The sun., 1909-01-10
CAUSE OF THE EARTHQUAKE
RUIN AT MESSINA or TECTONIC
ORIGIN, DIR. HOVEY THINKS.

ORIGIN, DIR. HOTEY THINKS.
The Shock Not or Great Geological I'm
portance as Measured by the seismo
Eraph-Science Unable at Present II
Make Predictions About Earthquakes
DR.. Edmund Otis Hovey, curator ir
the geological department vi the Amer.
can Museum Of Natural History and ar
authority on the subject Of volcanoes
and earthquakes, was asked to give hif
opinion Of the geological Importance OF
the Messina earthquake. DR.. Hovey
said:
From what we have been able to learn
up TO the present time the earthquake
does not seem TO be OF great geological
Importance. I was In Baltimore at the
time the news Of the earthquake reached
this country. and visited on Monday
evening, December 2s. the Johns Hop
kins University. where the seismograph
had recorded the disturbance. It did moi
indicate as much disturbance during
the Messina quake as during tbe San
Francisco earthquake in 1909. Tho geo
logical Importance OF an earthquake.
you understand, has nothing to do with
the amount Of Of damage caused by the
shock
There are two principal classes OF
earthquakes-those which are Of vol
canic origin and those which arise from
tectonic or mountain building move
ments OF the earth's crust.
THE volcanic quakes, as the name
implies, occur in districts OF active vol
canism and they rginate not fa- 1elow
the surface, probably at depttg OF less
than two miles. A we'll know example
Of this class is the earthquake Of Casa
micciola, which devasted the island Of
Ischia, Italy. in 1888 and cost the lives
Of about 900 people.
This quake was scarcely felt in Naples,
only twenty-five miles distant, and the
depth OF it's focus Is calculated to have
been scarcely more than half q mile
below the surface OF the earth. The
characteristics OF volcanic quake are
rather definite point OF origin or CEN
trum. comparatively restricted area
Of disturbance. and the usual absence Of
secondary aftershocks.
@Teotonic or dislocation earthquakes
are practically confined to those por-
tions 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 be
assigned to deepseated origin as COM
pared with the volcanic quakes.
"They are characterized by an in
definite or linear centrum. by a great
radius Of influence. and by succeeding
subordinate shocks, which sometimes
recur for months or even years. The
great destructive earthquakes which
have occurred within historic periods have
been assigned for the most part to this
class Of 1uakes.
Elm this class evidently without ques-
ton belongs the recent Messina earth
quake. Mount Etna or Mount yesuvius
had nothing whatever to do with this last
disturbance:
Earthquakes are propagated by waves,
which are Of four kinds: O normal. In
which the vibrations are forward and back
along radii emanating from the centrum
OF the quake; (2) transverse. in which the |
vibrations are perpendicular to the radii; !
(s) surface in which the movement is very |
slow. comparatively speaking, and is
horizontal in character; (D epifocal.
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 greater or less degree, and although
they are propagated with different voloci-
ties they are not always distinguishable,
even by aid Of seismographs
"The waves OF the fourth class, the
epifocal. are those which are conspicuous,
terrifying and destructive. They occur
only in the regions over and near the
centra or foci Of great earthquakes and
they are not felt far outside of these cpl
focal districts.
"They are caused when strong wav es
Of the first two classes pass from highly
elastic rocks into adjacent almost inelas-
tic soil and unconsolidated sediments.
These are the waves which eyewitnesses
Of great earthquakes have described as
the surface Of the ground TO rise
and fall like the pillows Of the acean, Open
ing cracks in the crests and closing them
a gain in the troughs as the motion passes
along. 1t is these epifocal waves that
caused the damage in the Messina quake,
as the conditions described above were |
present in that r.-'on,
THAT section is right within the earth !
quake belt. as is evidenced by the hun
ireds Of shocks that have occurred there. |
The earths crust there Is constantly |
changing, and you have the boundary |