like undulations travel over
or disturb the surface of the globe.
These movements, which have escaped our attention on account of their
slowness in period, for want of another term I call earth pulsations.
The existence of movements such as these may be indicated to us by
changes in the level of bodies of water like seas and lakes, by the
movements of delicate levels, by the displacement of the bob of a
pendulum relatively to some point on the earth above which it hangs,
and by other phenomena which will be enumerated.
_Indication of pendulums._—Pendulums which have been suspended for the
purposes of seismometrical observations have, both by observers in
Italy and Japan, been seen to have moved a short distance out from, and
then back to, their normal position.
This motion has simply taken place on one side of their central
position, and is not due to a swing. The character of these records
is such that we might imagine the soil on which the support of the
pendulum had rested to have been slowly tilted, and slowly lowered.
They are the most marked on those pendulums provided with an index
writing a record of its motions on a smoked glass plate, which index is
so arranged that it gives a multiplied representation of the relative
motion between it and the earth. As motions of this sort might be
possibly due to the action of moisture in the soil tilting the support
of the pendulum, and to a variety of other accidental causes, we cannot
insist on them as being certain indications that there are slow tips in
the soil, but for the present allow them to remain as possible proof of
such phenomena.
Evidence of displacement of the vertical, which are more definite than
the above, are those made by Bertelli, Rossi, Count Malvasia, and other
Italian observers, who, whilst recording earth tremors, have spent so
much time in watching the vibrations of stiles of delicate pendulums
by means of microscopes. As a result of these observations we are
told that the point about which the stile of a pendulum oscillates
is variable. These displacements take place in various azimuths, and
they appear to be connected with changes of the barometer. I have made
similar observations in Japan.
From this, and from the fact that it is found that a number of
different pendulums differently situated on the same area give similar
evidence of these movements, it would hardly seem that these phenomena
could be attributed to causes like changes in temperature and moisture.
M. S. di Rossi lays stress on this point, especially in connection with
his microseismograph, where there are a number of pendulums of unequal
length which give indications of a like character. The direction in
which these tips of the soil take place—which phenomena are noticeable
in seismic as well as microseismic motions—Rossi states are related to
the direction of certain lines of faulting.
_Indications of levels._—Bubbles of delicate levels can be easily seen
to change their position with meteorological variations; but Rossi also
tells us that they change their position, sometimes not to return for
a long time, during a microseismic storm. Here again we have another
phenomenon pointing to the fact that microseismic disturbances are the
companions of slow alterations in level.
One of the most patient observers of levels has been M. Plantamour,
who commenced his observations in 1878, at Sécheron, on the Lake of
Geneva. He used two levels, one placed north and south, and the other
east and west. During the summer of 1878 the east end rose, but at the
end of September a depression set in. The diurnal movements had their
maximum and minimum at 6 and 7.45 A.M. and P.M. The total amplitude was
4·89″. The variations of the east and west level appeared to be due to
the temperature, but the movements of the north and south level were
dependent upon an unknown cause.
Between October 1, 1879, and September 30, 1880, the east end fell
rapidly, from the middle of November up to December 26, amounting to
88·71″. It then rose 6·55″ to January 5, and then fell again. On
January 28 it reached 89·95″, after which it rose.
Between October 4, 1879, and January 28, 1880, the movement was 95·8″,
against 28·08″ of the previous year.
These movements were not due alone to temperature. The north and south
level, which was not influenced by the cold of the winter, moved
4·56″. In the previous year 4·89″.[144]
From February 17 to June 5, 1883, the author observed in Tokio the
bubbles of two delicate levels, one placed north and south, and the
other east and west. They were placed under glass cases on the head of
a stone column. The column, which is inside a brick building, rests
on a concrete foundation, and is about ten years old. It is in no way
connected with the building. The temperature of the room has a daily
variation of about 1° Fahr.
