Belmont chronicle. [volume] (St. Clairsville, Ohio) 1855-1973, 1885-05-21
How Earthquakes Are Caused.

hurricanes which followed the Spanish
earthquakes were not produced by those
subterranean disturbances, and all ex-
planning electricity has been called upon
to explain how earth throes might have
caused atmospheric disturbances. 1
know of no way in which such conse-
quences could have followed from dis
placement of the earth's crust To me
it seems far more natural to conclude
that the hurricanes and earthquakes
were alike produced (the hurricanes
chiefly, the earthquakes partially by the
atmospheric compression which preceded
the subterranean disturbances. This
compression indicated heaping of air
over the disturbed region; the earth's
crust yielded under this increase of pres
sure, combined with the action of other
forces. and earthquakes followed; the
compressed air swept away to regions of
less pressure, and the rarefaction fol-
lowing led in the usual way to the in
draught which precedes a cyclonie dis
turbaHce in the air.
But while the action of atmospheric
pressure in helping to excite subterra-
mean activities must not be overlooked.
the varying pressure exerted by seas and
oceans is more potent disturbing fac
tor. Atmospheric pressure is distributed
in such way that through the weight
of air on any given area is continually
changing, there are no sharply defined
lines, at any time, which separate regions
of less pressure from regions of greater
pressure. 1t is otherwise with the sea
along a shore line. Here we have the
sea acting with constantly varying in
tensity, as its level changes, on the sea-
ward side of the shore line, while on the
landward side there are no such varia-
tions of pressure. Let US consider what
this means. Take tolerably straight
shore line 500 miles in length, and sup
pose that along this shore line a region
of ocean 100 miles broad rises through a
height of three feet under the combined
action of sun and moon raising tidal
wave, and favering strong winds urging
the water shoreward. Then we have
$4,000 square miles of sea water, three
feet deep, added as so much dead weight
to that part of the earth's crust which
underlies the seas along that shore. Each
square mile contains in round numbers
100000 square yards, or 27,0OO,O00
square feet. The additional weight COR
responds, then (as the added LAYER is
three feet deep, to $4,000 times 81 ,OOO.-
900 cubic feet of water, each weighing
6415 pounds, or to 116,OOOO0O, 600 tons.
1t is clear that the addition of so enor-
nous weight as this to the submerged
part of. the earths crust, outside the
shore line, may well produce strains too
great to be resisted. 1t must be remem-
5ered that the very existence of pre-
cipitous shore line Gas distinguished from
one where the land above water and the
parts submerged form one great slOpe)
indicates the comparative weakness Of
the crust along that coast. 1t has yielded
on one side to pressure thrusting it up
ward above the sea level, and on the
other side to the pressure of the water
forcing it down. 1t is true, the actual
line of yielding may not coincide with
the existent shore line. For the action
of the sea waves may (and generally
must) have altered the position of the
coast from that which it occupied when
first formed. But it may be taken for
granted that not far from every precip-
itOus shore line lies line of weakness,
where the crust has given way in the
past, and may give way again. In this
consideration undoubtedly we find part
of the explanation of the observed fact
that almost all the great regions of sub
terranean activity on the earth lie near
the sea shore.
But while the changes of atmospheric
and oceanic pressure are potent factors
in the production of earthquakes, and
are probably in the great number of
cases their direct occasion, it is, of
course,to the subterranean regions them
selves that we must look for the forces
at work in upheaving the crust of the
earth. The forces acting on the outside
are as the pull on the trigger; the i'm
prisoned gases and vspOrs generated by
internal heat are as the powder by whose
explosion the missile is elected.
Yet even in considering the earth'5
subterranean activities we still have to
look outside for part at least of the
causes of disturbance. The air perhaps
may in this respect be neglected. but the
water is all important. 1t has been said,
indeed, and probably with a nearer ap-
proach to truth than usual in the case
of generalizations Of the sort, Without
water there can be no volcano" and a
similar rule. mot quite so generaD sp-
plies to earthquakes; few probably oc>
cur, possibly none, save through the ae-
ton of water in some way or other. All
active volcanoes except one tin mid
Asial are by the sea shore. Nearly all
the great earthquakes recorded 10y his
tory have taken place, and have appar-
emily had their center of disturbance,
near the sea.
There can be very little doubt, indeed,
that the direct cause of every great sub
terranean disturbance is water in the
| form of steam-steam superheated. un-
der great pressure, ana therefore possess
ing much greater expansive power than
steam at ordinary temperatures.-Rich
ard A. Proctor, in Harper's Magazine for