Geology

Michell

THE NATURE AND ORIGIN OF EARTHQUAKES

1760

From Philosophical Transactions of the Royal Society of London, Vol. LI, pp. 566–74, 1760.

It has been the general opinion of philosophers, that earthquakes owe their origin to some sudden explosion in the internal parts of the earth. This opinion is very agreeable to the phaenomena, which seem plainly to point out something of that kind. The conjectures, however, concerning the cause of such an explosion, have not been yet, I think, sufficiently supported by facts; nor have the more particular effects, which will arise from it, been traced out; and the connexion of them with the phaenomena explained. To do this, is the intent of the following pages; and this we are now the better enabled to do, as the late dreadful earthquake of the 1st of November 1755 [the Lisbon earthquake] supplies us with more facts, and those better related, than any other earthquake of which we have an account.

That these concussions should owe their origin to something in the air, as it has sometimes been imagined, seems very ill to correspond with the phaenomena. This, I apprehend, will sufficiently appear, as those phaenomena are hereafter recounted; nor does there appear to be any such certain and regular connexion between earthquakes and the state of the air, when they happen, as is supposed by those who hold this opinion. It is said, for instance, that earthquakes always happen in calm still weather: but that this is not always so, may be seen in an account of the earthquakes in Sicily of 1693, where we are told, "the south winds have blown very much, which still have been impetuous in the most sensible earthquakes, and the like has happened at other times."

Other examples to the same purpose we have in an account of the earthquakes that happened in New England in 1727 and 1728: the author of which says, that he could neither observe any connexion between the weather and the earthquakes, nor any prognostic of them; for that they happened Mike in all kinds of weather, at all times of the tides, and at all times of the moon.

If, however, it should still be supposed, notwithstanding these instances to the contrary, that there is some general connexion between earthquakes and the weather, at the time when they happen, yet, surely, it is far more probable, that the air should be affected by the causes of earthquakes, than that the earth should be affected in so extraordinary a manner, and to so great a depth; and that this, and all other circumstances attending these motions, should be owing to some cause residing in the air.

Let us then, rejecting this hypothesis, suppose, that earthquakes have their origin under ground, and we need not go far in search of cause, whose real existence in nature we have certain evidence of, and which is capable of producing all the appearances of these extraordinary motions. The cause I mean is subterraneous fires. These fires, if a large quantity of water should be let out upon them suddenly, may produce a vapour, whose quantity and elastic force may be fully sufficient for that purpose. The principal facts, from which I would prove, that these fires are the real cause of earthquakes, are as follow.

First, the same places are subject to returns of earthquakes, not only at small intervals for some time after any considerable one has happened, but also at greater intervals of some ages.

Both these facts sufficiently appear, from the accounts we have of earthquakes. The tremblings and shocks of the earth at Jamaica in 1692, at Sicily in 1693, and at Lisbon in 1755, were repeated sometimes at larger, and sometimes at smaller intervals, for several months. The same thing has been observed in all other very violent earthquakes. At Lima, from the 28th October 1746, to the 24th February 1747 (the time when the account of them was sent from thence), there had been numbered no less than 451 shocks, many of them little inferior to the first great one, which destroyed that city.

The returns of earthquakes also, in the same places, at larger distances of time, are confirmed by all history. Constantinople, and many parts of Asia Minor, have suffered by them, in many different ages: Sicily has been subject to them, as far back as the remains even of fabulous history can inform us of: Lisbon did not feel the effects of them for the first time in 1755: Jamaica has frequently been troubled with them, since the English first settled there; and the Spaniards, who were there before, used to build their houses of wood, and only one story high, for fear of them: Lima, Callao, and the parts adjacent, were almost totally destroyed by them twice, within the compass of about sixty years, scarce any building being left standing, and the latter being both times overflowed by the sea: nor were these the only instances of the like kind, which have happened there; for, from the year 1582 to 1746, they have had no less than sixteen very violent earthquakes, besides an infinity of less considerable ones; and the Spaniards, at their first settling there, were told by the old inhabitants, when they saw them building high houses, that they were building their own sepulchres.

Secondly, those places that are in the neighbourhood of burning mountains, are always subject to frequent earthquakes; and the eruptions of those mountains, when violent, are generally attended with them.

Asia Minor and Constantinople may be looked upon as in the neighbourhood of Santerini. The countries also about AEtna, Vesuvius, mount Haecla, &c. afford us sufficient proofs to the same purpose. But, of all the places in the known world, I suppose, no countries are so subject to earthquakes, as Peru, Chili, and all the western parts of South America; nor is there any country in the known world so full of volcanoes: for, throughout all that long range of mountains, known by the name of the Andes, from 45 degrees south latitude, to several degrees north of the line, as also throughout all Mexico, being about 5000 miles in extent, there is a continued chain of them.

Thirdly, the motion of the earth in earthquakes is partly tremulous, and partly propagated by waves, which succeed one another sometimes at larger and sometimes at smaller distances; and this latter motion is generally propagated much farther than the former.

The former part of this proposition wants no confirmation; for the proof of the latter, viz. the wave-like motion of the earth, we may appeal to many accounts of earthquakes: it was very remarkable in the two, which happened at Jamaica in 1687–8 and 1692. In an account of the former, it is said, that a gentleman there saw the ground rise like the sea in a wave, as the earthquake passed along, and that he could distinguish the effects of it, to some miles distance, by the motion of the tops of the trees on the hills. Again, in an account of the latter, it is said, "the ground heaved and swelled, like a rolling swelling sea," insomuch, that people could hardly stand upon their legs by reason of it.

The same has been observed in the earthquakes of New England, where it has been very remarkable. A gentleman giving an account of one, that happened there the 18th November 1755, says, the earth rose in a wave, which made the tops of the trees vibrate ten feet, and that he was forced to support himself, to avoid falling, whilst it was passing.

The same also was observed at Lisbon, in the earthquake of the 1st November 1755, as may be plainly collected from many of the accounts that have been published concerning it, some of which affirm it expresly: and this wave-like motion was propagated to far greater distances than the other tremulous one, being perceived by the motion of waters, and the hanging branches in churches, through all Germany, amongst the Alps, in Denmark, Sweden, Norway and all over the British isles.

Fourthly, it is observed in many places, which are subject to frequent earthquakes, that they generally come to one and the same place, from the same point of the Compass. I may add, also, that the velocity, with which they proceed, (as far as one can collect it from the accounts of them) is the same; but the velocity of the earthquakes of different countries is very different.

Thus all the shocks, that succeeded the first great one at Lisbon in 1755, as well as the first itself, came from the north-west. This is asserted by the person, who says, he was about writing a history of the earthquakes there: all the other accounts also confirm the same thing: for what some say, that they came from the north, and others, that they came from the west, cannot be looked on as any reasonable objection to this, but rather the contrary. The velocity also, with which they were all propagated, was the same, being at least equal to that of sound; for they all followed immediately after the noise that preceded them, or rather the noise and the earthquake came together: and this velocity agrees very well with the intervals between the time when the first shock was felt at Lisbon, and the time when it was felt at other places, from the comparison of which, it seems to have travelled at the rate of more than twenty miles per minute.

An historical account of the earthquakes, which have happened in New England, says, that, of five considerable ones, three are known to have come from the same point of the compass, viz. the north-west: it is uncertain from what point the other two came, but it is supposed that they came from the same with the former. The velocity of these has been much less than that of the Lisbon earthquakes: this appears from the interval between the preceding noise, and the shock, as well as from the wave-like motion before-mentioned.