In both these levels diurnal changes are very marked. Occasionally they
are enormously great. Thus, on March 25, the readings of the south end
of the north south level were as follows:—
Time Readings.
h. m.
25th. 4 00 P.M. 104·5
4 5 „ 103
4 10 „ 102
4 25 „ 101
4 30 „ 100
4 40 „ 98
4 42 „ 99·5
4 45 „ 100
4 50 „ 101
4 55 „ 101
5 00 „ 100
26th. 7 00 A.M. 105
Usually this level moves through about three divisions per day.
From March 25 to May 4 it travelled from 98 to 127. Since then, to June
5, it has descended to 116. During this period the east west level
has been _comparatively_ quiet. One division of the north south level
equals about 2″ of arc.
Many of these changes may be due to changes in temperature, variations
in moisture, and other local actions. Some of them, however, are hardly
explicable on such assumptions. The fact that the general direction in
change of the vertical, as indicated by a tromometer standing on the
same column with the levels, showed that the change which was taking
place was rather in the column than in the instruments.
The fact also that at the time of a barometrical depression a
_pulse-like surge_ can be seen in the levels, having a period averaging
about three seconds and sometimes amounting to about one second of
arc, is a phenomenon hardly to be attributed to sudden fluctuations in
moisture or temperature, but indicates real changes in level.[145]
In addition to variation in the bubbles of levels which come on more or
less gradually, we have many recorded instances of _sudden_ alterations
taking place in these instruments.
Examples of what may have been a slow oscillating motion of the earth’s
crust are referred to by Mr. George Darwin in a Report to the British
Association in 1882.
One of them was made by M. Magnus Nyrén, at Pulkova, who, when engaged
in levelling the axis of a telescope, observed spontaneous oscillation
in the bulb of the level.
This was on May 10 (April 28), 1877. The complete period was about
20 seconds, the amplitude being 1·5″ and 2″. One hour and fourteen
minutes before this he observes that there had been a severe earthquake
at Iquique, the distance to which in a straight line was 10,600
kilomètres, and on an arc of a great circle, 12,500 kilomètres.
On September 20 (8), in 1867, Mr. Wagner had observed at Pulkova
oscillations of 3″, seven minutes before which there had been an
earthquake at Malta. On April 4 (March 23), 1868, an agitation of the
level had been observed by Mr. Gromadzki, five minutes before which
there had been an earthquake in Turkestan. Similar observations had
been made twice before. These, however, had not been connected with any
earthquakes—at least, Mr. Darwin remarks—with certainty.
_Phenomena analogous to the pendulum and level observations._—As
examples of phenomena which are analogous to those made on pendulums
and levels, the following may be noticed. On March 20, 1881, at 9 P.M.
a watchmaker in Buenos Ayres observed that all his clocks oscillating
north and south suddenly began to increase their amplitude, until some
of them became twice as great as before. Similar observations were
made in all the other shops. No motion of the earth was detected.
Subsequently it was learnt that this corresponded with an earthquake in
Santiago and Mendoza.[146]
Another remarkable example illustrating the like phenomena is furnished
by the observations which were made on December 21, 1860, by means of a
barometer in San Francisco, which oscillated, with periods of rest, for
half an hour. No shock was felt, nor is it likely that it was a local
accident, as it could not be produced artificially. On the following
day, however, a violent earthquake was experienced at Santiago.[147]
At the time or shortly after the great Lisbon earthquake, curious
phenomena were observed in distant countries, which only appear to be
explicable on the assumption of the existence of earth pulsations.
Thus at Amsterdam and other towns, chandeliers in churches were
observed to swing. At Haarlem water was thrown over the sides of tubs,
and it is expressly mentioned that no motion was perceived in the
ground.
At the Hague a tallow chandler was surprised at the clashing noise
made by his candles, and this the more so because no motion was felt
underfoot.
