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scent is rapid to the coast at La Guayra and thence to the
depths of the Caribbean. Throughout the Windward and
Leeward Islands the frequency is high, though the intensity
is seldom great. The earthquake of 1867 at St. Thomas,
with its great sea-wave, however, will be recalled in this
connection, and also the fact that just north of this island is
the deepest abyss of the Atlantic Ocean within a distance
of about forty miles. San Domingo, Jamaica, the eastern
part of Cuba, and Porto Rico are also frequently shaken, and
their seismicity may be put into relation with the fact that
the sea-bottom in the vicinity of those islands is one of the
most rugged and highly diversified in its profiles of any
part of the earth.

1 An interesting account of the earthquakes of Caraccas is given by Mr. H.
D. Warner in the Atlantic Monthly of March, 1883, in an article entitled "A
City of Earthquakes."



CHAPTER XVI

SEAQUAKES

The Sea has its Quakes as Well as the Land Difficulty of Securing Well-
Observed Data Dr. Rudolph's Investigations of Seaquakes De-
scription of the Quake on the Water Varying Intensity of Seaquakes
Sounds Heard in Seaquakes Explanation of Tremors in the Water,
which can Transmit only Normal Waves Energy of Sound- Waves in
Water Other Disturbances than Normal Vibrations are Rare A Few
Instances of Extraordinary Agitation of the Water Not Attributable to
Normal Vibration Submarine Seismic Regions in the Atlantic St.
Paul's Rocks The Equatorial District Submarine Quakes near the
Azores In the West India Deep Sea- Waves of Seismic Origin Those
of the Peruvian-Chilian Coasts The Great Arica Quake and Sea- Wave
of August 13, 1868 Its Record on the Tide Gauges of Japan, Australia,
and California Its Speed of Propagation Causes of Such Waves
Sudden Uprise and Downfall of Considerable Areas of the Ocean-Bed
Downfalls More Frequent The Simoda Wave of December 29, 1854
Bay of Bengal, December 31, 1881 Dr. Rudolph's Views of the
Causes Krakatoa Wave

THE sea has its quakes as well as the land. Our oppor-
tunities for recognising and studying the two categories
are, however, widely contrasted. By means of delicately
poised instruments we can detect and measure the minutest
amplitudes and the slowest and gentlest periods of the land-
quake, while on the water the use of such instruments is
impossible. The seaquake can be detected only when the
water is put into a state of vibration sufficiently energetic
to cause the ship and its loose objects to tremble and thus

270



SEAQUAKE'S 2? I

affect the senses. The vast waves rolled in upon the shores
of continents are much too flat and too slow in period to be
perceptible to the senses of the traveller on shipboard. But
sometimes the vibration of the water imparted to the ship
is sharp enough to arouse the attention of passengers and
crew. Sometimes it is forcible enough to cause alarm or
terror. In very rare cases it causes utter dismay and the
feeling that destruction is at hand.

Dr. Emil Rudolph has devoted much time and labour to
the research of reports from ships' logs and other sources
bearing upon the subject of quakes observed at sea. 1 From
the very large number of reports he has brought together
we may select a few which will illustrate the kind of sensa-
tions experienced by the officers and crews of vessels during
a seaquake.

Captain Gales, of the ship Florence Nightingale, reports
that

" on January 25, 1859, while in N. Lat. o 48' and W. Long. 29
16', St. Paul's Rocks being about ten miles N. W. by N. of us, we
felt a strong shock of an earthquake. It began with a rumbling
sound like distant thunder and lasted about forty seconds. I
was quite well acquainted with earthquakes, as I had experienced
a good many on the west coast of America, but never had I felt so
severe a one. Glasses and dishes rattled so vigorously that I was
surprised to find them uninjured. A good many objects fell
down and it was as if the ship were grinding upon a reef. At once
arose from all sides the cry, 'The ship has struck!' and the
watch came tumbling up in hot haste. I was much alarmed and
looked over the side of the ship in order to see the reef, but
quickly formed my opinion and quieted the commotion by the

1 These researches are embodied in two long articles in Beitrage zur Geo-
physik, vols. i. and ii., and form very interesting and instructive reading.



2/2 EARTHQUAKES

explanation that it was only an earthquake." Petermann's
Geogr. Mittheil., xv., 1869, p. 97.

