DISTRIBUTION OF EARTHQUAKES IN SPACE AND TIME.
General distribution of earthquakes—Occurrence along lines—Examples
of distribution—Italian earthquake of 1873—In Tokio—Extension
of earthquake boundaries—Seismic energy in relation to
geological time; to historical time—Relative frequency of
earthquakes—Synchronism of earthquakes—Secondary earthquakes.
_General distribution of earthquakes._—The records of earthquakes
collected by various seismologists lead us to the conclusion that
at some time or other every country and every ocean in the world
has experienced seismic disturbances. In some countries earthquakes
are felt daily, and from what will be said in the chapter on earth
pulsations it is not unlikely that every large earthquake might with
proper instrumental appliances be recorded at any point on the land
surfaces of our globe. The area over which any given earthquake extends
is indeterminate. The area over which an earthquake is sensible is
sometimes very great. The Lisbon shock of 1755 is estimated as having
been sensible over an area of 3,300 miles long and 2,700 miles wide,
but in the form of tremors and pulsations it may have shaken the whole
globe.
The regions in which earthquakes are frequent are indicated in the
accompanying map, which, to a great extent, is a reproduction of a map
drawn by Mallet. The regions coloured with the darkest tint are those
where great earthquakes are the most frequent. The actual number of
earthquakes which have been felt in the differently coloured areas are
given, when speaking of the relation of seismic energy to season.
When looking at this chart it must be remembered that if we were
to make a detailed map of any one of the different countries where
earthquakes are frequent, we should find in it all the differences that
we observe in the general chart. For instance, one portion of Japan,
where perhaps sixty shocks are felt per year, would be coloured with
a dark tint, whilst other portions of the same country, where there
is only one slight shaking felt every few years, would be left almost
uncoloured. The black dots indicating the position of volcanic vents
are even more general in their signification than the tinted areas.
Professor Haughton gives for the world a list of 407 volcanoes, 225 of
which are active. These numbers are the same as those given by A. von
Humboldt. Of the active volcanoes 172 are on the margin of the Pacific,
and of the total number eight are in Japan. From my own observations
in Japan independently of the Kurile Islands, I have enumerated
fifty-three volcanoes which are either active or have been active
within a recent period. In a few years’ time this list will probably be
increased. I mention this fact to show how very imperfect our knowledge
is respecting the number of volcanic vents existing on our globe. If we
were in a position to indicate the volcanoes which had been in eruption
during the last 4,000 years, the probability is that they would number
several thousands rather than four or five hundred.
An inspection of the map shows that earthquakes chiefly occur in
volcanic and mountainous regions. The most earthquake-shaken regions of
the world form the boundaries of the Pacific ocean. It may be remarked
that these boundaries slope beneath the neighbouring ocean at a much
steeper angle than the boundaries of countries where earthquakes occur
but seldom. The coasts of South America, Kamschatka, the Kuriles,
Japan, and the Sandwich Islands, for example, have slopes beneath the
Pacific from one in twenty to one in thirty. The coasts of Australia,
Scandinavia, and the eastern parts of South America, where earthquakes
are practically unknown, have slopes from one in fifty to one in two
hundred and fifty. Many earthquakes have taken place in mid-ocean. In
the Atlantic Ocean M. Perrey has given about 140 instances of such
occurrences.
The majority of the earthquakes which shake Japan appear to have their
origin in the neighbouring ocean. If we could draw a map of earthquake
origins, it is probable that the greater number of the marks indicating
these origins would be found to be suboceanic and along lines parallel
to the shores of continents and islands which rise steeply from the
bed of deep oceans. In countries like Switzerland and India, our marks
would hold a relationship to the mountains of these countries.[89]
Looking at the broad features of the globe, we see on its surface
many vast depressions. Some of these saucer-like hollows form land
surfaces, as in central Asia. The majority of these, however, are
occupied by the oceans. Active volcanoes chiefly occur near the rim of
the hollows which have the steepest slopes. The majority of earthquakes
probably have their origin on or near the bottom of these slopes. To
these, however, there are exceptions, as for instance the earthquakes
in the Alps, in the hills of Scotland, and the shakings which are
occasionally felt in countries like Egypt. The earthquakes which shake
the borders of the Pacific have their origins in, and their effects are
almost exclusively felt on, the sides of the bounding ridge facing this
ocean. In Japan it is the eastern sides of the islands which suffer,
the western side being almost as free from these convulsions as England.
Similar remarks may be made about the eastern side of South America,
especially the southern portion of the continent. At Buenos Ayres, for
example, there has been no disturbance since Mendoza was destroyed,
some twenty years ago. In British Guiana slight shocks are occasionally
felt in the low delta which forms the settled portion of the colony,
but they are extremely rare.
_Disturbances in lines or zones._—It has often been observed that
disturbances are propagated along the length of mountains or valleys,
and it is but seldom that earthquakes cross them transversely. Thus the
valleys of the Rhone, the Rhine, and the Danube are lines along which
disturbances travel.
