Explanations may sound simple, as do most discussions of the Great Elements. They accord with the "scientific," as it were, worldview then prevalent in India. 
This worldview had multiple sources. It was provided from the vantage point of space (by royal ministers, advanced celestial visitors such as Sakka and the Four Great Kings, and rishis) and was discerned more deeply and directly through mysticism.
Moreover, the Buddha used quakes (rare, hair-raising events) as a momentous teaching moment. Using events in this way was common for many Buddhist teachings.
1. "This great Earth is estab-lished upon liquid, the liquid upon the atmosphere, and the atmosphere upon space. When mighty atmospheric disturbances take place, the liquid is agitated. With the agitation of the liquid, tremors of the Earth arise. This is the first reason, the first cause for the arising of mighty earthquakes.
2. "When an ascetic or [meditator] of great power, one who has gained mastery of mind, or a deva (radiant being) who is mighty and potent, develops intense concentration (jhana) on the delimited aspect of the earth element, and to a boundless degree on the liquid element, one, too, causes the Earth to tremble, quiver, and shake. This is the second reason, the second cause for the arising of mighty earthquakes.
3. "When a bodhisattva [a buddha-to-be] departs from the Tusita realm and descends into a mother’s womb, mindfully and clearly comprehending;
4. "When a bodhisattva comes out of his mother’s womb, mindfully and clearly comprehending;
5. "When a Tathagata becomes fully-enlightened in unsurpassed, supreme enlightenment;
6. "When a Tathagata sets rolling the Wheel of the Dharma;
7. "When a Tathagata renounces his will to live on;
8. "When the Tathagata comes to pass away into the state of Nirvana in which no element of clinging remains — this great earth trembles, quivers, and shakes.
"These are the eight reasons, the eight causes for a great earthquake to arise." Information excerpted from a much longer sutra (DN 16).
- BOOK: The Last Days of the Buddha (DN 16)
