both hemispheres, alike on land and at sea, the two first-named kinds of
motion have often appeared to me to occur simultaneously. The mine-like
explosiion -- the vertical action from below upward -- was most strikingly
manifested in the overthrow of the town of Riobamba in 1797, when the bodies
of many of the inhabitants were found to have been hurled to Cullea, a hill
several hundred feet in neight, and on the opposite side of the River Lican.
The propagation is most generally effected by undulations in a linear
direction,* with a velocity of from twenty to twenty-eight miles in a
minute, but partly in circles of commotion or large ellipses, in which the
vibrations are propagated with decreasing intensity from a center toward the
circumference.
[footnote] *[On the linear direction of earthquakes, see Daubeney 'On
Volcanoes', p. 515.] -- Tr.
There are districts exposed to the action of two intersecting circles of
commotion. In Northern Asia, where the Father of History,* and subsequently
Theophylactus Simocatta,** described the districts of Scythia as free from
earthquakes, I have observed the metalliferous portion of the Altai
Mountains under the influence of a two-fold focus of commotion, the Lake of
Baikal, and the volcano of the Celestial Mountain (Thianschan).***
[footnote] *Herod, iv., 28. The prostration of the colossal statue of
Memnon, which has been again restored (Letronne, 'La Statue Vocale de
Memnon', 1835, p. 25, 26), presents a fact in opposition to the ancient
prejudice that Egypt is free from earthquakes (Pliny, ii., 80); but the
valley of the Nile does lie external to the circle of commotion of
Byzantium, the Archipelago, and Syria (Ideler ad Aristot., 'Meteor.', p.
584).
[footnote] **Saint-Martin, in the learned notes to Lebeau, 'Hist. du Bas
Empire', t. ix., p. 401.
[footnote] ***Humboldt, 'Asie Centrale', t. ii., p. 110-118. In regard to
the difference between agitation of the surface and of the strata lying
beneath it, see Gay-Lussac, in the 'Annales de Chimie et de Physique', t.
xxii., p. 429.
When the circles of commotion intersect one another -- when, for instance,
an elevated plain lies between two volcanoes simultaneously in a state of
eruption, several wave-systems may exist together, as in fluids, and not
mutually disturb one another. We may even suppose 'interference'
p 205
to exist here, as in the intersecting waves of sound. The extent of the
propagated waves of commotion will be increased on the upper surface of the
earth, according to the general law of mechanics, by which, on the
transmission of motion in elastic bodies, the stratum lying free on the one
side endeavors to separate itself from the other strata.
Waves of commotion have been investigated by means of the pendulum and the
seismometer* with tolerable accuracy in respect to their direction and total
intensity, but by no means with reference to the internal nature of their
alternations and their periodic intumescence.
[footnote] *[This instrument, in its simplest form, consists merely of a
basin filled with some viscid liquid, which, on the occurrence of a shock of
an earthquake of sufficient force to disturb the equilibrium of the building
in which it is placed, is tilted on one side, and the liquid made to rise in
the same direction, thus showing by its height the degree of the
disturbance. Professor J. Forbes has invented an instrument of this nature,
although on a greatly improved plan. It consists of a vertical metal rod,
having a ball of lead movable upon it. It is supported upon a cylindrical
steel wire, which may be compressed at pleasure by means of a screw. A
lateral movement, such as that of an earthquake, which carries forward the
base of the instrument, can only act upon the ball through the medium of the
elasticity of the wire, and the direction of the displacement will be
indicated by the plane of vibration of the pendulum. A self-registering
apparatus is attached to the machine. See Professor J. Forbes's account of
his invention in 'Edinb. Phil. Trans.', vol. xv., Part i.] -- Tr.
In the city of Quito, which lies at the foot of a still active volcano (the
Rucu Pichincha), and at an elevation of 9540 feet above the level of the
sea, which has beautiful cupolas, high vaulted churches, and massive
edifices of several stories, I have often been astonished that the violence
of the nocturnal earthquakes so seldom causes fissures in the walls, while
in the Peruvian plains oscillations apparently much less intense injure low
reed cottages. The natives, who have experienced many hundred earthquakes,
believe that the difference depends less upon the length or shortness of the
waves, and the slowness or rapidity of the horizontal vibrations.* than on
the uniformity of the motion in opposite directions.