Another report from a locality not far from the above is
the following: "On January 28, 1883, in N. Lat. i 38', W.
Long. 27 40', in clear weather and a light sea, suddenly we
heard, about 7.47 P.M., a strange submarine noise not unlike
distant thunder or still more like the distant firing of heavy
guns. At the same time there was a vibration of the ship
as though the anchor had been let go, or as if one were
standing on the after-deck of a screw steamer. The entire
phenomenon lasted about a minute. A peculiar sensation
came upon everybody as if electrified. The crew thought
there must be a large stick of timber rubbing alongside.
The lookout thought that the ship had struck bottom."

The foregoing are representative of the large majority of
the reports of seaquakes. The ship quivers, vibrates ; loose
objects chatter and tremble. There is a strange noise in the
sea like distant thunder or distant artillery. The first im-
pression is as if the ship were grinding upon the bottom,
and there is an instinctive rush of the crew to the deck and
the bulwarks to see if the ship is not aground or on a reef.
But the situation is soon recognised. The ship is seen to
move steadily onward with unchecked speed, she rises and
falls to the swell of the sea without shock, the water is dark
and fathomless. The tremor soon passes and the nature of
the phenomenon is at length apparent.

Although the trembling of the ship and the strange roar
from the sea are the most common and exclusive indications
of the seaquake, there occur more forcible indications in a
few instances. As might be expected there are degrees of



SEAQUAKES 2/3

intensity in seaquakes just as there are in landquakes,
though the means and agencies by which they are made
sensible are much more limited. Among many hundreds of
" reports from ships at sea which Dr. Rudolph has collected
are a few which indicate intensities of a high order. Thus
one master of a vessel reports: "We felt a shock so strong
that the entire crew was brought to its feet at once; the
wheel flew from the hand of the steersman and I myself was
flung down upon the deck." He quotes Virlet d'Aoust, a
French geologist, who in a paper on earthquakes states that
in an earthquake experienced on the coast of Asia Minor:
"Our ship was over the epicentre and was so severely shaken
that at first the Admiral feared the complete destruction of
the corvette. " Heavy objects including cannon and their
carriages were thrown up from the deck. The corvette
itself seemed to be hurled upwards. The statement that
heavy objects have been lifted from the deck and the vessel
itself lifted as if projected upwards is by no means unique,
for Dr. Rudolph has collected a considerable number of
them. The exact amount of credence we ought to con-
cede them or the precise interpretation we ought to give
them is another matter. We seem to be justified in believ-
ing that in rare cases the power of the shocks may be great
enough to render standing on the deck as difficult as it
sometimes is on land. It may even be great enough to
cause the fear that the vessel is being shaken to pieces.

The tremors imparted to the vessel from the water and the
strange sounds from ocean depths are readily explained.
The only form of elastic wave which a fluid medium can
transmit is the normal wave. This mode of vibration the



2 74 EAR TH Q UA KES

water of the ocean receives from the materials which con-
stitute its bed. These vibrations transmitted through the
earth may, before reaching the under-surface of the water,
be either normal or transverse. A transverse wave passing
through a solid elastic medium and reaching a bounding
surface which separates it from a different solid elastic me-
dium is split up into four portions: i, a transmitted trans-
verse wave passing into the new medium ; 2, a transmitted
normal wave; 3, a reflected transverse, and 4, a reflected
normal wave. Also a normal wave reaching a similar
boundary is split into four portions: a transmitted normal,
a transmitted transverse, a reflected normal, and a reflected
transverse wave. But there are exceptions depending upon
the angle of incidence, i. e. , the angle at which a ray of the
original wave meets the bounding surface. When this angle
is less than a certain critical value there is no transmitted
wave, but a total reflection.

When the bounding surface separates a solid from a fluid
medium only the normal component of the wave can pass
from the former to the latter. But it may have been the
component of either an original normal or of an original
transverse wave.

When it has once entered the water it is in a sensibly
perfect, i. e., perfectly elastic, medium, and is propagated
through it like any ordinary sound-wave in water or in air.
If the vibration is continuous, it is maintained as a con-
tinuous tremor in the water like the continuous roar of a fog-
horn in air or the hum of the locomotive. But, as we have
already had occasion to note, the vibrations of earthquakes
are innumerable and of many periods, and all are simul-



SEAQUAKES

taneous and superposed. Those whose periods are with-
in the limits of audible sound can be heard, but as a
confused murmur, due to the superposition of innumerable
periods.

