Savannah morning news. [volume] (Savannah) 1868-1887, 1886-09-04
WHAT EARTHQUAKES ARE.
An Interesting Article by Prof. W.
lie Comic Stevens.
Eron. a Neao York WIZ.

From. Tho Ne'O York World.
The earthquake shock which has done
so much damage during the last thirty
ak hours at the South, especially in the
stricken city of Charleston, 5. c., is the
seyarest that has been felt on the eastern
coast of America during the present GEN.
fury, and scarcely, if at all, surpassed in
violence by any CO the Pacific coast OF our
continent. Its effect seems to have been
felt over nearly the whole area OF the
Tinted States east of the Rocky Noun
tains. The intense popular interest Just
now excited in phenomena OF this class
makes it important that brief general
discussion of earthquakes be given in con-
nectlon with the recital Of these terrible
events that will be remembered through
out the lifetime OF all whose homes have
been desolated or whose friends have
been exposed to peril.
An earthquake is cnmmotion prOpa-
gated through the earth's crust in all di-
rectlons from some source OF disturbance,
in waves that OutcrOp on the surface, and
are manifested In the perceptible motion
of the ground. They vary in intensity
from gentle tremblngs, requiring delicate
instruments for their detection, to conVul-
sions of terrific violence, capable of de-
strOying the most substantial products Of
human labor.
During the last thirty wears the study
OF earthquakes has been developed into a
special branch Of science to which the
name seismology has been given. The
first man who gave the word any reliable
scientific discussion of such phenomena
was Mr.. Robert MalIett, of England, who
visited Naples Immediately after tue
earthquake which devastated Southern
Italy in 1857. In his subsequent labors
ne received important aid from his son,
Dr. John w. Matletr, whose name has
since become prominently associated with
the progress of chemical science in
America. The number of investigators
in recent years has been greatly multi
piled. instruments have been devised
for measuring the intensity of shocks and
for recording the direction and register
ing The time of transit. In regions where
earthquakes are of frequent occurrence
such seIsmometers are distributed at
many points and kept in constant readl-
ness. 1t is from systematic records from
these that We derive our most important
information to-day.
REGIONS AFFECTED.
America is fortunately not often shak-
en-, and the number of seismOmeters kept
In readiness in our country is too small TO
assure us of accurate knowledge regard
ing the recent shock. Earthquakes occur
in all parts of the world, but nre most
frequent in mountainous regions, and es-
pecially those of volcanic activity. In
many cases the shocks are directly the
consequence of volcanic eruption, but
they occur also in places and under cir
cumstances that Indicate not the least
connection with volcanoes. This seams
to be true of the recent Charleston earth
quake.
The Andes region of South America and
the southern part OF Italy include some
of the worst shaken districts in the world,
and some of the most active volcanoes.
But among them earthquakes often occur
without the exhibition of any unusual
activity. The earthquake which
was so destructive to lie aud property a
few days ago in Greece and Italy was im-
mediately followed by commOtion in the
crater of Vesuvius. 1t is safe to say that
the same cause may give rise to both
volcanoes and earthquakes, but neither |
Of these implies the other necessarily.
The Himalayas and table lands OF Central |
Asia are subject TO violent earthquakes, |
but almost free from volcanoes. The |
Alps and Pyrenees are often shaken, bug |
include no volcanoes that are not eX-
tinct. The same is true of the American |
hCihC COdSb range oOuJu VI buG Obluul-
bia river. NOn-vOlcanIc basins, such as |
the valley Of the Mississippi and the basin !
CT the Baltic Sea, have been roughly |
shaken. Earthquakes are most rare in
Russia, which consists mostly of low |
plain free from mountains.
The region most shaken within the past |
two days is near the southern extremity |
Of the Allegheny range. 1t is too early |
yet to form any reliable estimate of the |
point where the disturbance originated. |
Mr. Mallett collected a catalogue Of be- |
tween 5000 and 1000 earthquakes record |
ed between the periods b. &. 1906 and A. !
d. 1850. This is obviously mere frac- |
ton of the number that occurred between |
those dates. Since 1850 the number of |
earthquakes recorded has greatly in !
creased, though there Is no reason to be- |
lleye that there is any increase in actual |
frequency Of shocks. BY the use of seie- !
mometers in Italy, Japan and elsewhere |
II has been proved that in such regions |
scarcely day passes without a notice. |
able earth tremor. Probably not mo- |
ment elapses without an earthquake IN
some part OF the world. The surface is !
continually in q CULVER, hut fortunately !
such titanic agUes as those that throw |
down cities are comparatively rare.
CAUSE OF EARTHQUAKE.
No one cause can be properly an
hounced as fully accounting for all earth |
quakes. BY far the most frequent is the |
breaking OF the earth's crust under the |
same surface contraction that squeeZes |
out lava from volcanoes and folds the
regions OF weakness into mountain
ranges. The earth continues to-day TO
give out into space the heat which was
locked in its interior during the early
stages Of its planetary history, The rate
is now slow, but even though almost IN.
nnlteslmal, it involves contraction. The
settling OF the crust upon a diminishing
interior necessitates bending OF the strata.
ana the rock is put into condition of
gradually increasing strain, until at
some point it suddenly yields with a vio-
tent concussion. A assure is produced,
one wall OF which may slide past tue
oIner, producing "iault' aud giving
strong impulse to the adjacent rock. All
Ioc& is more or less elastic, and the I'M
pulse IS propagated in all directions rs
ah earth wave. The violence of an OBJ.
served earthquake shock may therefore
uepend upon the following conditions:
l. The degree OF strain preceding the
concusson.
= The extent Of rock that yields.
~ o. The nature 81 tho rock transmitting
tae Impulse.
- The distance of tho observer from
'us starting point OF the eartn wave.
laults have been frequently found
Muero the ore wall has slipped past the
tVe'uet' not merely several feet, but many
SSNousands Of feet. One or more such are
rouud IN toe Appalachian mountGin chain