Earthquakes, more than any other phenomena, bring home to us the
relative insignificance of man, and his helplessness, in the presence
of any of the titanic activities of nature. The recent earthquakes
which have occurred in Japan have brought this subject to the fore, and
have set scientists to work afresh on the study of their causation.
Broadly speaking, it may be said that earthquakes are caused by
relatively slight changes in the earth’s crust. The earth is
continuously shrinking, and, as it does so, readjustments of the land
and water surfaces of the globe take place. When an earthquake occurs
under the ocean, this is less noticeable to us, for the reason that the
fluid water rapidly adjusts itself to the changed contour of the ocean
bed; but land earthquakes are not only more obviously noticeable to
us, but cause vast destruction of lives and property.
Certain areas of the land surface of the globe are particularly
susceptible to earthquakes. Japan and Central and South America may
be mentioned as countries where earthquakes are most frequent. The
tremors of earthquakes are transmitted at varying speeds through the
earth’s crust, which speeds can be measured by an instrument known as
a “Seismograph.” Sometimes these earth-tremors are slight, and are
transmitted across only a superficial and limited area of the earth
crust. Sometimes they are vast, and are transmitted across practically
the whole globe, so that it may be said that in such cases, the whole
earth trembles.
Land earthquakes which occur near the seashore or in the ocean-bed near
the coast-line are often the most destructive, being followed by tidal
waves of enormous height. During an earthquake in Alaska in 1899, the
land swayed and waves rose and fell 80 feet every few minutes. Great
destruction was wrought up to 40 feet above sea-level.
During a great earthquake shock in 1835, on the Chilean coast the sea
retired from the bay, so that sand banks usually from 40 to 50 feet
beneath the surface of the sea were exposed. Then a monstrous wave
returned 30 feet above high water mark, and swept all before it. This
was followed by a second and third, each exceeding the previous one in
power, and it was three days before the tremblings of the earth finally
came to rest. These shocks were felt 6,000 miles away, and caused
violent floods upon islands in the Mid-Pacific. The whole of the ocean
oscillated in its bed.
The Carnegie Institute of Washington has recently undertaken the
protracted study of the ocean bed along the Chilean coast, with a
view to discovering the causes of these earthquakes. The American
destroyers--the Hull and the Corry--have been equipped with novel
apparatus, by means of which they will make a complete map of the ocean
floor along the western American coasts. This apparatus is based upon
the invention of Professor Harvey C. Hayes, of the Naval Engineering
Station, at Annapolis. It is based upon the principle of the speed with
which sound can be reflected from the ocean-bed, so that its depth may
be immediately ascertained. In this way a chart can be made of the
bottom of the ocean, and possible earthquake-areas readily detected.