The sun. [volume] (New York [N.Y.]) 1833-1916, 1907-07-21
FIFTEEN EARTHQUAKES I DAY
That u the Average for This shsky Ter
restrial Ball.

There are two principal classes OF earth
quakes; those which are of volcanic origin
and those which arise from t~ectonic, or
mountain building, movements Of the earth's
crust.
The volcanic quakes. as the name i'm
plies. occur in districts OF active volcanism
and originate not far below the surface,
probably at depths Of less than two miles.
The characteristics Of a volcanic quake.
says Discovery. are a rather definite point
Of origin. or centrum" a comparatively
restricted area Of disturbance and the usual
absence Of secondary after shocks.
Tectonic or dislocation earthquakes are
practically confined to those portions Of the
earth's crust which are still undergoing
changes of elevation due to the action Of
mountain building forces. Such quakes
are usually to he assigned to a deep seated
origin. as compared with the volcanic
quakes.
They are characterized by an indefinite
or linear centrum, q great radius VI influence
aud by succeeding subordinate shocks.
which sometimes recur for months OF even
years. The great destructive earthquakes
which have occurred within the historic
period have been assigned for the most
part to this class Of quakes.
Earthquakes are ~propagated by waves.
which are Of four kinds "a Normal. in
which the vibrations are forward and back
along a emansting from the centrum
Of the quake; 90 transverse. IN which the
vibrations are perpendicular 10 the radii;
go surface. in which the movement is very
slow. comparatively speaking and lie hori-
zontal in character; () epifocai, Waves of
the first three classes depend for their
amplitude on the elasticity of the rocks
through which they pass. They are seldom
visible. though they are felt TO a greater
on less degree. and although they are prop
agsted with different yelocities. they
are not always distinguishable. even by
aid OF seismographs.
The waves OF the fourth class, the epif-
ocal. are those which are conspicuous.
terrifying and destructive. They are
caused when strong waves OF the first two
classes pass from highly elasticrocks into
adjacent almost inelastic soil and uncon-
soIidated sediments. These are the waves
which eye witnesses OF great earthquakes
have described as causing the surface OF
the ground to rise and fall like the pillows
Of the ocean, opening cracks In the crests
and closing them again in the troughs
as the motion passes along.
The present period has been supposed
by many, especially those not versed in
geology, to be a season OF rest In the action
Of earth building forces. This. however,
IL not the case. We know OF q gentle up
ward movement OF tnc chrlu cftiSt III
the Hudson Bay region, in New York and
the eastern Great lakes. and Of the sub
sidence OF parts OF he Atlantic coast and
the elevation OF other parts, but these are
slow and their connection with earthquakes
has not been established. Certain areas.
however. present definite breaks in the
rocks with surfaces polished or striated
by friction. indicating ancient movements
which must have been accompanied by
great earthquakes
The frequency Of earthquake shocks.
considering those Of all a~mplitudes. IL
not generally realized. The globe, indeed.
NAV be said hardly ever To be free from
seismic disturbances OF some kind some.
where. for the average OF all recorded
shocks is more than Is a day, the
bare enumeration Of those occurring In
1908 fills a book OF 600 tabulated ages.
About 90 heavy shocks occur every year
on an average. The Count de Montessus
de Ballore has plotted upon maps Of the
h~emispheres the positions OF the I5e. 7s+
independent quakes which had beea re-
corded up to 1908. IL was found by de
Ballore that 06 per cent OF all the recorded
ouakes had occurred within the limits OF
certain well defined zones, which are along
lines where the crust Is bent downward
forming great troughs, though the troughs
are onfy 10 be recognized geologically;
that Is, by the relation OF n rock strata
10 another. without reference to the surface
CONFEDERATION Of the earth.