Los Angeles herald., 1906-04-19
SOME Of WORLDS
GREATEST QUAkES
CAUSES AND RESULTS OF BIG
DISTURBANCES
Scientific Article Treats on History
of Most Violent Earthquakes
During Last Five
Centuries


earthquakes, which explains their
cause and gives in detail an account
of some OF the greatest shocks in the
worlds history, will prove of interest:
Earthquakes consist of a series of un-
dulatory impulses or elastic waves
transmitted through the rocks of the
crust of the earth in somewhat the
same way that sound is transmitted
through the air. These waves are Of
two classes; first compression waves
propagated parallel to the direction of
transmission, second distortion waves
normal to the direction of transmits
sion. The speed with which these
waves are transmitted varies greatly.
1t is greatest near the starting point
and diminisheS somewhat as the waves
die out. The speed is also affected
by differences in the elasticity and
homogeneousness of rock masses. In
general it varies from several hundred
to several thousand feet per second.
The waves of the Charleston earth
quake, August 81 1886, had the great
speed of nearly 10000 feet per second.
The destructive effects of earthquakes
are due to the high velocity of the
unduIatory impulses rather than to
the amplitude of the vibrations, since
the latter is usually only very small
fraction of an inch and but rarely
is as much as two inches. The violence
Of earthquakes varies greatly. Deli
cate instruments, seismographs, con-
structed to show variations in the STA
bility of the earth's crust show that
the crust is almost continually tremb-
ling even in regions where shocks of
some magnitude are rarely felt.
Seismic Center
Earthquakes are sometimes classified
as central and linear. In the first the
undulatory impulses radiate from a
center of origin or focus in all direc-
tions. In the second the strong i'm
pulses are felt along narrow strips of
the earth's crust. If rocks were of
equal elasticity and an earthquake
focus were a point the undulatory i'm
pulses would travel outwardly in all
directions, and at the surface Of the
earth points equally distant from the
focus or seismic center would feel the
shock at the same moment and the
isoseisms or lines drawn through
places simultaneously affected would
form circles. The point over the seis-
mic center when the shock is vertical
is termed the epicentrum. The results
from the study of earthquake data are
determination of the focus Is not an
easy matter. In general, the seismic
center has been found to be not
point but an area, and in the case OF
the Agram earthquake in Hungary,
1880, the area must have been large
since vertical impulses were felt over
a considerable region. 1t also is known
that the seismic center is very sell
dom thirty miles below the surface and
piprhf
miles.
Extent of Shocks
The seismic center of the Charleston
earthquake was at a depth of about
twelve miles. An earthquake tremor
may be felt on the surface distinctly,
and not in the lower levels of deep
mine and vice versa. The size of the
area when the trembling Is perceptible
probably bears some proportion to the
violence of the initial shock. The great
Lisbon earthquake, Its, was distinctly
felt from Madeira to Great Britain, or
over the whole southwest Europe and
northwest Africa. The Charleston
earthquake was felt from Northern
Florida to New England and as far
west as Iowa, or over an area approx
imately 1000 miles long and 300 miles
wide. In the Lisbon earthquake the
tremors lasted but about six minutes.
In the Calabrian earthquakes, lfS3 ana
1887, the trembling was practically
continuous for four years. When an
earthquake occurs beneath the sea the
ocean above is uplifted and the shock
spreads outwardly in low, long waves
that travel rapidly but are slower than
the earthquake.
High Speed Of Waves
In mid ocean such waves are of no
importance, but as they approach shore
the drag Of the sea bottom decreases
their speed and length, but greatly in
creases their height, and what are err
roneousy termed tidal waves result.
on low lying coast such waves may
be very destructive. The explosion of
Krakatoa in, August, 1883. caused sea
waves that went around the world at a
speed of 400 miles an hour At distant
points like the coast of Europe the
waves were slight and were detected
by tide gauges. On the coast of Java
the waves were over 60 feet high and