Evening star., 1923-09-13
EARTHQUAKES ROLL
OR HIT 0UTWARDLY
Geographic Society Bulletin
Explains Waves and How
They Start.

Exactly what is an earthquake?
a bulletin from the local headquar-
.ters of the National Geographic So
ciety answers the question: 1T is
Quaking of the earth or part of it;
and theoretically any vibration at all
nils the bill. The. quake in so far
as man experiences it is the coming
10 the surface of waves in the earth.
Ihese waves cannot be compared
simply to the up-and-down waves or
r1ppled water, nor to the back-and-
forth waves in the air that make
sound. Even the simplest wave mo-
ton Is none too easy to understand:
and the waves in the varying earth
materials that make themselves known
TO man as earthquakes are probably
Ihe most complex waves that exist.
They are at once motions 'up-and-
down like sea waves; crosswise like
the waves in q tapping flag; back
and-forth like sound waves or the
motion in a long coiled spring suS-
penoed with q weight popping al It's
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 fin 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 constantly grow
outward. Soon, however, because of
the difference in the rigidity of differ
ent rock materials, the form is not a
sphere at all. but a very irregular
curved solid instead, more nearly that
pf a. potato, perhaps.
Two Kinds of Waves.
'The two principal types of earth
waves, those that travel like the pull
sations in bobbing spring, straight
out; and those that travel like ripples
and the waves in a flag, with side
wise motion, move at different speeds.
The pulSing waves are the swifter
and of course the place where they
make themselves feat first and most
: atrongly is the surface area straight
above their starting point. This is
technically the 'epicenter' or 'epicen-
Lrum. The pulsing waves around
the epicenter 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 surfaces
or q very few inches on soft shell,
the violence may be like that from in
Vulnerable tremendous blows and Suf-
fcient to shake down buildings.
The 'sideWise' waves follow along
almost Immediately after the first
pulsfng waves, and the two, with
perhaps twisting waves as well, then
operate together. The ground in the
cpicentral area, therefore, gets in
Vulnerable thrusts and falls and at
the same time is jerked sidewise and
back for tiny distances in every direc-
tion.
"At points some distance from the
epicenter the pulsing waves strike the
surface at a slant and so accentuate
the effects of the 'sidewise' waves
and create new surface undulations
The shaking at the epicenter, too,
throws the earth's surface there into
undulatory waves like rippIes on a
pond, and these also travel outward
in widening circles. These surface
fwaves are those of greatest motion,
I but least speed. They register the
heaviest vibrations on the seismo-
Hgraphs of the world.
u The second important question in
3Tegard to an earthquake is: What
starts the waves?
4 No one has ever seen an eagth-
lQuake wave start outward from its
4center, and it is safe enough to say.
that no one ever will. But science