The Coolidge examiner., 1931-04-10
prepared by the National Geographic
Society. Washington. d. C)
(WNU Service0

tRrepdre0 Oy the national Geographic
Society. Washington. d. CD
(WNU Service1
TINE occurrence of a destruc
tive earthquake such as thai
which recently practically de
strOyed the city of Napier, New
Zealand, sets the world to thinking
of and perhaps to fearing these de-
vaStating tremOrs. 1t is wrongly as
sumed by many people that an
earthquake is likely to occur al
any place.
Up to 1906, il is computed by an
eminent scientist, COmpte de Bal.
lore. there had been 159,7S2 record
ed earthquakes. In later years
when more accurate records have
been kept, they have averaged
about GO a year. There is comfort
to the dwellers in most of the
world to know that 94 per cent of
recorded shocks have occurred in
two narrow, well-defined belts
one called the Mediterranean. with
53 per cent to its credit, and the
other the Circum-Pacific, with Al
per cent-while the remainder of
the, world has only 6 per cent
widely distributed.
The United States has been sin
gularly free from recorded seismic
disturbance, perhaps the most dis
aStrOus being in 1911, when a very
sissippi valley south of the Ohio,
which was felt in New York in
one direction and in the West In
dies in another. This earthquake
changed the face of the earth. A
vast extent of land was sunk, lakes
were formed, and even the course
of the Mississippi river was OBJ
structed for time.
Exactly what is an earthquake
and what causes it?
Specifically 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 heavy truck
passing along the street or a blast
in a nearby quarry to the most vio-
lent cOnvulsiOn that slides hills
from their bases and opens yawn-
ing chasmS in the earth.
Waves 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 it 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; anG the waves
in the varying earth materials that
make themselves known to man as
earthquakes are probably the most
complex waves that exist. They
are at once motions "up-and-down"
like sea waves; crosswise like the
waves in Tapping flag; back-and-
forth like sound waves or the mo-
tion in a long cOiled spring sus-
pended with weight bObbing at
its end; and in addition may have
twists about imaginary 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 rigidity of different rock ma
terialS, the form is not a sphere
at all but a very irregular curved
solid intsead. more nearly that of
a potato, perhaps.
The two principal types of earth
waves, those that travel like the
pulsations in a bobbing spring,
straight out; and those that travel
like ripples and the waves in a
flag. with a sidewise motion, move
at different speeds. The pulsing
W oNCS IhC o"ItOls anu vi NOuI SC
the place where they make them
selves felt first and most strongly
is the surface area straight above
their starting point. This is tech
nically the 'epicenter" or "epicen-
trum." The pulsing waves around
the epicenter alternately push up
the surface of the earth and every
thing on it, and let them fall back.
When the Waves Combine.
The "sidewiSe" waves follow
along Immediately after the first
pulsing waves, and the two, with
perhaps twisting waves" as we'll
then operate together. The ground
in the epicentral area, therefore
gets innumerable thrusts and falls
and at the same time is jerked
sidewise and back for tiny dis
tanceS in every direction.
At points some distance from the