The New York herald., 1921-01-25
Catching Earthquakes.
The Origin and Dirstion Shown by
ClOckwork and Eleric Devices.

J. j. ShdiD in the Londs Daily Mail.
There is something faSnating in set
ting a seismograph lilSet mouse trap
and waiting for an earthaake to come
along. One must see tht the dock
mechanism and electric deices are al
ways up to concert pitc, o. after
weeks and months of waitinla fine rec
ord may be lost.
Earthquakes are ruptures inhe earths
crust due to bending, crumling and
thrusts, stresses which are coamon at
the foot Of mountain chais. The
stresses accumulate, the strain >ecomes
too great, finally the rock strata tactur
like an overbent stick, and hge vi
bration Is set up sufficient to shaks
down all structures In the vicirit or.
IL suboceanic. to set UP tidal wavesan
shake the crockery on board ships as
happens In a house on land.
A shock sets up different types of
waves. which rush outward at different
speeds. There are three principal tpes
designated "P," "S." and "II."--pri-
mary" secondary" and "large"-w,ie'l
attain to a maximum. NJ. then die
away. 1t is from the time that elases
between the arrival Of the primary
waves and the secondary waves thatthr
distance is calculated. Thus: One un-
dred and sixty-six seconds for 1Doe
miles, 394 seconds for 5000 miles. 5s5
seconds for 5000 miles.
The secondary waves rarely survive
after travelling 5000 miles. when tt .
time between "P." and a." or "P."lnd
M" though much less reliable. ha to
be resorted to. The only dificultyli lies
in determining from the record which
out of some hundreds of waves Is The
particular one required.
The chief seismographs In use ae
Milne, British; Milne-Shaw, Rritin
Galitzin, Russian; Weichert, Germn
Mainka, German Omori, Japanese. d
these the Galitzin and the Milne -Shm
are the most sensitive. The frsl thin,
record by a ray of light being phot.
graphed on sensitized paper: the In,
three by glass fibre writing on smoke
paper
Clockwork keeps the recording surfac
always moving. and the time Is marke
on electrically every minute. Locatin
tiiS pidSc VI VI ~ IA uiojt OuI'
cult. It's direction can be sometime
determined from one station when tn
first impulse is well defined. but muc
more reliable Is the three station method
The big shock of December Is wi
serve as an example. The "S.-P. diff
ference" as the indicating time is called
at West Bromwich was 555 second.
representing 1885 miles. Chicago. re
Gently equipped with Milne-Shaw sci,
mographs. stated their distance to \,
5000 miles. Circles drawn with radi
of 1885 and 1000 miles from wcs
Bromwich and Chicago respectively a.
tersect at two points-namely. j,s'
south OF Alaska, 151 w. "a NJ, az.
northwest Brazil. SO w. N
A well placed third station with
third circle is necessary to defermn.
which Of the two intersections Is cor
rect. No third station In a suitable n,
sition has 30 far reported. but the III '
that Aberdeen received the first wav,
10 seconds earlier than West Brotn
wich. and oxford 1 second later, shows
that the earthquake was in northefl'
direction. definitely deciding--if all dat
be correct--that the earthquake was II
the bed OF the Pacific Ocean. 500 mile
south OF Alaska and / 900 west Of Brit
ish Columbia.