_Unusual disturbances in bodies of water._—At the time of large
earthquakes it would appear that earth pulsations are produced, which
exhibit themselves in countries where the actual shaking of the
earthquake is not felt, by disturbances in bodies of waters like lakes
and seas.
Some remarkable examples of these disturbances are to be found in the
records of the great Lisbon earthquake. This earthquake, as a violent
movement of the ground, was chiefly felt in Spain, Portugal, northern
Italy, the south of France and Germany, northern Africa, Madeira, and
other Atlantic islands. In other countries further distant, as, for
instance, Great Britain, Holland, Scandinavia, and North America,
although the records are numerous, the only phenomena which were
particularly observed was the slow oscillations of the waters in lakes,
ponds, canals, &c. In some instances the observers especially remark
that there was no motion in the soil.
Pebley Dam, in Derbyshire, which is a large body of water covering
some thirty acres, commenced to oscillate from the south. A canal near
Godalming flowed eight feet over the walk on the north side.
Coniston Water, in Cumberland, which is about five miles long,
oscillated for about five minutes, rising a yard up its shores. Near
Durham a pond, forty yards long and ten broad, rose and fell about one
foot for six or seven minutes. There were four or five ebbs and flows
per minute.
Loch Lomond rose and fell through about two and a half feet every
five minutes, and all the other lochs in Scotland seem to have been
similarly agitated.
At Shirbrun Castle, in Oxfordshire, where the water in some moats and
ponds was very carefully observed, it was noticed that the floods
began gently, the velocity then increased, till at last with great
impetuosity they reached their full height. Here the water remained for
a little while, until the ebb commenced, at first gently, but finally
with great rapidity. At two extremities of a moat about 100 yards long,
it was found that the sinkings and risings were almost simultaneous.
The motions in a pond a short distance from the moat were also
observed, and it was found that the risings and sinkings of the two did
not agree.
During these motions there were several maxima.
These few examples of the motions of waters, without any record of the
motions of the ground, at the time of the Lisbon earthquake, must be
taken as examples of a very large number of similar observations of
which we have detailed accounts.
Like agitations, it must also be remembered, were perceived in North
America and in Scandinavia, and if the lakes of other distant countries
had been provided with sufficiently delicate apparatus, it is not
unlikely that similar disturbances would have been recorded.
Besides these movements in the waters of seas and lakes, at or about
the time of great earthquakes, we have records of like movements, which
take place as independent phenomena.
Thus we read that on October 22, 1755, the waters of Lake Ontario rose
and fell five and a half feet several times in the course of half an
hour.[148] On March 31, 1761, Loch Ness rose suddenly for the period
of three-quarters of an hour.[149]
As another example of the disturbance of water at the time of a great
earthquake in districts where the earthquake was not felt, may be
mentioned the swelling of the waters of the Marañon, in 1746, on the
night when Callao was overwhelmed.
Sudden variations in the level of the water have been many times
observed in the North American lakes. The changes in level which
sometimes take place in the Genfer and Boden lakes are supposed to have
some relation to the condition of the atmosphere. A rising and falling
of especial note took place on April 18, 1855.
In Switzerland these sudden changes are known as ‘seiches’ or ‘rhussen.’
From the observations and calculations of Prof. Forel it would seem
that the period of the ‘seiches’ depends upon the dimensions of the
lakes; the calculated periods dependent on the depths of the lakes
being approximately equal to the observed periods.[150]
W. T. Bingham, writing on the volcanoes of the Hawaiian Islands,
remarks that it is not unusual for the sea to be agitated by great and
unusual tides, and that such sea waves have not been attended with
volcanic eruptions or seismic disturbances. Thus in May 1819 the tide
rose and fell thirteen times. On November 7, 1837, there was an ebb and
flow of eight feet every twenty-eight minutes. Again, on May 17, 1841,
like phenomena, unaccompanied by any other unusual occurrences, were
recorded.[151]
Phenomena which may possibly hold a relationship to earth pulsations
are the periodical swellings of the ocean on the coast of Peru. Dr. C.