The energy involved in sound-waves in water far exceeds,
ordinarily, the energy of sound-waves in air. This is be-
cause the energy, which is proportional to the mass of the
water particle multiplied by the square of its vibratory
speed is many times greater than the correlative energy of
the air particle. Moreover, the elasticity of water is many
times greater than that of air. It is not surprising, there-
fore, that the quicker vibrations of the earthquake, impart-
ing normal vibrations to the water, should supply them
with energy enough to make a ship tremble. If exception-
ally strong they may, by cumulative vibration, upset loose
objects and cause grave alarm. But that they can ever,
under any circumstances, become energetic enough to lift
the ship, or throw up heavy objects on the deck, is doubt-
less an illusion.

Besides this trembling of ships, sometimes taking a vigor-
ous and most alarming form, and the roaring of the sea due
to the normal waves rushing through it from the earth be-
neath, there are other phenomena whose accounts Dr.
Rudolph has collated and which are of interest in this con-
nection. Very rarely do the reports of seismic occurrences
at sea speak of any disturbance of the water other than that
.due to the winds and waves. Usually they state distinctly
that there was no other. But a few reports have been
gathered where the agitation of the water is spoken of as
being marked or even extreme. The descriptions seem to



2/6 EAR THQ UAKES

imply nothing less than submarine volcanic eruptions of
great power, and vessels sailing over the spot appear to have
been caught in the swirl of the waters sometimes with dis-
astrous results. These occurrences are extremely rare.
But with the tens of thousands of ships ever sailing the seas
it is well within the limits not only of possibility but even
of probability that such a mishap should at long intervals






FIG. 63. Dr. Rudolph's Contours of Ocean-Bed in the St. Paul's Rocks and
Equatorial Seismic Districts.

occur. Among the most valuable results of Dr. Rudolph's
researches is the establishment of numerous submarine dis-
tricts or regions which are seismically sensitive and give
origin to an unusual number of seaquakes. Among the
first of these to be determined were two, located in the
Atlantic Ocean very near the equator, nearly midway be-
tween Cape Palmas on the south-eastern coast of Liberia and
Cape St. Roque, Brazil. Near Lat. i N. and Long. 30



SEAQUAKES

W. is the volcanic reef known as St. Paul's Rocks. It is
the peak, almost awash, of a submarine ridge planted upon
a plateau in the ocean-bottom. On all sides the sea-bed
descends rapidly into deep water. In the vicinity of these
rocks an extraordinary number of seaquakes has been felt
in passing vessels. Farther east, on both sides of the
equator and on both sides of the meridian of 20 W., is
another region from which an unusual number of seaquakes
has been reported. Dr. Rudolph has charted them and
put them into relation with bathymetric curves obtained
from deep-sea soundings. He indicates for apparently
sound reasons two entirely distinct districts separated by a
wide surface interval which has furnished no reports of
quakes, and separated in the depths by a strongly marked
and rugged topography. To the western field he gives the
name of the St. Paul's Rocks district and to the eastern
the name of the Equatorial district.

Between 1845 an< ^ 1893 there are thirty-seven quakes
cited by Dr. Rudolph from ships in the neighbourhood of
St. Paul's Rocks, and forty-nine from the Equatorial dis-
trict between 1747 and 1890. A considerable number in
the latter district, however, are reported by two or more
vessels which felt the tremors simultaneously from the same
disturbances. The degree of seismicity thus indicated for
both these places will upon full consideration be regarded
as a very high one. It will occur to us that these are
merely the reports of passing vessels, which chanced to have
fallen into the records of navigation bureaus, while the ex-
periences of other vessels which may have felt the same or
other shocks failed to be preserved, or, if anywhere pre-



278 EARTHQUAKES

served, failed to be discovered. It is not probable that any
such tremors were noted at all unless under the following
conditions: 1st, they must be of a rather high degree of in-
tensity and derived from quakes of very much more than
average power to be felt under any circumstances; 2nd, they
must have occurred only when the ocean was very quiet, and
would hardly have been felt in rough weather. The equator,
it is true, is ordinarily a belt of calms; but, on the other
hand, the ocean is never still. All things considered,
the probability of any quake other than one of consider-
able power being felt by a passing vessel at sea is a very
small one, and the probability of its record finding a resting-
place where it may in after-years reach the eye of some
investigator is also a very small one. Thus the number of
quakes recorded in these two oceanic regions must be but
a minute fraction of those which have probably occurred
there.

