Between 469 and 463, a tremendous earthquake shook Sparta, and was considered to be of epoch making importance, because it is cited by historical sources as one of the key events that led up to the First Peloponnesian War. In 465 the Athenians besieged the island of Thasos in the northern Aegean where they met fierce resistance. The Spartans had secretly made a promise to the Thrasians to aid them by invading Attica. They were only prevented by doing so by the greatest earthquake that had ever happened at Sparta. It did great damage to the countryside and destroyed almost the whole of the city of Sparta, ostensibly killing 20 000 people. The earthquake caused the revolt of the helots, the slave class of Spartan society, and various Messenian subjects of Sparta.. The Helots who after defeating the Spartans with loss of 300 men on the plain of Stenyclarus fortified themselves on Mount Ithome.
They defied the Spartans who appealed for assistance to Athens, since the Athenians were skilled in siege operations.The Athenians still formally tied to the Spartans by the Greek alliance against Persia sworn in 481, came to their assistance. 4000 Athenian hoplites were sent under the command of the pro-Spartan strategos Cimon to help in the siege of Ithome. But Ithome was not easily to be taken, and the Spartans, perhaps suspecting treason, suddenly and insultingly dismissed the Athenian troops. This incident caused a political revolution in Athens and ultimately a diplomatic revolution in Greece. The indignation at Athens was intense and Cimon, along with his pro-Spartan policy, was ostracized in 461. The antin-Spartan group led by Pericles and Ephialtes, which had opposed sending help to Peloponnese, withdrew from the old alliance with Sparta, and made a new alliance with Sparta’s old and bitter enemy, Argos. After the ostracism of Cimon, the imperialist Pericles stood alone at the helm of the Athenian state for the next 30 years. The revolt stopped in 455, when the besieged Helots could hold out no longer. The Spartans allowed them to leave the Peloponnese under a truce, provided they never return. The Athenians settled them as a group at a strategic site on the north shore of the Corinthian Gulf, in the city of Naupactus .
Though recent studies show a tendency to date the earthquake to 464, ancient writers are not in the agreement on the matter. Thucydides gives 465-464, Diodorus 469-468, Plutarch 466-465 and Pausanias 464-463. Hammond, following Diodorus’ account, suggest that a first series of shocks occurred around 469-468.
Geophysical studies in Greece support the occurrence of this earthquake: satellite images and fieldwork has identified a 20 km long normal fault scarp a few kilometres east of the ancient city of Sparta. Armijor et al. have estimated that its magnitude was around 7.2.
On a side note, Cicero and Pliny the Elder report the same earthquake but during Anaximander of Miletus lifetime (c. 610 – c. 546 BC), a pre-Socratic philosopher. According to them, Anaximander warned the Spartans to be careful of their city and dwellings because an earthquake was impending. Subsequently the city was ruined, and a part of Mt Taygetus, which had projected in the shape of a ship’s stern, broke off. Though it seems a doublet to the earthquake of 464, some modern writers consider that the accounts of Cicero and Pliny refer to a destructive earthquake in Sparta in the middle of the 6th century BC.