F. Winslow, who made a long period observation upon the coast of Peru,
found ‘the highest tides to prevail at Callao and Paita in December
and January,’ and ‘also a series of enormous waves or sea-swells to
be thrown from time to time upon the coast, varying from twenty-four
to twenty-seven hours in continuance, accompanied by unusual height
of the tide during the same period.’ During June and July the ocean
was unusually tranquil. These phenomena do not appear to be connected
with great atmospheric storms, nor do they hold any relation to the
prevailing wind. They increase with and accompany the swelling of the
tides, and occur generally, but not always, about full moon.
Sometimes they break suddenly upon the coast. ‘_They are annual and
constant in their periodicity._’
The periodical swellings are most noticeable between Tumbez 3° S.L. and
the Chincha Islands 14° S.L.
These oceanic phenomena synchronise with the periodic intensity of
earthquake phenomena in that part of the globe, and these with tidal
movements.[152]
_Other phenomena possibly attributable to earth pulsations._—If we
assume that earth pulsations have an existence, these many phenomena
which are otherwise difficult to understand meet with an explanation.
The curious effects which were produced in the springs at Toplitz at
the time of the Lisbon earthquake may have been due to a pulse-like
wave. The flow of the principal spring was greatly increased. Before
the increase it became turbid and at one time stopped. Subsequently it
became clear and flowed as usual, but the water was hotter and more
strongly mineralised. Sudden changes in the flow of underground waters
which from time to time are observed may be attributed to like causes.
Secondary earthquakes such as occurred after the Lisbon earthquake,
as for instance in Derbyshire, may have been produced by pulsations
disturbing the equilibrium of ground in a critical state.
The falling in of subterranean excavations is also possibly connected
with these phenomena.
_Possible causes of earth pulsations._—Mr. George Darwin, in a report
to the British Association (1882), has shown that movements of
considerable magnitude may occur in the earth’s crust in consequence of
fluctuations in barometrical pressure. (A rise of the barometer over an
area is equivalent to loading that area with a weight, in consequence
of which it is depressed. When the barometer falls, the load is removed
from the area, which, in virtue of its elasticity, rises to its
original position. This fall and rise of the ground completes a single
pulsation.)
On the assumption that the earth has a rigidity like steel, Mr. Darwin
calculates that if the barometer rises an inch over an area like
Australia, the load is sufficient to sink that continent two or three
inches.
The tides which twice a day load our shores cause the land to rise and
fall in a similar manner. On the shores of the Atlantic, Mr. Darwin
has calculated that this rise and fall of the land may be as much as 5
inches. By these risings and fallings of the land the inclination of
the surface is so altered that the stile of a plummet suspended from a
rigid support ought not always to hang over the same spot. There would
be a deflection of the vertical.
In short, calculations respecting the effects of loads of various
descriptions, which we know are by natural operations continually
being placed upon and removed from the surface of various areas of the
earth’s surface, indicate that slow pulsatory movements of the earth’s
surface must be taking place, causing variations in inclination of one
portion of the earth’s crust relatively to another.
Although it is possible that phenomena like the surging of levels may
be attributable to causes like these, we can hardly attribute the other
phenomena to such agencies.
Rather than seek an explanation from agencies exogenous to our earth,
we might perhaps with advantage appeal to the endogenous phenomena of
our planet. When the barometer falls, which we have shown corresponds
to an upward motion of the earth’s crust, we know, from the results of
experiments, that microseismic motions are particularly noticeable.
As a pictorial illustration of what this really means, we may imagine
ourselves to be residing on the loosely fitting lid of a large
cauldron, the relief of the external pressure over which increases the
activity of its internal ebullition—the jars attendant on which are
gradually propagated from their endogenous source to the exterior of
our planet. This travelling outwards would take place much in the same
way that the vibrations consequent to the rattle and jar of a large
factory slowly spread themselves farther and farther from the point
where they were produced.
