Harrisburg telegraph. [volume] (Harrisburg, Pa.) 1879-1948, 1919-01-02
Why the Earth
Ouakes
by Prof. &. Il. seryiss

QcahEo-it Is HeueEdllg supposed
TO Be Because II Is Slowly Coolins
OF Within but there is Curious
Theory Ascribing the Trouble to the
'Pear-shape' of the Earth.
What causes earthQuakes?"-R
NJ. c.. Brooklyn"
The principal causes are: Cd
The movement of underground rocb
masses yielding to the strain TO which
the crust OF the globe is subjected by
unbalanced forces, This is the cause
of the majority of great earthquakes
~ Volcanic eruptions sometimes
cause earthquakes by upsetting the
equilibrium of the rock strata in the
neighborhood tSl Local earth.
auakes. of no great intensity. may be
caused by the fall of subterranean
cavern roofs. or the sudden subsid-
cnce of an increasingly heavy deposit
OF soil washed from the mountains bv
water. (4 Explosions of steam;
formed from sea-water that has
penetrated into heated rocks, may
also cause earthquakes. Prof.
T. !. J. See regards this as the
chief cause of earthuakes. (5l The
unsteadiness of the earth's axis of
rotation. whose poles wander about
irregularly, few yards from the
positions that they would continually
occupy ii the axis were rigidly fixed
has also been assigned as possible
cause of earthquakes.
Statistics show that when the de-
parture of the poles from their mean
places is greatest. or when rapid
Ia .n1- ,Nln t., hA Il ian
OF their movement, making a decided
'wobble." earthquakes are more fire.
Quent than when the great whirling
globe sleeps" on a steadier axis. like
perfectly balanced top. (6) The
tidal strain of the attraction of the
sun and the moon on the earth is also
considered by some 10 have an effect
in inducing earthquakes,
You will perceive, from all this.
what a delicately balanced thing this
huge globe of ours is. Its vast magni-
tude and immense weight make it
tremble. with molecular as we'll as
molar vibrations. like heavily loaded
elastic floor, which, though strong
enough as whole to bear its burden,
shivers with almost imperceptible
undulations a. every shifting of
weight, and impresses the sensitive
observer with a consciousness of the
strain that is being borne, con-
sciousness that sometimes becomes
startlingly vivid when a cracking
indicates a quick readjustment of
the opposing forces
The fact that most earthquakes
occur along certain well-known belts
of the earth's surface proves that
within these belts the crust is less
solidly adjusted than elsewhere.
Lines of weakness" exist there. with
assures and faults where. when the
rocks begin to buckle under tha
strain, slips and downfalls take place
which produce the effect of hammer
strokes on the solid crust and sends
vibrations through the earth as if it
were a giant bell. The origin of the
strains which bring about these ef-
fects must be found ultimately in the
changes of figure that the globe un-
dergoes as it slowly shrinlls within
while its crust wrinkles like the skin
of drying apple.
That the earth is very hot insiIe
and at no very great depth from the
surface is certain, and equally GER
tain is 1t that the heat must. howPv-r
slowly, escape into surrounding
space, and this escape of heat must,
in turn, bring about shrinkage suffi-
cient to prevent the soliditfed crust
from settling into a state of perma-
nent equilibrium.
One curious suggestion derived
from studies of the precise shape OF
the earth is that the crushing and
buckling forces which disturb the
crust may arise, at least in part. from
the tendency of the planet to pass
under the pull of its own gravitation
from form slightly resembling the
outlines of :; pear to truly spherical
shape. AS long as the pear shape con-
tinues. the crust can never be at rest.
1t is Professor ]. H. JeanS who has
assigned this peculiar form to the
earth According to his interpreta-
tion OF the measures made by various
methods the small end of the earth
pear is in the southern hemisphere,
and includes the lofty Antarctic conti-
nent. as we'll us part of Australia.
while the broader end is in the mid
die of the North Atlantic Ocean and
the belt or middle part of the pear
includes the regions where the great
est continental masses of the earth
exist.
Professor Jeans points out that the
lines along which the chief earth
quake regions lie follow the belt or
equatorial circumference of the pear,
where the disturbing forces would be
most active.
If YOU will look at a school globe
of the earth you will see the location
of these regions. But, even if you
had all exact model of the earth as
large as house you could not clearly
see the pear-shape because the rela
tive elevations and depressions on
which it depends are very slight when
compared with the size of the earth
as a whole. This is complicated, too,
with the fiattening about the poles of
the axis of rotation.
Many other curious conclusions
have been drawn from studies of the
earth's shape, all of which have some
bearing on the question of the origin
of disturbances within its mass, for
when you have huge body revolving
swiftly, as the earth does, il cannot
20 with perfect steadiness if it lacks
symmetry. Upon the whole we may
regard ourselves as very fortunate
that the earth behaves as we'll as it
does with regard 10 our comfort.
SMASHED BY HUN FIRE,