Eaˊrthquake [of eorth,earth and cwacian,to quake]
a violent shock or concussion of the earth, or some parts of it; caused by fire or hot vapours pent up in the bowels or hollow parts of it, which force a passage, and frequently produce dreadful effects, as the destruction of whole cities, the swallowing up, or overturning of mountains, &c.
Naturalists, some of them, ascribe Earthquakes to water, others to fire, and all of them with some reason. Nay,
1. The earth itself may be the cause of its own shaking, when the roots or basis of some large mass being dissolved or wore away by a fluid underneath, it sinks into the same, and by its weight causes a tremor, produces a noise, and frequently an inundation of water.
2. The subterraneous waters may occasion earthquakes by their cutting out new courses, &c. or the water being heated or rarify’d by the subterraneous fires, may emit fumes, blasts, &c. and may cause great concussions.
3. The air may be the cause of earthquakes; for the air being a collection of fumes and vapours raised from the earth and water, if it be pent up in too narrow viscera of the earth, either the subterraneous heat, or its own native one rarifying and expanding it, the force wherewith it endeavours to escape, may cause a shaking of the earth.
4. Fire is a principal cause of earthquakes, both as it produces the subterraneous air or vapours before mentioned; and as this aura, air or spirit, from the different matter and composition of which, sulphur, bitumen, and other inflammable matters do arise, takes fire, by either some other fire it meets withal, or from its collision against hard bodies, or by its being intermixed with other fluids; by which means bursting out into a larger compass, the space becomes too narow for it, and so pressing against it on all sides, it causes a shaking of the contiguous parts, till having made itself a passage, it spreads itself in a volcano.
There being much sulphur and bitumen, and such like combustible matter in many places of the bowels of the earth, it is no hard matter to imagine how it should inkindle, which tho’ it may be done several ways, I shall instance but in one. Since the earth contains such different matters in it, it may be easily imagined that there are caverns in some places, which are filled with no other matter but gross airs, and sulphureous or bituminous vapours, and it may so happen that a flint shall drop from the arch of the cavern to another flint below, and strike fire out of it, which shall either enflame the vapour, or the sulphureous and bituminous matter thereabouts, which when they have once taken fire, keep it in very long, and communicate it to other bodies of the like nature, and when these get vent, they burst out in very violent eruptions, as has been seen in Ætna, Vesuvius, and other places.
But when it so happens, that in vast caverns the vapours and thicker matter take fire all at once, the air in such a motion cannot rarify and disperse, but it must give a sudden concussion to the upper part of the caverns, and make all the ground above it to tremble, and cause an earthquake; and the deeper the mine lies, and the larger the quantity of matter is, which takes fire at one time, the more violent and extensive is the earthquake.
But if the cavern happens to be near the surface of the earth, there are many times eruptions of fire that consume the bowels of it, so that the ground sinks in; and where the opening is wide enough, trees and houses are swallow’d up in it, as it happened in Jamaica in the year 1692.
And this is not bare conjecture, but is confirmed by experience, for the great eruptions of the famous burning mountains are always attended with an earthquake in the neighbourhood, as they in Naples and the places thereabouts have experienced.

Definition taken from The Universal Etymological English Dictionary, edited by Nathan Bailey (1736)

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