More Gibbon, this time a tsunami.
From vol.3 of the Bury edition of The Decline and Fall of the Roman Empire, chapter 26:
IN the second year of the reign of Valentinian and Valens, on the morning of the twenty-first day of July, the greatest part of the Roman world was shaken by a violent and destructive earthquake. The impression was communicated to the waters; the shores of the Mediterranean were left dry by the sudden retreat of the sea; great quantities of fish were caught with the hand; large vessels were stranded on the mud; and a curious spectator amused his eye, or rather his fancy, by contemplating the various appearance of valleys and mountains which had never, since the formation of the globe, been exposed to the sun. But the tide soon returned with the weight of an immense and irresistible deluge, which was severely felt on the coasts of Sicily, of Dalmatia, of Greece, and of Egypt; large boats were transported and lodged on the roofs of houses, or at the distance of two miles from the shore; the people, with their habitations, were swept away by the waters, and the city of Alexandria annually commemorated the fatal day on which fifty thousand persons had lost their lives in the inundation. This calamity, the report of which was magnified from one province to another, astonished and terrified the subjects of Rome, and their affrighted imagination enlarged the real extent of a momentary evil. They recollected the preceding earthquakes, which had subverted the cities of Palestine and Bithynia; they considered these alarming strokes as the prelude only of still more dreadful calamities; and their fearful vanity was disposed to confound the symptoms of a declining empire and a sinking world.
I found a reference to this here which puts the quake (AD 365) at 8.5 on the Richter scale. That being pretty much a maximum for earthquakes, apparently.