[footnote] * "Tutissimum est cum vibrat crispante Aedificiorum crepitu; et
cum intumescit assurgens alternoque motu residet, innoxium et cum
concurrentia tecta contrario ictu arietant; quoniam alter motus alteri
renititur. Undantis inclinatio et fluctus more quaedam volutatio investa
est, aut cum in unam partem totus se motus impellitae -- Plin., ii., 82.
The circling rotatory commotions are the most uncommon, but, at the same
time, the most dangerous. Walls were observed to be twisted, but not thrown
down; rows of trees turned from their previous parallel direction;
p 206
and fields covered with different kinds of plants found to be displaced in
the great earthquake of Riobamba, in the province of Quito, on the 4th of
February, 1797, and in that of Calabria, between the 5th of February and the
28th of March, 1782. The phenomenon of the inversion or displacement of
fields and pieces of land, by which one is made to occupy the place of
another, is connected with a translatory motion or penetration of separate
terrestrial strata. When I made the plan of the ruined town of Riobamba,
one particular spot was pointed out to me, where all the furniture of one
house had been found under the ruins of another. The loose earth had
evidently moved like a fluid in currents, which must be assumed to have been
directed first downward, then horizontally, and lastly upward. It was found
necessary to appeal to the 'Audiencia', or Council of Justice, to decide
upon the contentions that arose regarding the proprietorship of objects that
had been removed to a distance of many hundred roises.
In countries where earthquakes are comparatively of much less frequent
occurrence (as for instance, in Southern Europe), a very general belief
prevails, although unsupported by the authority of inductive reasoning,*
that a calm, an oppressive
p 207
heat and a misty horizon, are always the forerunners of this phenomenon.
[footnote] *Even in Italy they have begun to observe that earthquakes are
unconnected with the state of the weather, that is to say, with the
appearance of the heavens immediately before the shock. The numerical
results of Friedrich Hoffmann ('Hinterlassene Werke', bd. ii., 366-376)
exactly correspond with the experience of the Abbate Scina of Palermo. I
have myself several times observed reddish clouds on the day of an
earthquake, and shortly before it on the 4th of November, 1799, I
experienced two sharp shocks at the moment of a loud clap of thunder.
('Relat. Hist.', liv. iv., chap. 10.) The Turin physicist, Vassalli Eaudi,
observed Volta's electrometer to be strongly agitated during the protracted
earthquake of Pignerol, which lasted from the 2d of April to the 17th of
May, 1808; 'Journal de Physique', t. lxvii., p. 291. But these indications
presented by clouds, by modifications of atmospheric electricity, or by
calms, can not be regarded as 'generally' or 'necessarily' connected with
earthquakes, since in Quito, Peru, and Chili, as well as in Canada and
Italy, many earthquakes are observed along with the purest and clearest
skies, and with the freshest land and sea breezes. But if no meteorological
phenomenon indicates the coming earthquake either on the morning of the
shock or a few days previously, the influence of certain periods of the year
(the vernal and autumnal equinoxes), the commencement of the rainy season in
the tropics after long drought, and the change of the monsoons (according to
general belief), can not be overlooked, even though the genetic connection
of meteorological processes with those going on in the interior of our globe
is still enveloped in obscurity. Numerical inquiries on the distribution of
earthquakes throughout the course of the year, such as those of Von Hoff,
Peter Merian, and Friedrich Hoffmann, bear testimony to their frequency at
the periods of equinoxes. It is singular that Pliny, at the end of his
fanciful theory of earthquakes, names the entire frightful phenomenon a
subterranean storm; not so much in consequence of the rolling sound which
frequently accompanies the shock, as because the elastic forces, concussive
by their tension, accumulate in the interior of the earth when they are
absent in the atmosphere! "Ventos in causa esse non dubium reor. Neque
enim unquam intemiscunt terre, nisi sopito mari, coeloque adeo tranquillo,
ut volatus avium non pendeant, subtracto omni spiritu qui vehit; nec unquam
nisi post ventos conditos, scilicet in venas et cavernas ejus occulto
afflatu. Neque aliad est in terra tremor, quam in nube toonitruum; nec
hiatus aliud quam cum fulmen erumpit, incluso spiritu luctante et ad
libertatem exire nitente." (Plin., ii., 79.) The germs of almost every
thing that has been observed of imagined on the causes of earthquakes, up to
the present day, may be found in Seneca, 'Nat. Quaest.', vi., 4-31.
