Probably no part of the earth’s surface is entirely free from vibration,
but, fortunately, destructive earthquakes are confined to comparatively
limited regions. In most cases each shock lasts only a few seconds, but
the tremblings that follow may be continued for days, weeks, or even
months. Noises of sundry kinds usually precede, accompany, or succeed an
earthquake. Some earthquakes, however, are not attended by any
subterranean sounds. This has been the case with some of the most
destructive South American disturbances. Thus at the time of the
terrible shock which destroyed Riobamba in Ecuador in 1797, a complete
silence reigned. On the other hand, subterranean sounds may be heard
without any earth-tremor being perceived.
The sound which accompanies many earthquakes is due to the transmission
to the air of vibrations in the soil. To produce sound-waves in the air,
the ground must vibrate like a drumhead. Hence no sound will be heard
when the oscillations are horizontal.
The velocity of propagation of an earthquake is very variable. Thus in
the case of the earthquake of Lisbon in 1755, it seems to have
considerably exceeded one thousand feet per second, while in the Lisbon
earthquake of 1761 the rate was three times greater. At Tokio, in 1881,
the velocity, as estimated by Professor Milne, varied between four
thousand feet and nine thousand feet per second.
DEPTH OF EARTHQUAKES. Various attempts have been made to estimate the
depth at which earthquakes originate. Mallet was of opinion that the
centrum of the Neapolitan earthquake of 1857 was probably five and
one-half miles from the surface. The same eminent physicist thought that
an earthquake centrum probably never exceeded a depth of thirty
geographical miles. According to Professor Milne, the angles of
emergence of the earth-waves obtained during the Yokohama earthquake of
1880 showed that the depth of origin of that earthquake might be between
one and one-half and five miles; and he gives a table, compiled from the
writings of various observers, which exhibits the mean depths at which
certain earthquakes have originated. These estimated depths range from
17,260 feet to 127,309 feet.
The area disturbed by an earthquake is generally proportionate to the
intensity of the shock. The great earthquake of Lisbon disturbed an
area four times as great as the whole of Europe. In the form of tremors
and pulsations, Mr. Milne remarks, it may have shaken the whole globe.
In a violent submarine earthquake the ordinary earth-wave and sound-wave
are accompanied by sea-waves. These waves may be twenty, sixty or even
eighty feet higher than the highest tide, and are usually more dreaded
than the earthquake shock itself in such regions as the maritime
districts of South America. The greatest sea-wave on record is that
which in 1737, is said to have broken near Cape Lopatka, at the south
end of Kamchatka, two hundred and ten feet in height.
=NOTABLY DESTRUCTIVE EARTHQUAKES=
79. One accompanied by the eruption of Vesuvius; the cities of
Pompeii and Herculaneum buried.
742. Awful one in Syria, Palestine, and Asia; more than 500 towns
were destroyed and the loss of life surpassed all calculations.