Perth Amboy evening news., 1923-10-11
The What and Why
OF An Earthquake

Exactly what is an earthquake
and what causes it?
These two questions very natural
ly pop into the minds of millions of
persons when q catastrophe such as
that which recently occurred in
Japan oVertakeS some great city
and causes thousands of deaths.
A bulletin from the Washington,
d. c., headquarters of the National
Geographic Society answers these
questions:
"SIperficially the name itself tells
the story. 1t is no more nor less than
a quaking of the earth or a part of
it; and theoretically any vibration
at all fills the bill, from the tremor
caused by a heavy truck passing
along the street or a blast in a near
by quarry to the most violent con-
vulsion that slides hills from their
bases and opens yawning chasms in
the earth.
"But going beyond the superficial
there is more that needs to be said
ever by way of a bare definition of
an earthquake. The quake in so far
as man experiences 1t is the coming
to the surface of waves in the earth.
These waves cannot be compared
simply to the up and down waves
of rippled water nor to the back
and-forth waves in the air that
make sound. Even the simplest
waV9 motion Is mons too easy to
understand: and the waves in the
varying earth materials that make
themselves known to man as earth
quakes are probably the most COM
pick waves that exist. They are al
once motions 'up-an-down' like sea
waves; crosswise like the wavs in
n Tapping flag: back-and-forth like
sound waves or the motion in a long
cooled spring suspended with a
weight bobbing at its end; and in
addition may have twists about i'm
aginary pivot lines in all of the
three dimensions.
THE earth waves originate in
some region down in the earth and
start out in every direction. The
waves from a pebble dropped into a
pond form growing circles on the
surface of the water. But the
waves in the earth being in a solid,
attempt to form spheres that con-
stantly grow outward. Soon, how
ever, because of the difference in
the rightly of different rock mate
rails, the form is not a sphere at all
but a very Irregular curved solid
instead. more nearly that of a pota-
to. perhaps.
"The two principal types of earth
waves. those that travel like the
pulsations in a bobbing spring,
straight out; those that travel like
ripples and the waves in a flag,
with a sidewise motion, move at
different speeds. The pulsing waves
are the swifter, and of course the
place where they make themselves
felt FIRST and noarf efrnnnJu - kEa
surface area straight above their
starting point. This is technically
the 'epicenter' or 'epicentrum.' The
~pulsating waves around the epicen-
ter alternately push up the surface
of the earth and everything on 1t,
and let them fall back. This is done
very rapidly and though the actual
rise and fall may be only a fraction
of an inch on rocky surface or a
very few inches on soft soil. the
violence may be like that from in
Vulnerable tremendous blows, and
sufficient to shake down buildings.
The 'sidewise' waves follow along
almost Immediately after the first
pulsing waves, and the two, with
perhaps twisting waves as well,
then operate together. The ground
in the epicentral area therefore.
gets innumerable thrusts and falls
and at the same time is jerked side
wise and back n forth for tiny
distances in every direction.
AT points some distance from the
onicerter the pulsing waves strike
the surface at a slant and so accen-
tuate the effects of the 'sidewlse'
waves and create undulations. The
shaking at the epicenter, too, throws
the earth's surface there into undu-
latory ave like ripples on a pond,
and these also travel outward In
widening circles. These surface
are those OF greatest motion
but least speed. They register the
heaviest vibrations on the seismo-
graphs of the world.
THE second important question
in regard to an earthquake Is: What
starts the waves?
mo one has ever seer an earth
quake wave start outward from its
center and it is s1fo enough to saw
that no one ever will. But science
has built certain hypothesis about
the matter that seem to account
satisfactorily for the happenings.
and which are accepted pretty gem
orally.
Hold a dry stick as big as your
thumb in your hands and bend 1t
until it breaks. At the snap waves
will travel to your hands and usu-
ally give you quite painful sensa-
ton.
Hold an iron bar by one end
and strike the other sharply with
- H. - :. -.