The term earthquake is applied to any tremor or
shaking of the ground. Many earthquakes are so gentle as to pass almost
unrecognized, other are sufficiently pronounced to excite general alarm, while
some spread enormous destruction. Destructive earthquakes are usually confined
to limited regions.
The usual phenomena recorded in well-known
earthquakes are first a trembling, next one or more serve shocks, and then a
trembling which gradually dies away. In most cases, each shock lasts only a few
seconds, but the trembling’s that follow may continue for days, weeks or even
months. Noises of various kinds usually accompany an earthquake. They have been
likened to the growling of storm, the growling of thunder, the clanking and
clashing of iron chains, or the rumbling of heavy wagons along a road. Such
noises are conducted through the ground, or they ma travel though the sea or
air, and are often heard at great distances from the place where the shock is
felt. Some earthquakes however are not accompanied by these noises. At the time
of the terrible chock which destroyed Riobamba in Ecuador on February 4, 1797,
a complete silence reigned.
Many changes are produced on the earth’s surface”
by earthquakes. They cause landslips and cracks in the earth, which will
sometimes alter the drainage system of a country. They are frequently
accompanied by great sea waves, which will often...sweep rocks and sand great
distances inland. Permanent elevations and depressions of land are sometimes
caused. After the great earthquakes of 1750, the coast of Chili was found to
have been permanently raised from three to four feet. Well-known examples of
permanent depressions are those of the Run of Cuts and the coast lands near
Chittagong, which suddenly sank during the Bengal earthquake of 1762.
Earthquakes are most common is volcanic and
mountainous regions, and many of them are no doubt due to volcanic action.
These appear to originate in the sea, and may be due to the flashing into steam
of the water which finds its way down through cracks to the underlying heated
rocks. Tethers appear to originate in volcanoes themselves, being due to the
explosion of vapors which expand. Many other causes are ascribed, of which two
may be mentioned. Some earthquakes may be due to the collapse of hollows
beneath the ground, and others again to the snapping of strata which has been
subjected to too great a strain. It is noticeable that most earthquakes occur
during the cold months or winter.
Among destructive earthquakes in modern times may
be mentioned the one that altered the Straits of Messing between Italy and
Sicily in 1908. And the terrible upheaval in Japan in 1925, which destroyed
whole towns and caused the death of thousands of people.