Earthquakes

In his Letter to Pythocles (105), Epicurus posits two possible explanations for the cause of earthquakes:

Atmospheric "flatulence": Caverns inside the earth may either be penetrated by winds that vibrate as they enter the earth from the outside, or existing air already trapped inside is set into vibration when pieces of earth fall down in cavernous places.

Structural Collapses: In this scenario, a double shockwave is caused, the first one when large clods of earth break off from the ceilings of caverns, and the next one when the fall is broken upon impact at the bottom of the cavern.

As always, Epicurus also leaves open "many other possible causes."