THE DISTRIBUTION OF EARTHQUAKES
Earthquakes may occur at any part of the earth's surface, at any time
of the day, or at any season of the year, yet they are more frequent
at certain parts, certain hours, certain seasons.
Since some earthquakes are unquestionably connected with volcanic
eruptions, a map or chart of the volcanoes of the earth would also,
to a certain extent, show the parts of the earth that are likely to
be visited by earthquakes. Since, however, by far the most severe
earthquakes are not directly connected with volcanoes, but are due to
sudden slips of faulted strata, a volcanic chart would necessarily
fail to indicate accurately the principal earthquake regions.
In the preparation of a map showing the distribution of earthquakes
over the earth's surface, Mallet adopted the plan of colorings or
tintings in such a manner that the depth of the colors would represent
not only the parts shaken, but also the relative number of times
shaken, as well as the intensity of the shocks. In order to determine
the depth of tint to be employed, Mallet divided earthquakes into the
following classes according to their intensity:
_Great earthquakes_, or earthquakes of the first class; or those in
which the area affected is of great size, in which many cities have
been overthrown, and many people killed, and parts of the surface
greatly altered.
_Intermediate earthquakes_, or those in which, although the area
affected is great, yet the destruction of buildings, or loss of life,
has been comparatively small.
_Minor earthquakes_, or those which, although capable of producing
small fissures in the crust, generally leave but few or no traces of
their occurrence.
The greatest distance to which earthquake waves of the first class
extend is taken by Mallet as being over a diameter of 1,080 miles;
those of the second class over a diameter of about 360 miles, and
those of the third class over a diameter of about 120 miles.
According to the Rossi Forel scale already given, earthquake shocks
are divided according to their relative intensity into ten separate
classes, viz.: I. The micro-seismic; II. The extremely feeble; III.
The very feeble; IV. The feeble; V. The moderately intense; VI.
The fairly strong; VII. The strong; VIII. The very strong; IX. The
extremely strong; X. Shocks of extreme intensity.
An earthquake map prepared according to Mallet's scale would show a
greater depth of color or tint in the neighborhood of the volcanic
districts of the earth and especially in the neighborhood of the
mountain regions, where tectonic quakes are most frequent. Oceanic
areas would be left almost untinted, not because earthquakes do not
occur on the bed of the ocean, but because of the difficulty of
observing such earthquakes at great distances from the land. So far
from earthquakes being absent on the bed of the ocean it is most
probable that they are more frequent there than elsewhere.
Prepared in this way, Mallet's map would show a preponderance of
earthquakes along the borders of the continents, especially along the
"Great Circle of Fire" on the borders of the Pacific Ocean.
Dutton as well as some others assert that the "Great Circle of Fire"
on the shores of the Pacific has in reality no existence; that,
instead of there being a continuous region of volcanoes, there is in
reality nothing more than a considerable number of volcanoes arranged
in groups along the borders of this ocean, but separated by spaces
containing no marked volcanic activity. We do not think this a tenable
position, since it is well known that volcanoes lie along great lines
of fissures at different points or openings which are kept open
by subsequent volcanic activity, while the remaining portions are
closed soon afterwards; and, moreover, in parts of these so-called
non-volcanic regions, there are probably extended regions of extinct
volcanoes.
Since the time of Mallet many maps have been made to show the
distribution of earthquakes. Among the best of such is that by M. de
Montessus de Ballore.
Some idea of the great amount of work required for the preparation of
Montessus' map may be formed when one learns that the catalogue of
earthquakes collected by him for this purpose included for the years
1880 to 1900, 131,292 quakes.
De Montessus' earthquake map divides the grand divisions of the earth
into numerous sub-divisions, too numerous, indeed, for even brief
description in a work of this kind. From the map he thus laboriously
prepared De Montessus drew the following general conclusions:
1. The parts of the earth that are most apt to be shaken by
earthquakes are those which possess the greatest differences of relief
between their highlands and lowlands, and that in such regions the
most pronounced earthquakes are found on the steepest slopes.
2. Earthquakes are most common along those parts of the crust that are
thrown up in huge wrinkles, or mountain ranges, whether these masses
be above the level of the sea or are covered by it.
[Illustration: FIG. 55. DAVISON'S EARTHQUAKE MAP OF JAPAN]
3. Earthquakes are more common in mountainous districts than in
plains. But not all mountains are characterized by earthquakes nor
are all plains free from them. Sometimes the plain at the base of
the mountain appears to be especially liable to shocks, probably by
reason of slips along faults at these points.
The great mountain ranges of the world are generally characterized
by unequal slopes, the long gentle slope facing the interior of the
continents, and the short, abrupt slopes being turned towards the
coast. Now, Montessus points out that volcanoes are the most frequent
on the short, abrupt slopes. In some cases, however, where the long
slopes are the roughest, it is these slopes that are most frequently
shaken.
The beds of the ocean that lie along rapidly descending lines,
especially when they lie on the borders of large mountain ranges, are
especially liable to earthquakes.
Dr. Charles Davison has made a map of the earthquakes of Japan in
which he had adopted the plan of representing the origin or centres
of earthquakes by a series of contour lines like those employed on
topographical maps. The advantage of this type of map over that
employed by Mallet is just this: Davison's earthquake map of Japan in
which the active volcanoes are marked by dots, and the earthquakes
by contour lines surrounding the points of origin, discloses the
interesting fact that here the positions of the volcanoes and the
earthquake centres coincide, since the mountainous districts where the
active volcanoes are numerous are singularly free from earthquakes.
This can be seen from an inspection of Fig. 55.