The fallacy of this popular opinion is not only refuted by my own
experience, but likewise by the observations of all those who have lived
many years in districts where, as in Cumana, Quito, Peru, and Chili, the
earth is frequently and violently agitated. I have felt earthquakes in
clear air and a fresh east wind, as well as in rain and thunder storms. The
regularity of the horary changes in the declination of the magnetic needle
and in the atmospheric pressure remained undisturbed between the tropics on
the days when earthquakes occurred.*
[footnote] *I have given proof that the course of the horary variations of
the barometer is not affected before or after earthquakes, in my 'Relat.
Hist.', t. i., p. 311 and 513.
These facts agree with the observations made by Adolph Erman (in the
temperate zone, on the 8th of March, 1829) on the occasion of an earthquake
at Irkutsk, near the Lake of Baikal. During the violent earthquake of
Cumana, on the 4th of November, 1799, I found the declination and the
intensity of the magnetic force alike unchanged, but, to my surprise, the
inclination of the needle was diminished about 48 degrees.*
[footnonte] *Humboldt, 'Relat. Hist.', t. i., p. 515-517.
There was no ground to suspect an error in the calculation, and yet, in the
many other earthquakes which I have experienced on the elevated plateaux of
Quito and Lima, the inclination as well as the other elements of terrestrial
magnetism remained always unchanged. Although, in general, the processes at
work within the interior of the earth may not be announced by any
meteorological phenomena or any special appearance of the sky, it is, on the
contrary, not improbable, as we shall soon see, that in cases of violent
earthquakes some effect may be imparted to the atmosphere, in consequence of
which they can not always act in a purely dynamic manner.
p 208
During the long-continued trembling of the ground in the Piedmontese valleys
of Pelis and Clusson, the greatest changes in the electric tension of the
atmosphere were observed while the sky was cloudless. The intensity of the
hollow noise which generally accompanies an earthquake does not increase in
the same degree as the force of the oscillations. I have ascertained with
certainty that the great shock of the earthquake of Riobamba (4th Feb.,
1797) -- one of the most fearful phenomena recorded in the physical history
of our planet -- was not accompanied by any noise whatever. The tremendous
noise ('el gram ruido') which was heard below the soil of the cities of
Quito and Ibarra, but not at Tacunga and Hambato, nearer the center of the
motion, occurred between eighteen and twenty minutes 'after' the actual
catastrophe. In the celebrated earthquake of Lima and Callao (28th of
October, 1746), a noise resembling a subterranean thunder-clap was heard at
Truxillo a quarter of an hour after the shock, and unaccompanied by any
trembling of the ground. In like manner, long after the great earthquake in
New Granada, on the 16th of November, 1827, described by Boussingault,
subterranean detonations were heard in the whole valley of Cauca during
twenty or thirty seconds, unattended by motion. The nature of the noise
varies also very much, being either rolling, or rustling, or clanking like
chains when moved, or like near thunder, as, for instance, in the city of
Quito; or, lastly, clear and ringing, as if obsidian or some other vitrified
masses were struck in subterranean cavities. As solid bodies are excellent
conductors of sound, which is propagated in burned clay, for instance, ten
or twelve times quicker than in the air, the subterranean noise may be heard
at a great distance from the place where it has originated. In Caracas, in
the grassy plains of Calabozo, and on the banks of the Rio Apure, which
falls into the Orinoco, a tremendously loud noise, resembling thunder, was
heard, unaccompanied by an earthquake, over a district of land 9200 square
miles in extent, on the 30th of April, 1812, while at a distance of 632
miles to the north-east, the volcano of St. Vincent, in the small Antilles,
poured forth a copious stream of lava. With respect to distance, this was
as if an eruption of Vesuvius had been heard in the north of France. In the
year 1744, on the great eruption of the volcano of Cotopaxi, subterranean
noises, resembling the discharge of cannon, were heard in Honda, on the
Magdalena River. The crater of Cotopaxi lies not only 18,000 feet higher
than Honda, but these two points are separated by the colossal
p 209
mountain chain of Quito, Pasto, and Popayan, no less than by numerous
valleys and clefts, and they are 436 miles apart. The sound was certainly
not propagated through the air, but through the earth, and at a great depth.
