RELATION OF SEISMIC TO VOLCANIC PHENOMENA.
Want of synchronism between earthquakes and volcanic
eruptions—Synchronism between earthquakes and volcanic
eruptions—Conclusion.
_Connection between earthquakes and volcanic eruptions._—Insomuch as
it is a recognised fact that regions which are characterised by their
seismic activity are chiefly those which are also characterised by
the number of their volcanoes, it is generally assumed that these
two phenomena have an intimate relation. The residents in a volcanic
country, when seeking for the origin of an earthquake, invariably turn
towards the volcanoes which surround them. If a neighbouring volcano
is in a state of activity, it is often regarded as a safeguard against
seismic convulsions, in other cases it is looked upon as being the
cause of such disturbances. In certain instances both of these views
have apparently been corroborated. When we consider that an earthquake
and a volcanic eruption may both be the result of some great internal
convulsion, and that first one and then the other may take place in the
same neighbourhood, it is natural to expect that when these internal
forces have expended themselves in the production of one of these
phenomena, it is not so likely that they should exhibit themselves in
the other. The inhabitants of Sicily and Naples, we are told, regard
eruptions of Etna and Vesuvius as safeguards against earthquakes. A
similar belief is to be found in portions of South America with regard
to the volcanoes for which that country is so celebrated.
From an examination of the records of the large earthquakes and the
volcanic eruptions which have taken place in Japan during the last
2,000 years, Dr. Naumann found that there was often an approximate
coincidence between the times of the occurrence of these phenomena,
suggesting the idea that the efforts which had been sufficient to
establish the volcano had at the same time been sufficient to shake the
ground.
Of destructive earthquakes which have occurred at the time of volcanic
eruptions, and of examples when these phenomena have occurred at widely
separated intervals, the records are extremely numerous.
_Want of synchronism between earthquakes and volcanic eruptions._—Many
of the great earthquakes of South America do not appear to have been
connected with volcanic eruptions.
The great earthquakes of the world, like those of Calabria and Lisbon,
which took place in regions which are not volcanic, have not, Fuchs
tells us, taken place in conjunction with volcanic outbursts.
In Japan, as in the Sandwich Islands and in many other parts of the
globe, the small earthquakes which occur almost daily do not appear to
show any marked connection with volcanic disturbances.
In 1881, during the eruption of Natustake, a volcano lying about a
hundred miles north of Tokio, there was neither an increase nor a
decrease in the earthquakes which were felt in Tokio. Similar remarks
apply to the state of seismic activity of 1876–77, when Oshima, a
volcanic island about seventy miles to the south of Tokio, was in
eruption. In the Sandwich Islands Mauna Loa seems to have its eruptions
independently of the disturbances which shake these islands.[115]
_Synchronism of earthquakes and volcanic eruptions._—Although many
examples like the above may be quoted, which apparently show an utter
want of connection between earthquakes and volcanoes, we must not
overlook that class of earthquakes which almost invariably accompany
all great volcanic disturbances. In fact the sudden explosions which
take place at volcanic foci, as, for instance, at the commencement
of an eruption, are enumerated as one of the causes which produce
earthquakes. Earthquakes like these usually continue until the pressure
of the steam and lava have found for themselves an opening. As compared
with the total number of earthquakes which are recorded, they form but
an insignificant portion.
The direct connection which exists between these phenomena has, no
doubt, done very much to spread the popular belief that all earthquakes
may be connected with volcanic eruptions. As examples where this
connection has existed we might quote from almost all the volcanic
countries in the world.
Thus, Fuchs tells us that on October 6, 1737, almost the whole of
Kamschatka and the Kurile Islands were disturbed by movements which
were simultaneous with the outbreak of the great volcano Klutschenskja
of North Kamschatka.
One of the earliest records of a severe earthquake and a volcanic
eruption occurring simultaneously is found in the accounts of the
destruction of Herculaneum and Pompeii. The throwing up of Monte Nuovo
in the neighbourhood of Pozzuoli was accompanied with a dreadful
earthquake.[116]
In 1868 the earthquake of Arequipa was accompanied by the opening of
the volcano Misti, on its north side. The distance to the volcano is
about fourteen miles.
At the time of the eruptions of Kilauea in 1789 the ground shook and
rocked so that persons could not stand.
The first eruption of the volcano Irasu, in Costa Rica (1783), was
accompanied by violent earthquakes.[117] The smoke and flames which
are said to have issued from the side of Mount Fojo at the time of the
Lisbon earthquake are regarded by some as having been volcanic. Others
thought that the phenomena, rather than being on the side of Fojo,
which showed no traces of volcanic action, had taken place in the ocean.
