The Salt Lake herald. [volume] (Salt Lake City [Utah]) 1870-1909, 1899-10-04
POPULAR LECTURES
IN PHYSICAL SCIENCE.
XII. EARTHQUAKES.
BY WILLIAM j. HOPKINS, S. B.

| DI WILLIAM y. MIND p ID. d
The earth beneath us. which we are
accustomed to regard as naturally firm
and stable, is in reality In a state of
continual vibration. Some of these
motions, it is true, are so exceedingly
! slow and of such gentle character
that records of past ages are needed to
detect them. Others are 50 slight and
| faint that no impression is made on our
imperfect senses, which note only those
shocks of exceptional magnitude and
violence. The footballs of an animal or
the blows of falling raindrops may pro-
duce jars of the earth which>can be de-
tected by sensitive instruments, while
! the passage of railroad train or even
of a street car may shake the house,
and such a catastrophe as the recent
magazine explosion at Toulon would
| produce upon the adjacent country the
effects of moderate earthquake, These
! tremblings, of all degrees of violence,
are so continuous that some investiga-
| tons on the disturbance of gravity by
| the moon, undertaken some years ago.
had to be given up, The earth vibra-
I! lions overpowered the effects to be ex-
| amined.
OF the slow and gentle movements of
/ the earth referred TO there are several
The continents are, in general, rising
! at very slow rate, just about sufficient
to make up for the wear of the land by
the weather. Such an exceedingly SLOW
motion as this can be detected only by
referring to geological records, but
there are other movements of a. similar,
nature which, although very slow when
reckoned by human standards, are
geoogically rapid. Parts Of the eastern
shore of North America appear to be
now sinking at the rate of a foot or so
in a century, while on other shores an
uplifting is observable. Some coasts
have within historic times made two
or three oscillations of this character.
The best known example is in the bay
of Naples, where the land has sunk
twenty feet or more, risen again by
about the same amount, and is now
again sinking at the rate Of an inch
or more a year, all since the beginning
of the Christian era.
The imposition of a great glacial
sheet of ice causes sinking of the
land, followed by a corresponding rise
when the ice melts away. The unknown
antarctic continent, under its burden
of q sheet of ice whose thickness is
measured in miles, must be depressed
by that burden many feet below the
level it would assume II that ice were
removed. Slight changes occur also,
much more rapid but yet too gentle to
be perceived except by the most deli
cate instruments. These swayings may
complete their cycle in few seconds or
"a few hours, and are perhaps due to
changes in the weight Of the atp1os-
phere. They have been called puisa-
tions.
+ In addition to these slow and gentle
oscillations there are continual slight
Earrings or tremblings of the earth,
varying in frequency, from time TO
time These can be detected only BY
such delicate instruments as the NIL
crophone or the seisniograph, when rs
an instrument designed to show by the
records of pendulumlike oscillations the
degree of violence and the direction OF
earth movements These tremors oil the
earth differ in no respect, except in
violence. from earthquakes. They are
most marked in regions subject TO
earthquake shocks and NEST noticeaple
and continuous just before violent
shocks.
characteristics of Earthquakes.
Strictly speaking, the term earth
quake" might be applied to any
trembling or Migratory movement OF
the earth. 1t s. customary, however, to
annJv the name to those shocks only
which are severe enough teforce 'tne1n-
selves upon our attention, ranging IN
violence from the gentle shaking which
not everyone notices to the shock
which destroys cities in moment, WIFI
thousands of human lives
The shock of the earthquake CONSISTS
in the passage of a wave of pressure
through the solid ground just as
wave travels in water ANY sudden
movement or conclusion in the body OF
the earth's crust would be the source OIL
one or more such waves. ROCKS are
somewhat elastic, some of them great
lv so. although they differ much in this
respect, according to their structure
and nature. A sudden blow or copcus-
sion therefore sets up vibrations In the
rocks. and these vibrations are sent
outward in all directions, with veloci-
ties which may be different in different
directions. or may even change at
every point of the path in accordance
with the nature of the rocks and soil
through which they pass ana the fela-
tions of the rocks to each other. In
these changes the waves of shock are
reflected or turned aside and may be-
come, at little distance from the
source. very much confused. The wave
from the same source might, therefore
reach any point at different times by
different paths. Well recognized exper-
fences of this kind are recorded in
which both shocks came through the
earth In any case, there Will be sep
arate shocks through the earth, the
water and the air, arriving at different
times. although produced by the same
conclusion. The air wave causes the
noise which sometimes accompanies the
earthquake, and the water wave, if the
point is on, the shore, accunuiates until
it may rush in, many feet high, and
overwhelm the ruin which the previous
land shock has accomplished.
Earthquakes often appear to be ca
PRECIOUS in their behavior, certain small
tracts being unaffected while the. whole
country Immediately about is violently
shaken In some cases the shock tray
els outward in all directions from
center, with some approach to equality,
While iii other cases it seems to pursue
a well defined track in a band or zone.
without much departure from it later
ally This class of earthquake is typi-@
cal of great mountain chains, and the
best examples are the numerous great
Of country lying between the Andes
South America.
The wave of shock constituting Han
earthquake has, at any point where it
energies upon the surface, a generally
definite direction. Where accurate in
struments are placed for the purpose
the direction of travel of the wave is
exactly recorded, and where there are
no such sensitive indicators the diree-
ton may be estimated approximately
by the sensation and BIZ the direction
of the rents in the&tWalls of buildings
or of the cracks prodused in the ground.
These rents of cracks are formed near
~ at right angles ID the line Of travel
of the wave, and by following the line