Chapter 36: On earthquakes

On earthquakes.⁹⁴⁴

1 · ⟨Aëtius⟩: Plato

Plato says that every movement has six possible directions: up and down, to the right and to the opposite side, forward and backward.⁹⁴⁵ ⟨But⟩ the earth cannot move in any of these directions. Lying at the point that is lowest from every side, it remains motionless, since it has no special reason to incline in one direction rather than another. Particular regions of it, however, are shaken because of their porosity.⁹⁴⁶

2 · ⟨From the epitome of Arius Didymus⟩

Aristotle’s account.⁹⁴⁷ Aristotle says that earthquakes, rumblings, and fissures arise when dry exhalation flows into porous spaces beneath the earth and, rushing in en masse, produces trembling and upheaving in it. For just as the exhalation creates a violent wind when it rushes outward, so, when imprisoned inside, the force of its movement shakes the regions in which it is trapped.⁹⁴⁸

Air held beneath the surface

⟨And⟩ this force is held within the earth by the compactness of its surface, by cooling, and by dryness. Hence earthquakes generally occur when the air is still, because the winds’ impetus has turned inward.⁹⁴⁹

Night, dawn, and midday

They occur more often at night, because of the cold, and around dawn: the winds then begin to blow and turn back toward the earth ⟨the⟩ current of air rising with the exhalation. In daytime they sometimes occur around noon, since the sun overpowers the exhalation and confines it within the earth.⁹⁵⁰

Seasons and drought

They are also more frequent in spring and autumn (for these seasons are changeable; summer and winter make the air motionless, the one through frost and the other through heat, because summer is excessively dry and winter excessively cold).⁹⁵¹ They occur also during droughts, when the air is most full of wind; for drought is precisely a mass of dry exhalation.

Imprisoned vapor and heavy rain

Thus, when vapor is driven into the earth and imprisoned, a throbbing and trembling arise in it. Earthquakes occur also during heavy rains, when moisture and cold seal the earth, and the exhalation inside is increased and compressed by the rains, so that its impact moves and shakes the earth violently.⁹⁵²

Cavities and neighboring seas

They occur also in places whose underground regions are cavernous and spongy, or which have a sea with strong currents beside them. Such places admit more air from outside; moreover, the sea, as it strikes against them, drives back into the earth the air that would naturally breathe out from it and disperse. Thus there are many great earthquakes around the Peloponnese and Achaia, and in the valleys of Euboea.⁹⁵³

A comparison with the body

For just as the breath that we breathe outward often turns back inward and causes palpitation and throbbing in our bodies, so the air enclosed in the earth, jostled about its passages and interrupted in its movement, produces convulsions and earthquakes.⁹⁵⁴

Subterranean rumbling

Rumblings sometimes accompany earthquakes and sometimes occur without them. The air resounds, so that a noise and a very prolonged booming can be heard, whenever it runs back and forth through the underground tunnels, rubbing against them without escaping.⁹⁵⁵

Fissures and escaping air

Fissures, too, accompany earthquakes. Indeed, some earthquakes have not ceased until the regions where they occurred split open, as the enclosed air forced its way out like a squall. Such were the earthquake around Heracleia on the Pontus and the still earlier one around the island of Aeolus called Sacred.⁹⁵⁶

Swelling ground and cinders

For in these places earthquakes produced great swellings of the ground. When these burst open, they blew out a great quantity of wind and cinders, covering the nearby city of the Liparians with ash.⁹⁵⁷

Islands in the open sea

The islands called open-sea islands shake less than those near the mainland, because the great mass of sea cools the exhalation from the earth.⁹⁵⁸

Sound and a dim sun

Evidence that air flows beneath the earth comes, first, from the warning it gives by resounding in the places where earthquakes occur,⁹⁵⁹ and then from what happens to the sun. For the sun appears dimmer and misty, though there is no cloud, when the current that thins and disperses the air begins to retreat beneath the earth.⁹⁶⁰

Calm and cold before dawn

Further evidence is the calm and cold that precede early-morning earthquakes: the calm occurs because the winds flow back inward, and the cold because the warm exhalation is absent.⁹⁶¹

So much for earthquakes and the other disturbances affecting the earth.