During the violent earthquake of New Granada, in February, 1835,
subterranean thunder was heard simultaneously at Popayan, Bogota, Santa
Marta, and Caracas (where it continued for seven hours without any movement
of the ground), in Haiti, Jamaica, and on the Lake of Nicaragua.
These phenomena of sound, when unattended by any perceptible shocks, produce
a peculiarly deep impression even on persons who have lived in countries
where the earth has been frequently exposed to shocks. A striking and
unparalleled instance of uninterrupted subterranean noise, unaccompanied by
any trace of an earthquake, is the phenomenon known in the Mexican elevated
plateaux by the name of the "roaring and the subterranean thunder)
('bramidos y truenos subterraneos') of Guanaxuato.*
[footnote] *On the 'bramidos' of Guanaxuato, see my 'Essai Polit. sur la
Nouv. Espagne', t. i., p. 303. The subterranean noise, unaccompanied with
any appreciable shock, in the deep mines and on the surface (the town of
Guanaxuata lies 6830 feet above the level of the sea), was not heard in the
neighboring elevated plains, but only in the mountainous parts of the
Sierra, from the Cuesta de los Aguilares, near Marfil, to the north of Santa
Rosa. There were individual parts of the Sierra 24-28 miles northwest of
Guanaxuata, to the other side of Chichimequillo, near the boiling spring of
San Jose de Comgngillas, to which the waves of sound did not extend.
Extremely stringent measures were adopted by the magistrates of the large
mountain towns on the 14th of January 1784, when the terror produced by
these subterranean thunders was at its height. "The flight of a wealthy
family shall be punished with a fine of 1000 piasters, and that of a poor
family with two months' imprisonment. The militia shall bring back the
fugitives." One of the most remarkable points about the whole affair is the
opinion which the magistrates (el cabildo) cherished of their own superior
knowledge. In one of their 'proclamas', I find the expression, "The
magistrates, in their wisdom (en su sabiduria), will at once know when there
is actual danger, and will give orders for flight; for the present, let
processions be instituted." The terror excited by the tremor gave rise to a
famine, since it prevented the importation of corn from the table-lands,
where it abounded. The ancients were also aware that noises sometimes
existed without earthquakes. -- Aristot., 'Meteor.', ii., p. 802; Plin.,
ii., 80. The singular noise that was heard from March, 1822, to September,
1824, in the Dalmatian island Meleda (sixteen miles from Ragusa) and on
which Partsch has thrown much light, was occasionally accompanied by shocks.
This celebrated and rich mountain city lies far removed from any active
volcano. The noise began about midnight on the 9th of January, 1784, and
continued for a month. I have been enabled to give a circumstantial
p 210
description of it from the report of many witnesses, and from the documents
of the municipality, of which I was allowed to make use. From the 13th to
the 16th of January, it seemed to the inhabitants as if heavy clouds lay
beneath their feet, from which issued alternate slow rolliing sounds and
short, quick claps of thunder. The noise abated as gradually as it had
begun. It was limited to a small space, and was not heard in a basaltic
district at the distance of a few miles. Almost all the inhabitants, in
terror, left the city, in which large masses of silver ingots were stored;
but the most courageous, and those more accustomed to subterranean thunder,
soon returned, in order to drive off the bands of robbers who had attempted
to possess themselves of the treasures of the city. Neither on the surface
of the earth, nor in mines 1600 feet in depth, was the slightest shock to be
perceived. No similar noise had ever before been heard on the elevated
tableland of Mexico, nor has this terrific phenomenon since occurred there.
Thus clefts are opened or closed in the interior of the earth, by which
waves of sound penetrate to us or are impeded in their propagation.
The activity of an igneous mountain, however terrific and picturesque the
spectacle may be which it presents to our contemplation, is always limited
to a very small space. It is far otherwise with earthquakes, which although
scarcely perceptible to the eye, nevertheless simultaneously propagate their
waves to a distance of many thousand miles. The great earthquake which
destroyed the city of Lisbon on the 1st of November, 1755, and whose effects
were so admirably investigated by the distinguished philosopher Emmanuel
Kant, was felt in the Alps, on the coast of Sweden, in the Antilles,
Antigua, Barbadoes, and Martinique; in the great Canadian Lakes, in
Thuringia, in the flat country of Northern Germany, and in the small inland
lakes on the shores of the Baltic.*
[footnote] *[It has been computed that the shock of this earthquake
pervaded an area of 700,000 miles, or the twelfth part of the circumference
of the globe. This dreadful shock lasted only five minutes: it happened
about nine o'clock in the morning of the Feast of all Saints, whien almost
the whole population was within the churches, owing to which circumstance no
less than 30,000 persons perished by the fall of these edifices. See
Daubeney 'On Volcanoes', p. 514-517.] -- Tr.