Admitting an action of this description to take place, it would then
follow that this extra liberation of gaseous material beneath the
earth’s crust would result in an increased upward pressure from within,
and a tendency on the part of the earth’s crust to elevation. If we
accept this as an explanation of the increased activity of a tremor
indicator, then such an instrument may be regarded as a barometer,
measuring by its motions the variations in the internal pressure of our
planet.
The relief of external pressure and the increase of internal pressure,
it will be observed, both tend in the same direction—namely, to an
elevation of the earth’s crust.
This explanation of the increased activity of earth tremors, which
has also been suggested by M. S. di Rossi, is here only advanced as a
speculation, more probable perhaps than many others.
We know how a mass of sulphur which has been fused in the presence of
water in a closed boiler gives up in the form of steam the occluded
moisture upon the relief of pressure. In a similar manner we see steam
escaping from volcanic vents and cooling streams of lava. We also
know how gas escapes from the pores and cavities in a seam of coal
on the fall of the barometrical column. We also know that certain
wells increase the height of their column under like conditions. The
latter of these phenomena, resulting in an increase in the rate of
drainage of an area by its tendency to render such an area of less
weight, facilitates its rise. If we follow the views of Mr. Mallet in
considering that the pressures exerted on the crust of our earth may in
volcanic regions be roughly estimated by the height of a column of lava
in the volcanoes of such districts, we see that in the neighbourhood of
a volcano like Cotopaxi the upward pressures must be enormously great.
Further, the phenomena of earthquakes and volcanoes indicate that these
pressures are variable. Before a volcano bursts forth we should expect
that there would be in its vicinity an upward bulging of the crust, and
after its formation a fall. Further, it is not difficult to conjecture
other possible means by which such pressures may obtain relief.
Should these pressures then find relief without rupturing the
surface, it is not difficult to imagine them as the originators of
vast pulsations which may be recorded on the surface of the earth as
wave-like motions of slow period.
As an explanation of the strange movements observed on seas and lakes,
Kluge brings forward the following strange and remarkable theory. The
oxygen of the air is magnetic, whilst water is diamagnetic and the
earth magnetic: we have, therefore, in our seas and lakes a diamagnetic
body lying between and being, consequently, repelled from two magnetic
bodies. By variation in temperature, the balance of repulsions
exerted by the air and the earth is destroyed. Thus, by an elevation
of temperature the air expands and flows away from the heated area,
where, in consequence, there is less oxygen. The result of this is,
that the repulsion of the air upon the waters is less than that of the
earth upon the waters, and the waters are in consequence raised up. By
a falling of temperature the waters may be depressed, and by either
of these actions waves may be produced without the intervention of
earthquakes or earth pulsations.
The more definite kinds of information which we have to bring forward,
tending to prove the existence of earth pulsations, too slow in period
to be experienced by ordinary observers, are those which appear to be
resultant phenomena of great earthquakes.
The phenomena that we are certain of in connection with earth
vibrations, whether these vibrations are produced artificially by
explosions of dynamite in bore-holes, or whether they are produced
naturally by earthquakes, are, first, that a disturbance as it dies out
at a given point often shows in the diagrams obtained by seismographs a
decrease in period; and, secondly, a similar decrease in the period of
the disturbance takes place as the disturbance spreads.
As examples of these actions I will quote the following.
The diagram of the disturbance of March 1, 1882, taken at Yokohama,
shows that the vibrations at the commencement of the disturbance had a
period of about three per second, near the middle of the disturbance
the period is about 1·1, whilst near the end the period has decreased
to ·46. That is to say, the backward and forward motion of the ground
at the commencement of the earthquake was six times as great as it was
near the end, when to make one complete oscillation it took between two
and three seconds. Probably the period became still less, but was not
recorded owing to the insensibility of the instruments to such slow
motions.[153]
We have not yet the means of comparing together diagrams of two or more
earthquakes, one having been taken near to the origin, and the other
at a distance. The only comparisons which I have been enabled to make
have been those of diagrams taken of the same earthquake, one in Tokio
and the other in Yokohama. As this base is only sixteen miles, and the
earthquake may have originated at a distance of several hundreds of
miles, comparisons like these can be of but little value.
