CHAPTER XVI.

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.