Los Angeles herald., 1906-04-27


earth's crust, due to internal causes. 1t
consists of a series of undulatory inn
pulses or elastic waves, transmitted
through the rocks of the crust in some
what the same way that sound is trans
mitted through the air. These waves
are of two classes: "a Compression
waves propagated parallel to the di-
rection of transmission. (2) Distortion
waves normal to the direction of trans
mission. 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
the differences in the elasticity ana
homogeneousnes 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, (the data were
carefully compiled, had the great speed
of nearly 10000 feet per second. Inc
destructive effects of earthquakes are
due to the high velocity of the undula-
tory impulses rather than to the am
plitude of the vibrations, since lhe
latter is usually only very small
fraction of an inch and but rarely Is
as much as two inches.
The violence OF earthquakes Noi 'N"
greatly. Delicate instruments, seismo-
graphs. constructed to show variations
i'm the stability of the earths crust,
show trembling, even in regions where
shocks of some magnitude are rarely
felt. Earthquakes are sometimes class
flied as central and linear; in the hrst
the undulatory impulses radlate from
a center of origin or focus in all d.
rections; in the second the strong I'M
pulses are felt along narrow Strips OF
the earth's crust. If rocks We're Oh
equal elasticity and an earthquahe
focus were a point, the undulatory I'M
nulses would travel outwardly in all
directions and at the surface of the
earth points equally distant from the
focus of seismic center would reel tne
shock at the same moment ana the
isoscisms or lines drawn through places
simultaneously affected would Iorih
circles. The point over the selsmic
center when the shock is vertical Is
termed the epicentrum.
The results from the study OF ear.li-
quake data are often variable ana puH-
ling and the determination of the focus
is not an easy matter. In general, the
seismic center has been found to be not
a point but an area, and in the case oil
the Aeram earthquake in Hungary
QSSO). the area must have been large
since vertical impulses were felt over
q considerable region. 1t is also known
that the seismic center is very seluoil
thirty miles below the surface and the
oommonest depth is eight or ten miles.
The seismic center of the Charleston
earthquake was at depth OF ABOUT
twelve miles.
An earthquake tremor may be felt on
nd
lower levels of a deep mine, and wee
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 earth
quake Q775) was distinctly felt from
Madeira to Great Britain or over the
whole of southwest Europe and north-
west Africa. The Charleston eafth-
quake was felt from northern Florida
to New England and as far west as
Iowa, or over an area approximately
1000 miles long and 800 miles wide. In
the Lisbon earthquake the tremors last-
ed but about six minutes. In the cana
brian earthquake 07S3-O the trembling
was practically continuous for four
years. When an earthquake occurs
beneath the sea the ocean is uplifted
and the shock spreads outwardly in low
long waves that travel rapidly, but are
slower than the earth 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 increases their
height. and what are enormously
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 speed of 400
miles an hour At the 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 sixty feet high and thousands of
people were drowned. An earthquake
in the North Pacific il June, 1896
caused waves forty feet high along the
northern coast of Japan that did great
damage, over $1,000 people being
drowned. Such an earthquake wave
on the coast of Peru once picked up a
gunboat of the United States navy and
| left it stranded a mile inland. Earth
quakes are usually commonest in re-
gions of present or recent volcanic
activity. In the United States they are
commonest in California, and in gen
eral earthquakes are more often felt
near the coast than inland, just as vol
canoes are more often felt near the sea-
At the same time there is a seismic
zone encircling the whole earth In
which earthquakes are more numerous
than elsewhere. and this zone does not
coincide with the distribution of vol
canoes. The zone includes Central
America. the West Indies the Azores
Italy, Syria. Persia, Afghanistan
Tibet. Japan and Hawaii. 1t is by some
regarded as the real earthquake cen
ter. since shocks in regions without the
| belt are sometimes traced to disturb
ances within it. Thus the earthquakes
of the Mississippi valley QS12) ceaset
when the volcano of st. Vincent, 20O'
miles distant, burst into activity. Th'
sounds that accompany earthquakes
have been Variously compared to th'
rumbling of, thunder, the clanking O
chains, etc; sometimes loud sounds
accompany Slight shocks.
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CAUSES OF EARTHQUAKES
AS earthquakes are frequently con
nected with the eruptions of Voloanoe
and are common in volcanic regions
many observers have thought this
earthquakes were volcanic phenomena
This view was first expressed nearl,
200 years ago by Pliny and during tn
first half of the nineteenth centur,
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