At the time of the great earthquake at Concepcion (1835), whilst the
waves were coming in, two great submarine eruptions were observed. One,
behind the Isle of Quiriquina, appeared like a column of smoke. The
other, in the bay of San Vicente, appeared to form a whirlpool. The
sea-water became black, and had a sulphurous smell, there being a vast
eruption of gas in bubbles. Many fish were killed.[118]
With this same earthquake, near to Juan Fernandez, about one mile from
the shore, the sea appeared to boil, and a high column of smoke was
thrown into the air. At night flames were seen.
In 1861, when Mendoza was destroyed and 10,000 inhabitants killed, a
volcano at the foot of which Mendoza is situated burst into eruption.
The earthquake of 1822 at Valdivia was accompanied by eruptions of the
neighbouring mountains, which only lasted a few minutes.
At the time of the Leghorn shocks (January 16–27, 1742) some fishermen
observed a part of the sea to rage violently, to raise itself to a
great height, and then rush landwards.[119]
In 1797, when Riobamba was destroyed, the neighbouring volcanoes were
not affected, but Mount Pasto, 120 miles distant, suddenly ceased to
throw out its usual column of water.
On the night of December 10, 1874, a strong shock was felt in New
England, whilst at 4.45 A.M. on December 11 a shock was felt in the Pic
du Midi, in the Pyrenees. In the middle of December there were volcanic
outbursts in Iceland.[120]
It is possible that these occurrences might be the results of some
widespread disturbance beneath the crust of the earth, or perhaps even
of widely extended earth pulsations. The probability, however, is that
these coincidences are accidental. When we remember that in a small
area like the northern half of Japan alone there are periods when there
are at least two shocks per day on the average, it is impossible for
these coincidences not to exist. Less frequently coincidences between
the larger disturbances must occur. Over and above these accidental
coincidences, it would appear that in the world’s history periods
have occurred when earthquakes were unusually frequent, and at such
times distant countries have suffered simultaneously. This approximate
coincidence in period, which has been referred to when speaking of the
distribution of destructive earthquakes in historical time, does not
imply an exact synchronism in the single shocks.
Small earthquakes, or, more properly speaking, local tremblings, are a
necessary accompaniment of almost all volcanic eruptions. Tremors of
this description are seldom, however, felt beyond the crater, or at the
most upon the flanks of the mountain where the eruption is going on.
They are due to the explosive action of steam bursting through the
molten lava.
_Volcanic eruption succeeding earthquakes._—Sometimes it has happened
that an earthquake, or a series of earthquakes, have terminated with
the formation of volcanic vents.
As an example of a volcanic outburst terminating a seismic disturbance,
may be mentioned the appearance of a new volcano in the centre of
Lake Ilopango, as a sequel to the shocks which had disturbed that
neighbourhood in 1879.[121]
In 1750 there were continuous shakings lasting over three months at
Manilla. These terminated with an eruption of a small island in the
middle of a neighbouring lake. Three days after the commencement of
this eruption, four other small islands rose in the same lake.[122]
Antonio d’Ulloa, when speaking of the Andes, remarks that after a
volcanic eruption the shocks cease.[123]
_Conclusion._—Looking at this question generally, insomuch as the
greatest number of volcanic eruptions appear, according to Fuchs,
to have taken place in summer, whilst the greatest number of its
earthquakes have apparently taken place in winter, it would seem that
the two phenomena are without any direct connection, unless it be that
both are different effects of a common cause.
Regarded in this manner, an earthquake may be looked upon as an
uncompleted effort to establish a volcano. To use the words of Mallet,
‘The forces of explosion and impulse are the same in both; they differ
only in degree of energy, or on the varying sorts and degrees of
resistance opposed to them.’[124]
Although we have many examples of earthquakes having occurred without
volcanic eruptions, and, on the other hand, of volcanic eruptions
without earthquakes, volcanoes may still be regarded as ‘safety-valves
of the earth’s crust,’ which, by giving relief to internal stresses,
guard us against the effects of earthquakes.
That many earthquakes are felt at Copiapo is attributed to the fact
that in the neighbouring mountains there are no volcanic vents.
We must not, however, overrate the protective influence of volcanoes.
In the Sandwich Islands we see the columns of liquid lava in
neighbouring mountains standing at different heights, indicating a
want of subterranean connection between these vents. In consequence
of this it would seem that enormous pressures might be generated in
the neighbourhood of one of these mountains without finding relief at
the other. When we have conditions like these, it would seem that the
eruption of a volcano may have little or no influence in protecting
neighbouring districts.
This may possibly be the explanation of the fact that in 1835
Concepcion was destroyed, notwithstanding there being an unusual
activity in the volcanic vents of the neighbouring mountains.