SEAQUAKES
As earthquakes are shakings of the earth's crust in places where it is
uncovered by the waters of the ocean, so _seaquakes_ are the shakings
of those portions that lie on the bed of the ocean.
Mallet points out that the earthquake wave may start either in the
interior of the continent, or on the bed of the ocean; that the latter
place is the more common, since on the land vents--rude safety-valves,
as it were,--are provided by the craters of the volcanoes; that, when
earthquakes start on the ocean bed, the impulses are conveyed in
different forms of waves, i. e., those through the solid earth, those
through the water, and those through the air, with varying sounds like
bellowings and explosions, or like the rolling of wagons over rough
roads.
To learn when quakes occur on the sea is a much harder task, since on
the land we can use seismoscopes, seismographs, or seismometers to
indicate, record, or measure the shakings of the crust, while on the
sea, where the water is always in more or less motion and the surface
so far from the ocean's bed this is impossible, or, rather shall it
be said, has hitherto been found so; for that the mind of man may
surmount this obstacle is not impossible to conceive.
To detect the wave produced by the quaking of the bed of the ocean
is exceedingly difficult, since those in very deep water are flat or
possess but a small height. Indeed, in the deepest parts of the ocean
this height is probably to be measured only by inches instead of feet.
When, however, the waves advance towards the shore they increase in
height, and when they reach the shallows near the coast, they begin
to curl over and break, thus creating the enormous waves mentioned so
often as attending great earthquakes in the ocean.
During the great earthquake of Simoda in Japan, 1854, the waters of
the bay were first greatly agitated, and then retreated, leaving the
bottom bare in places where the water was formerly thirty feet deep. A
wave, thirty feet high, then rushed in from the bay and, climbing the
land, swept away everything in its path, covering the town with water
to the tops of the houses. This monster wave then receded, but rushed
back five times.
In 1751, an earthquake wave suddenly entered Callao, the port of Lima,
Peru, sinking twenty-three vessels and driving a frigate inland, where
it was left high and dry. This wave extended across the Pacific to the
Hawaiian Islands, a distance of 6,000 miles.
On the 13th of August, 1866, an earthquake wave, that started a short
distance from shore, produced a number of earthquake waves sixty feet
high that reached the coast of Peru half an hour after the principal
earthquake shock. These waves reached Coquimbo, 800 miles distant,
in about three hours, and Honolulu, on the Sandwich Islands, 5,520
miles distant, in twelve hours, and the coast of Japan, more than
10,000 miles distant, on the next day. Le Conte remarks that these
waves would have encircled the earth, had it not been for the barrier
interposed by the Andes.
Another great seaquake, known as the Iquiqui seaquake, during 1868 in
the same neighborhood damaged severely the towns of north Chile and
southern Peru.
While, however, there is difficulty in readily observing the
earthquake waves that form in the deep ocean, yet such is at times the
violence of an earthquake that there is no difficulty in detecting its
presence, even in deep water. Dr. Rudolph has made a careful study of
the evidences of earthquakes produced in the deep sea, from a careful
examination of a great number of the logs of ships. Logs, as everybody
knows, are books in which the captain or commanding officer makes
careful entries of all important happenings to the vessel, conditions
of the weather and of the sea. From this source Dr. Rudolph obtained
considerable information of much value concerning these phenomena.
I have already called your attention to portion of the Atlantic Ocean
lying near the Equator, in the warmest part of the ocean, between
Africa and South America, as being a region especially liable to
submarine volcanic showers. While, generally speaking, there is
nothing in this region to indicate the probability of submarine
disturbance, yet suddenly, if a vessel happens to pass directly over
the point of origin of the quake, there ensues a great quaking or
quivering. Loose objects on the ship begin to shake and clatter.
Noises arise from some invisible point deep down in the ocean. The
disturbance grows, the noises begin to resemble distant thunder, the
ship trembles and staggers as though it had struck rocks, and many
believe she is about to go down; when, as suddenly as it began, the
commotion ceases, the noises stop, and the ship shapes her course as
calmly, and as gallantly, as before.
Rudolph gives two excellent examples of seaquakes in this region, both
of which were, doubtless, due to submarine eruptions.
On the 25th of January, 1859, as the ship _Florence_ was in lat.
0° 48' N., long. 29° 16' W., about ten miles N. W. by N. from St.
Paul's Rock, the people on board felt a sudden shock that began with a
rumbling sound like distant thunder. This lasted only forty seconds.