Remote springs were interrupted in their flow, a phenomenon attending
earthquakes which had been noticed among the ancients by Demetrius the
Callatian. The hot springs of Toplitz dried up, and returned, inundating
every thing around, and having their waters colored with iron ocher. In
Cadiz
p 211
the sea rose to an elevation of sixty-four feet, while in the Antilles,
where the tide usually rises only from twenty-six to twenty-eight inches, it
suddenly rose above twenty feet, the water being of an inky blackness. It
has been computed that on the 1st of November, 1755, a portion of the
Earth's surface four times greater than that of Europe, was simultaneously
shaken. As yet there is no manifestation of force known to us, including
even the murderous inventions of our own race, by which a greater number of
people have been killed in the short space of a few minutes: sixty thousand
were destroyed in Sicily in 1693, from thirty to forty thousand in the
earthquake of Riobamba in 1797, and probably five times as many in Asia
Minor and Syria, under Tiberius and Justinian the elder, about the years 19
and 526.
There are instances in which the earth has been shaken for many successive
days in the chain of the Andes in South America, but I am only acquainted
with the following cases in which shocks that have been felt almost every
hour for months together have occurred far from any volcano, as, for
instance, on the eastern declivity of the Alpine chain of Mount Cenis, at
Fenestrelles and Pignerol, from April, 1808; between New Madrid and Little
Prairie,* north of Cincinnati in the United States of America, in December,
1811, as well as through the whole winter of 1812; and in the Pachalik of
Aleppo, in the months of August and September, 1822.
[footnote] *Drake, 'Nat. and Statist. View of Cincinnati', p. 232-238;
Mitchell, in the 'Transactions of the Lit. and Philos. Soc. of New York',
vol. i., p. 281-308. In the Piedmonese county of Pignerol, glasses of
water, filled to the very brim, exhibited for hours a continuous motion.
As the mass of the people are seldom able to rise to general views, and are
consequently always disposed to ascribe great phenomena to local telluric
and atmospheric processes, wherever the shaking of the earth is continued
for a long time, fears of the eruption of a new volcano are awakened. In
some few cases, this apprehension has certainly proved to be well grounded,
as, for instance, in the sudden elevation of volcanic islands, and as we see
in the elevation of the volcano of Jorullo, a mountain elevated 1684 feet
above the ancient level of the neighboring plain, on the 29th of September
1759, after ninety days of earthquake and subterranean thunder.
If we could obtain information regarding the daily condition of all the
earth's surface, we should probably discover that the earth is almost always
undergoing shocks at some point of its superficies, and is continually
influenced by the reaction
p 212
of the interior on the exterior. The frequency and general prevalence of a
phenomenon which is probably dependent on the raised temperature of the
deepest molten strata explain its independence of the nature of the mineral
masses in which it manifests itself. Earthquakes have even been felt in the
loose alluvial strata of Holland, as in the neighborhood of Middleburg and
vliessingen on the 23d of February, 1828. Granite and mica slate are shaken
as well as limestone and sandstone, or as trachyte and amygdaloid. It is
not, therefore, the chemical nature of the constituents, but rather the
mechanical structure of the rocks, which modifies the propagation of the
motion, the wave of commotion. Where this wave proceeds along a coast, or
at the foot and in the direction of a mountain chain, interruptions at
certain points have sometimes been remarked, which manifested themselves
during the course of many centuries. The undulation advances in the depths
below, but is never felt at the same points on the surface. The Peruvians*
say of these unmoved upper strata that "they form a bridge."
[footnote] *In Spanish they say, 'rocas que hacen puente'. With this
phenomenon of non-propagation through superior strata is connected the
remarkable fact that in the beginning of this century shocks were felt in
the deep silver mines at Marienberg, in the Saxony mining district, while
not the slightest trace was perceptible at the surface. The miners ascended
in a state of alarm. Conversely, the workmen in the mines of Falun and
Persberg felt nothing of the shocks which in November, 1823, spread dismay
among the inhabitants above ground.