Other diagrams illustrating the same point are those obtained at three
stations in a straight line, but at different distances from the
origin of a disturbance produced by exploding a charge of dynamite in
a bore-hole. A simple inspection of these diagrams shows that at the
near station the disturbance consisted of backward and forward motions,
which, as compared with the same disturbance as recorded at a more
distant station, were very rapid. Further, by examining the diagram
of the motions, say, at the near station, it is clearly evident that
the period of the backward and forward motion rapidly decreased as the
motion died out.
These illustrations are given as examples out of a large series of
other records, all showing like results.
An observation which confirms the records obtained from seismographs
respecting the increase in period of an earthquake as it dies out
I have had opportunities of twice making with my levels. After all
perceptible motion of the ground subsequent upon a moderately severe
shock had died away, I have distinctly seen the bubble in one of these
levels slowly pulsating with an irregular period of from one to five
seconds.
Although we must draw a distinction between earth waves and water
waves, we yet see that in these points they present a striking
likeness. Let us take, for example, any of the large earthquake waves
which have originated off the coast of South America, and then radiated
outwards, until they spread across the Pacific, to be recorded in Japan
and other countries perhaps twenty-five hours afterwards, at a distance
of nearly 9,000 miles from their origin. Near this origin they appeared
as walls of water which were seen rapidly advancing towards the coast.
These have been from twenty to two hundred feet in height, and they
succeeded each other at rapid intervals, until finally they died out as
a series of gentle waves. By the time these walls of water traversed
the Pacific, to, let us say, Japan, they broadened out to a swell so
flat that it could not be detected on the smoothest water excepting
along shore lines where the water rose and fell like the tide. Instead
of a wall of water sixty feet in height, we had long flat undulations
perhaps eight feet in height, but with a distance from crest to crest
of from one to two hundred miles.
If we turn to the effects of large earthquakes as exhibited on the
land, I think that we shall find records of phenomena which are only
to be explained on the assumption of an action having taken place
analogous to that which takes place so often in the ocean, or an action
similar to that exhibited by small earthquakes, and artificially
produced disturbances, if greatly exaggerated.
The only explanation for the phenomena accompanying the Lisbon
earthquake appears to be that the short quick vibrations which had
ruined so many cities in Portugal had, by the time that they had
radiated to distant countries, gradually become changed into long flat
waves having a period of perhaps several minutes. In countries like
England these pulse-like movements were too gentle to be perceived,
except in the effects produced by tipping up the beds of lakes and
ponds.
The phenomenon was not unlike that of a swell produced by a distant
storm. It would seem possible that in some cases pulsations producing
phenomena like the ‘seiches’ of Switzerland might have their origin
beneath the ocean, or deep down beneath the earth’s crust. Perhaps,
instead of commencing with the ‘snap and jar’ of an earthquake, they
may commence as a heaving or sinking of a considerable area, which
may be regarded as an uncompleted effort in the establishment of an
earthquake or a volcano.
From what has now been said it would seem that earth pulsations
are phenomena with a real existence, and that some of these are
attributable to earthquakes. On the other hand, certain earthquakes
are attributable to earth pulsations. Some of the phenomena which
have been brought forward have only a possible connection with these
movements, and they yet require investigation. Elastic tides in the
earth’s crust have for long been realities in the minds of physicists.
These, however, are due to lunar and solar influences, and are regular
in their action. The tidal-like movements called pulsations are of
greater magnitude, and their goings and comings are irregular.