The major axes of the elliptical areas of disturbances which have
shaken India have a general direction parallel to the valley of the
Ganges along the flanks of the Himalayas.
The disturbances which have shaken London appear to have been chiefly
east and west, or along the valley of the Thames. In South America the
line of disturbance is along the western sides of the Andes. Another
line is along the northern coast of the continent through Andalusia
and Caraccas towards the Antilles and Trinidad. The shocks of the
Pyrenees are chiefly felt along the southern side of these mountains.
In the middle and on the northern side they are but seldom felt. This
propagation in lines or zones may in certain cases be apparent rather
than real. Thus the north and south ranges of mountains in Japan are
mountains almost simultaneously shaken along their eastern flanks,
giving the impression that an earthquake had originated simultaneously
from a fissure parallel to this line, or else, starting at one end,
had run down their lengths. Time observations have, however, shown
that such disturbances had their origin at some distance in the ocean,
and, travelling inwards, had reached all points on the flanks of these
mountains almost simultaneously. The same explanation will probably
hold for the so-called linear disturbances of western South America.
All earthquake disturbances have probably a tendency to radiate from
their source, and are only prevented from doing so by meeting with
heavy mountainous districts, which by their mass and structure absorb
the energy communicated to them. Much energy is also lost by emergence
on the open flanks of a range of mountains. Rather than say that
high mountains often bound the extension of an earthquake, or that
earthquakes appear to run along the flanks of such mountains, we might
say that earthquakes have boundaries parallel to the strike of the
rocks in a given district, that such a direction is the one in which
the propagation is the easier.
Rossi is of opinion that volcanic fractures play an important part in
governing the distribution of seismic disturbances. When a volcano is
formed, a series of starlike fractures are formed round the central
crater. Secondary craters may indicate the line of these fissures. The
mountains about Rome are regarded as typical of this radial structure.
The more distant the secondary craters are from the centre of the
system, the smaller will they be, and also the younger. If two fissures
intersect we get a larger crater at the junction. Earthquakes are
propagated along the direction of these fissures, whilst the rising
and falling of these lips throw off transverse waves. Rossi adduces
observations which appear to meet with explanation on such suppositions.
Suess, who has written upon the earthquakes of lower Austria, shows how
the majority of the disturbances have had their origin along certain
lines which form a break in the continuity of the Alps. One line runs
north-east from Bruck towards Vienna. Near Wiener Neustadt, where the
greatest number and heaviest shocks have occurred, this line is met by
a north-north-west line crossing the Danube and following the valley
of the river Kamp.[90] Hoeffer has drawn similar lines from the head
of the Adriatic, one set running north-north-east to intersect near
Litschau, and the other north-north-west to intersect near Frankfort in
the valley of the Rhine.[91]
_Examples of distribution._—A curious example of the distribution of
seismic movement is that of the earthquake of March 12, 1873, worked
out by Professor P. A. Serpieri. This earthquake appears to have been
simultaneously felt on the Dalmatian coast and in central Italy, in a
region lying north-east from Rome and south-east from Florence. In both
of these areas the motion was from south-east to north-west. The shock
then radiated from the central Italian regions, so that at places on
the western shore of the Adriatic it was felt after it had been felt on
the Dalmatian coast.
Many explanations might be offered for this peculiar distribution
of seismic activity. Possibly the shock originated at a great depth
beneath the bed of the southern part of the Adriatic, and by following
existing lines of weakness simultaneously reached the surface of the
earth in central Italy and Dalmatia.
In Tokio, which is built partly on a flat plain, partly in valleys
denuded from a low tableland, and partly on the spurs of the tableland
itself, the distribution of earthquakes is a subject yet requiring
attention. Sometimes it has happened that persons in one house have
been sufficiently alarmed to escape into the open air, whilst others,
not more than a mile distant, have not been aware that the city had
been shaken.
[Illustration: FIG. 34.
Areas almost simultaneously struck from S.E. to N.W. [graphic]
Subsequent radial disturbance [graphic]]
_Extension of earthquake boundaries._—Natural obstructions which may be
sufficient to retard small earthquakes may in certain instances not be
found sufficient to retard the larger disturbances. Thus the shocks of
Calabria are usually only felt on the western side of the Apennines,
but instances have occurred when they have crossed this barrier. In
1801 the earthquake of Cumana crossed a branch of the coast range.
Sometimes earthquake boundaries give way, and countries which they
sheltered subsequently become exposed to all disturbances. The true
explanation of this is probably in a shifting of the centre of seismic
activity. Thus up to December 14, 1797, although Cumana was often
devastated, the peninsula of Araya was not hurt. On this date Araya
commenced to suffer, and has continued to suffer ever since.
Fuchs gives an example of the movement of a seismic centre in the case
of the Calabrian earthquake. The first shock commenced near Oppiedo,
the second shock commenced four or five miles farther to the north, and
the third shock had its origin five or six miles still farther, near to
Girifalco.