Worcester Democrat and the ledger-enterprise. (Pocomoke City, Md.) 1921-1953, 1923-12-08
WHAT AND why OF.
AN EARTHQUAKE
Caused by Earth Waves 0rigi-
mating in Inner Regions.
TWO PRINCIPAL WAVE TYPES

Exactly what is an earthquake and
what causes it?
These two questions very naturally
pop into the minds of millions Of per-
sons when a catastrophe such as that
which recently occurred in japan over
takes some great city and causes thou
sands of deaths.
A bulletin from the Washington, d.
G., headquarters of the National Geo
graphic society answers these ques-
tons:
"Supericially the name itself tells
the story. It is no more nor less than
a quaking Of the earth or part of
lit; and theoretically any vibration at
: all fills the bill, from the tremor
caused by heavy truck passing along
the street or blast in nearby quarry
to the most violent CONFUSION that
slides hills from their bases and opens
yawning chasms in the earth.
Earth Waves Hold Secret.
"But going beyond the superficial.
there Is more that needs to be said
even by way Of bare definition Of an
earthquake. The quake in so far az
man experiences 1t Is the coming to
the surface of waves in the earth.
These waves cannot be compared sim
ply to the up-and-down waves Of rip
pied 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; and 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 q
tapping flag; back-and-forth like sound
waves or the motion in a long cOiled
spring suspended with a weight bob
bling at it's end. and in addition may
have twists about imaginary pivot lines
IN all of the three dimensions.
1.-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 constantly grow outward.
Soon, however, because Of the diff
ference in the rigidity Of different
rock materials, the form Is not
sphere at all but a very irregular
curved solid instead, more nearly that
Of 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 rip
pies and the waves in a flag, with
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 starting point.
This is technically the 'epicenter' or
'epicentrum.' The pulstng waves
around the epicenter alternately push
up the surface Of the earth and every
thing 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 a very few inches on soft soil, the
violence may be like that from innu-
merable tremendous blows, and suffi-
cient to shake down buildings.
"The 'sidewise' waves follow along
almost Immediately after the first
pulsing waves, and the two, with per-
haps twisting waves as well, then
operate. together. The ground in the
epicentral area, therefore, gets in
Vulnerable thrusts and falls and at the
same time is jerked sidewise and back
for tiny distances in every direction.
AT points some distance from the
epicenter the pulsing waves strike the
surface at 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 undula-
tory waves like ripples on pond, and
these also travel outward in widening
circles. These surface waves are those
Of greatest motion but least speed.
They register the heaviest vibrations
on the seismographs Of the world.
"The second important question in
regard to an earthquake is: What
starts the waves?
mo one has ever seen an earth
quake wave start outward from its
center, and 1t is safe enough to say
that no one ever will. But science
has built certain hypothesis about the
matter that seem to account satisfac-
torily for the happenings, and which
are accepted pretty generally.
Hold dry stick as big as yonr
thumb in your hands and bend 1t un-
TIL 1t breaks. At the snap waves will
travel to your hands and usually give
you a quite painful sensation.
Hold an iron bar by one end and
strike the other sharply with a ham,
mer. Again the vibrations carried to
your hand may he violent enough to
produce singing pain.
"And If you should strike a small
explosive can with the end of long