I'll quote myself from What were some Roman reactions to the eruption of Mt. Vesuvius and subsequent destruction of Pompeii?:

Cassius Dio, a historian in the second and third centuries AD, recounts the infamous eruption of Mount Vesuvius with some added details, perhaps the details you're seeking.

Cassius tells that some believed that the Titans had awakened from the earth and were preparing to rebel again. He reports that "many of their forms could be discerned in the smoke and, moreover, a sound as of trumpets was heard" (Book 66 23.1). He describes how the giants went from the mountain to the cities and left drought and earthquakes in their wake. He reports that others believed the universe to be dissolving into fire. Cassius tells that the reaction in Rome was that the sun was fleeing the earth and that the earth was being raised into the sky (along with the end of the world and Titanomachy Redux).

All of this is important because, while Cassius doesn't go to any great lengths to tell about it, Vesuvius erupted during his lifetime. With Pompeii still in relatively recent memory (only two or three generations removed), another eruption wouldn't be met with as much doomsday panic.

Master in Ancient history here.

Please excuse my english, it's my 2nd Language :)

Events like Pompeii would typically fall under the category of "prodigia". "Prodigia" were all kinds of unusual natural portends, such as the finding of a hermaphrodite, a talking cow, comets, earthquakes, vulcano eruptions etc. Prodigia were almost always "omira dira" or dire omens, and signaled that the "Pax deorum" or peace with the gods had been disturbed, and thus resulting in "ira deum" or Anger of the gods. The senate would ask one of the priest colleges (haruspices, augures, decemvirs, pontifices) for interpretation and proper cause of action. The distubance of pax deorum was always caused by human actions, and thus an expiation was required. Expiations could be sacrifices, singing of hymns, conscecration of a new temple or shrine, thanksgivings or even the colorful "lectisternium" (litteraly a dinner for the gods, where you would take the statues of the gods, lay them down on those roman chair-couch-bed-stool-things and serve them a grand feast, yes a litteral feast. with real food, dancers, singers, the works)

In the case of Pompeii, i would suspect a pretty huge expiation. Though no sources have survived, this is how the Roman normally reacted to unusual natural phenomena, in a religious sense. I wouldn't lend Dio too much credence in this, since his historia romana is political, also he was Greek ;). If you want to read further into the subject, from ancient sources, i can recommend:

Cicero: de divinatione - Cicero: De natura deorum - Livy: Ab urbe Condita - Pliny: Naturalis Historia - Julius Obsequens: Prodigium Liber -

You can also take a look in the Blackwell Companion to Roman Religion.