The glass and dishes of the vessel rattled so violently that it was
feared they would be broken. The shakings were so strong that several
objects on the vessel were thrown down. Everyone believed the ship
had struck on rocks. The captain leaned over the taffrail in order to
see the position of the reef, but soon saw that the vessel had struck
nothing, and informed his crew "it was only an earthquake shock."
Another of the log books examined by Rudolph was that of a ship in the
same part of the Atlantic Ocean. This record showed that suddenly on
a morning, in 1883, strange noises were heard that soon increased and
became not unlike the firing of great guns or the peals of distant
thunder. The ship vibrated as if its anchor had been suddenly let go,
and at the same time a feeling came over all the crew, as if they had
been electrified.
In some cases the vibrations were sufficiently severe to throw heavy
objects from the deck, as appears in an account given by a French
geologist of a quake in the Mediterranean off the shores of Asia Minor.
"Our ship was over the epicentre,"[5] he says, "and was so severely
shaken that at first the Admiral feared the complete destruction of
the corvette." He then makes the statement that the shocks which were
directly upwards were so strong as to throw heavy objects in the air;
for example, a heavy gun and its carriage. While it is possible, as
Dutton remarks, that this incident of the heavy gun and carriage was
grossly exaggerated, yet it should not be forgotten that in the case
of submarine eruptions such as that which resulted in the production
of the island of Sabrina, an immense column of water, weighing
probably many times more than a gun and its carriage, was observed to
be shot high into the air.
Where the seaquake is produced by a strong submarine volcanic
eruption, there is a violent commotion of the water itself, so that a
vessel passing over such a point may be greatly injured, and, indeed,
even destroyed. Such disasters, however, are fortunately exceedingly
rare.
Among other common effects of seaquakes is the destruction of fish
already mentioned by the sudden blow to the water stunning and killing
them, just as the explosion of dynamite or other high explosives does
in a lake or pond.
The most marked effect, however, of seaquakes is the starting of the
great wave on the coasts of continents and islands.
There are certain parts of the ocean that are especially liable to
seaquakes. Some of the more important of these, as shown by Rudolph's
researches, are:
The region already referred to in the narrowest parts of the Atlantic
Ocean between Africa and South America almost immediately under the
equator. Here there are two well marked regions. One is in lat. 1°
N., long. 30° W., where there is a submarine ridge, the tops of which
form what are known as St. Paul's Rock. The ocean here is very deep,
the slopes of the ridge descending rapidly. It is on these slopes
that earthquakes are very apt to occur just as they do on the steep
slopes of mountain ranges. The other region, called by Rudolph the
_Equatorial District_, lies a little further to the east on both sides
of the equator in long. 20° W.
It appears from Rudolph's researches that between 1845 and 1893 no
less than thirty-seven seaquakes were reported in the logs of ships
in the neighborhood of St. Paul's Rock, and between 1747 and 1890, in
the equatorial district, there were forty-nine seaquakes. It must not
be supposed, however, that these were all the quakes in the regions
during these times, since, of course, many shocks must have happened
that were not felt even by vessels in the neighborhood and many more,
when this portion of the ocean was free from any craft.
In the North Atlantic there is a portion of the ocean's bed known as
the _West Indies Deep_. Here the bed is marked by great depths and by
many irregularities and is, therefore, a region where seaquakes are
common.
Still another district is found in the North Atlantic in the
neighborhood of the Azores. This is the region in which the Lisbon
earthquake is believed to have started.
Another region where seaquakes are common is in the Pacific along the
coast of South America from the equator to 45° S. lat. "Here," says
Dutton, "especially in the vicinity of the angle where the Peruvian
and Chilian coasts meet have they been most numerous and formidable.
The harbors of Pisco, Arica, Tacua, Iquiqui, and Pisago have been
repeatedly subject to these destructive invasions."
There has been considerable discussion as to the exact manner in which
the earthquake waves are set up. Whatever be the cause or causes,
the action must be sudden, such as an upheaval of the bottom, or a
collapse of a large section of the ocean's bed, with a dropping of a
vast body of water. Or, possibly, a submarine volcanic eruption causes
the water to lift suddenly under pressure of steam generated by escape
of the lava and other hot volcanic products.
Dr. Rudolph attributes earthquake waves to submarine volcanic
eruptions alone. It would seem, however, as if each one of the other
things above referred to might at times be the direct cause.