Another district from which seaquakes have been re-
ported with exceptional frequency in the North Atlantic
is the neighbourhood of the Azores. Between these islands
and the coast of Portugal it may be remembered that the
great quake originated which on November I, 1755, de-
stroyed Lisbon, though probably its epicentre was much
nearer the continental shore than the islands. Although
seaquakes are common anywhere along this belt of sea-
bottom (between the Azores and Portugal) they appear to
cluster much more thickly near the islands or near the main-
land than midway between them. Whether this is due to
the fact that vessels which are likely to observe and report
them pass much more frequently near the two localities than



SEAQUAKES 279

between them, or is due to a real difference in the number
of quakes, is uncertain.

The West India Deep, that profound basin of the Atlantic
lying north of the Lesser Antilles and east of the Bahamas,
where the Atlantic has its greatest depths and where its
bottom has its greatest inequalities and reliefs, is another
district from which an unusual number of seaquakes has
been reported.

The most impressive accompaniments of seaquakes are
those gigantic waves in the ocean which are generated at
the same time and doubtless by the same kinetic causes as
the seaquakes themselves, and which break upon adjacent
coasts with disastrous results to cities and their inhabitants.
These have been known through a long period of history in
the eastern Mediterranean, where they have ravaged the
coasts of Syria and Asia Minor, as well as the shores and
islands of the ^Egean. It is not a little remarkable that the
introduction of the great work of Suess, Das Antlitz der
Erde, is an essay which attributes the Noachian Deluge to
a legend of a mighty sea-wave rolled in upon the lands of
Chaldea from the Persian Gulf. Although western Europe
and the eastern coasts of North America have had little ex-
perience of such visitations, the dreadful memory of the
destruction of Lisbon still survives. 1 In the West Indies,
though happily not common, the history of the last four

1 The literature of the Lisbon disaster is very copious and might form a
library of itself. The best succinct account of it that occurs to me is given by
Lyell in his Principles of Geology. There is also an interesting paper in vol.
xii., Trans. Seism. Soc. Japan^ 1888, communicated by E. J. Pereira, being
an abstract of a rare old pamphlet published in Lisbon in 1756 by an eye-
witness a few months after the catastrophe.



280 EARTHQUAKES

centuries has preserved accounts of several of them. But,
generally speaking, the Atlantic both north and south of
the equator has been remarkably exempt from them.

It is off the Pacific coast of South America from the
equator to 45 of south latitude that these waves originate
with greatest frequency and also in greatest power. Espe-
cially in the vicinity of the angle where the Peruvian and
Chilian coasts meet have they been most numerous and
formidable. The harbours of Pisco, Arica, Tacna, Iquiqui,
and Pisagua have been repeatedly subject to these destruc-
tive invasions. Usually they are preceded by a violent
earthquake, and the inhabitants, taking due warning there-
from, betake themselves to the hills. The sea-wave, how-
ever, does not always follow the earthquake; in fact, in a
great majority of cases, it does not. But it appears often
enough to arouse serious apprehension of its coming when-
ever the ground is strongly shaken. The first indication of
the coming disaster is the withdrawal of the sea from the
shore, leaving bare the bottom of the harbour. A few
minutes later the sea returns in a high wave of resistless
power which overflows the adjoining flats. Again it with-
draws and still again returns. The oscillations may con-
tinue through many hours and even for two or three days
at intervals, i. e., periods of about fifteen minutes, and with
slowly diminishing amplitudes (amount of rise and fall) until
they gradually die out.

The most memorable seaquake of this locality occurred
August 13, 1868. The coast of South America was shaken
all the way from Guayaquil in Ecuador to Valdivia in Chili,
the highest intensity being manifested in the neighbourhood



SEAQUAKES 28 1

of Arica. 1 The force of the quake in this town was very
great, throwing down most of the structures and producing
landslips. A few minutes later precisely how many
minutes is not known the sea was observed to retire slowly
from the shore so that ships anchored in seven fathoms of
water were left high and dry. A few minutes later still it
was seen returning in a great wall or "bore," which caught
up the ships in the roadstead and swept them inland as if
they were mere chips of wood. Among therrv was the U.
S. steamer Wateree, one of the improvised war-vessels of
the blockading fleet of the Civil War, which was carried in-
land nearly half a mile and left with little injury on shore
by the recession of the wave.

