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 a 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:
"SIperfcially 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
even 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
wave motion Is none too easy to
understand: n 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 at
once motions 'up-an-down' like sea
waves; crosswise like the WAYS in
q tapping flag: baek-and-forth like
sound waves or the motion in a long
coiled spring suspended with a
weight bobbing at its end; and in
addition nay 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
hwaves 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 rigiditv 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.
u lNO Pl iilSlBdi " pes VI GdIlll
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 most strongly is the
surface area straight above their
| Oh rtf.,- NAE..A nut l. -.
the ~'epicenter' or 'epicentrum.' The
pulsating waves around the epicen-
ter alternately push up the surface
OF the earth and everything on it,
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 3on soft soil. the
violence may be like that from in
Vulnerable tremendous blows, and
sufficient to shake down buildings
THE 'sidewise' waves follow alons
almost Immediately after the frs
pulsing waves, and the two. witt
perhaps twisting waves as we'll
then operate together. The GROUND
in the epicentral area therefore
gets innumerable thrusts and fall!
and at the same time is ierked side
wise and back and forth for tin,
distances In every direction.
AT points some distance from tn
epicerter the pulsing ave strik
the surface at a slant and so accen
tuate the effects of the 'sidewise
waves and create fluctuations. TIL
shaking at the epicenter. too, throw
the earth's surface there into undu
latory waves like ripples on a pond
and these also travel outward il
widening circles. These surfac,
wa'ras are those OF greatest motion
but least speed. They register tn
heaviest vibrations on the selsmo
graphs of the world.
THE second important question
m regard to an earthquake is: VIVA
starts the waves?
mo one has ever seer an earth.
quake wave start outward from it
n conter. and it is safe enough to sa,
that no one ever will. But scienc,
I has built certain hypothesis abou
the matter that seem to accoun
o satisfactorily for the happenings
and which are accepted pretty gen
: orally.
Hold a dry stick as big as you,
n thumb in your hands mi bend 1
until it breaks. At the snap wave
will travel to your hands and usu
ally give you quite painful sensa
ton.
Hold an iron bar by one en-,
and strike the other sharply witt
Ia hammer. Again n vihrationu oar