The wave in the ocean generated in this quake made
itself felt on the coasts of Australasia, Japan, Kamtchatka,
Alaska, Oregon, and California. In the harbour of Hako-
date, in Japan, a series of waves was registered upon the
tide gauge. The ordinary tide at that port is about 2\ to 3
feet. On this occasion the water rose and fell ten feet
(double amplitude) with a period of about twenty minutes
for a complete oscillation. It had taken the first wave
twenty-five hours to traverse the distance of about 7600
nautical miles. On May 9, 1877, another seaquake of
similar magnitude had its origin in the same neighbour-
hood. It is known as the Iquiqui quake. A vast sea-wave
invaded with disastrous effect the towns of northern Chili
and southern Peru, repeating the havoc of 1868. At Arica

1 A very full account of this great seaquake is given by F. von Hochstetter
in Sitz. der K. K. Akad. der Wiss., bd. Iviii., 1868, ii. abth., and it is sum-
marised in Petermann's Geogr. Mitt., 1869.



282 EARTHQUAKES

the stranded hulk of the Wateree was picked up and swept
farther inland. Like its predecessors, this wave was felt all
over the Pacific. At Samoa the height of the waves varied
from six to twelve feet ; in New Zealand and Australia from
three to twenty feet; in Japan, from five to ten feet.

It should be borne in mind, however, that as a sea-wave
progresses from deep water into' gradually shoaling water it
increases its height and the steepness of its front slope until
it begins to comb or break. The height measured by the
tide gauge in a seaport is therefore no measure of the height
of the wave far out in the ocean, and, in fact, the mid-ocean
height of the wave is doubtless measurable in inches while
the inshore height is measurable in feet.

Since a wave must travel its own wave-length in the time
indicated by its period, we shall have no difficulty in com-
puting the lengths of these giant waves as soon as we know
their periods and speeds of propagation. If it requires
twenty-five hours for a wave to travel from Arica to Hako-
date, a distance of 7600 sea-miles, we have a speed of about
three hundred miles per hour, or five miles per minute; and
if the period as shown on the tide gauge at Hakodate is
twenty minutes, we have a wave-length of one hundred
nautical miles. We have no means of knowing the height
of the wave in dep water. We can only say that it must
be much less than on a neighbouring shore. Obviously it n
greater near the origin than away from it.

Since the speed of propagation of water waves is depend-
ent upon the depth of the water, much computation has
been devoted in times past to the problem of deducing the
depths of the ocean from the speeds of sea-waves. The



SEAQUAKES 283

measurement of oceanic depths by actual soundings has
been one of the results of the demands of ocean telegraphy,
and it has been prosecuted so vigorously that we are rapidly
accumulating direct and positive knowledge on this point,
and the computation of depths from the speeds of sea-waves
has now become an academic problem rather than a practical
one.

There has also been much discussion of the specific modes
of action which have originated these waves. It must be
admitted that the varying opinions or views on this subject
appear to have been influenced often by preconceived views
of geodynamic action, and the discussions have in some in-
stances shown a tendency to mould facts to fit hypotheses ;
to belittle or explain away facts which may be adverse to
such preconceptions and unduly magnify those which may
seem to favour them.

There are, however, several facts or groups of facts which
admit of no dispute or qualification, (i) A great sea-wave
of this kind implies that somewhere in the depths of the
ocean, usually not very far from shore (say within one or two
hundred miles of it), a displacement of the under-surface of
the water occurs. It may be a sudden upri-ing of the sea-
bottom, lifting the overlying mass of water, or it may be the
sudden dropping of the bottom, carrying the water down
with it, or it may be a sudden volcanic outbreak discharging
lavas with their invariably associated contents of intensely
hot steam or other occluded gases into the overlying ocean
with sufficient expansive force to lift the water. In any
event there must be some lifting or depressing force acting
at the sea-bottom or on the littoral.



284 EARTHQUAKES

(2) The energy involved in the movement must be great
enough not only to lift an unknown thickness of sea-bottom,
but the overlying water several thousand metres in depth.
The area of displacement must also be a very large one,
i. e., hundreds and perhaps thousands of square kilometres


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