... there was an earthquake at Nicomedia/Izmit on 24 August 358 AD (that's approx. 20 years after Constantine died there).
Such were the events which transpired at Sirmium. It seemed at this period as if, from the fear of displeasing the emperor, the Eastern and Western Churches had united in the profession of the same doctrine. The emperor had determined upon convening a council at Nicaea to take into consideration the innovations introduced at Antioch, and the heresy of Aetius. As Basil, however, and his party were averse to the council being held in this city, because doctrinal questions had previously been agitated there, it was determined to hold the council at Nicomedia in Bithynia; and edicts were issued, summoning the most intelligent and eloquent bishops of every nation to repair thither punctually on an appointed day, so that it might be the privilege of all the priests of the state to share in the Synod and to be present at its decisions. The great number of these bishops had commenced their journey when the calamity that had come upon Nicomedia was reported, and that God had shaken the entire city to its foundations. Since the story of the destruction of the city everywhere prevailed and grew, the bishops arrested their journey; for as is usual in such cases, far more was rumored to those at a distance, than had actually occurred. It was reported that Nicaea, Perinthus, and the neighboring cities, even Constantinople, had been involved in the same catastrophe. The orthodox bishops were grieved immoderately at this occurrence; for the enemies of religion took occasion, on the overthrow of a magnificent church, to represent to the emperor that a multitude of bishops, men, women, and children fled to the church in the hope of their finding safety, and that they all perished. This report was not true. The earthquake occurred at the second hour of the day, at which time there was no assembly in the church. The only bishops who were killed were Cecropius, bishop of Nicomedia, and a bishop from the Bosphorus, and they were outside of the church when the fatal accident happened. The city was shaken in an instant of time, so that the people had not the power, even if they had the wish, to seek safety by flight; at the first experience of danger, they were either preserved, or they perished on the spot where they were standing.
It is said that this calamity was predicted by Arsacius. He was a Persian, and a soldier who was employed in tending the emperor's lions; but during the reign of Licinius he became a noted confessor, and left the army. He then went to the citadel of Nicomedia, and led the life of a monastic philosopher within its walls. Here a vision from heaven appeared to him, and he was commanded to quit the city immediately, that he might be saved from the calamity about to happen. He ran with the utmost earnestness to the church, and besought the clergy to offer supplications to God that His anger might be turned away. But, finding that far from being believed by them, he was regarded with ridicule, and as disclosing unlooked-for sufferings, he returned to his tower, and prostrated himself on the ground in prayer. Just at this moment the earthquake occurred, and many perished. Those who were spared fled into the country and the desert. And as happens in a prosperous and large city, there were fires in the brasiers and extinguishers of every house, and in the ovens of the baths, and in the furnaces of all who use fire in the arts; and when the framework fell in ruin, the flame was hemmed in by the stuff, and of course there was dry wood commingled, much of which was oily, -- this served as a contribution to the rapid conflagration, and nourished the fire without stint; the flame creeping everywhere, and attaching to itself all circumjacent material, made the entire city, so to speak, one mass of fire. It being impossible to obtain access to the houses, those who had been saved from the earthquake rushed to the citadel. Arsacius was found dead in the unshaken tower, and prostrated on the ground, in the same posture in which he had begun to pray. It was said that he had supplicated God to permit him to die, because he preferred death to beholding the destruction of a city in which he had first known Christ, and practiced monastical philosophy. As I have been led to speak of this good man, it is well to mention that he was endowed by God with the power of exorcising demons and of purifying those troubled by them. A man possessed with a demon once ran through the market-place with a naked sword in his hand. The people fled from him, and the whole city was in confusion. Arsacius went out to meet him, and called upon the name of Christ, and at that name the demon was expelled, and the man restored to sanity. Besides the above, Arsacius performed many other actions beyond the power and skill of man. There was a dragon, or some other species of reptile, which had entrenched itself in a cavity of the roadside, and which destroyed those who passed by, with its breath. Arsacius went to the spot and engaged in prayer, and the serpent voluntarily crept forth from its hole, dashed its head against the ground, and killed itself. All these details I have obtained from persons who heard them stated by those who had seen Arsacius.
As the bishops were deterred from continuing their journey by the intelligence of the calamity which had occurred at Nicomedia, some awaited the further commands of the emperor, and others declared their opinions concerning the faith in letters which they wrote on the subject. The emperor hesitates as to what measures ought to be adopted, and writes to consult Basil as to whether a council ought to be convened. In his reply, it appears, Basil commended his piety, and tried to console him for the destruction of Nicomedia by examples drawn from the Holy Scriptures; he exhorted him, for the sake of religion, to hasten the Synod; and not to drop such a proof of his zeal for religion, and not to dismiss the priests who had been gathered together for this purpose, and had already set forth and were on their way, until some business had been transacted. He also suggested that the council might be held at Nicaea instead of Nicomedia, so that the disputed points might be finally decided on the very spot where they had been first called in question. Basil, in writing to this effect, believed that the emperor would be pleased with this proposition, as he had himself originally suggested the propriety of holding the council at Nicaea. On receiving this epistle from Basil, the emperor commanded that, at the commencement of summer, the bishops should assemble together at Nicaea, with the exception of those who were laboring under bodily infirmity; and these latter were to depute presbyters and deacons to make known their sentiments and to consult together on contested points of doctrine, and arrive at the same decision concerning all points at issue. He ordained that ten delegates should be selected from the Western churches, and as many from the Eastern, to take cognizance of the enactments that might be issued, and to decide whether they were in accordance with the Holy Scriptures, and also to exercise a general superintendence over the transactions of the council. After further consultation the emperor enacted that the bishops should remain where they might be residing, or in their own churches, until it had been decided where the council was to be held, and until they received notice to repair thither. He then writes to Basil, and directs him to inquire by letter of the Eastern bishops, where they would advise the council to be held, so that a public announcement might be made at the commencement of spring; for the emperor was of opinion that it was not advisable to convene the council at Nicaea, on account of the earthquake which had recently occurred in the province. Basil wrote to the bishops of every province, urging them to deliberate together, and to decide quickly upon the locality in which it would be most expedient to hold the council, and he prefixed a copy of the emperor's letter to his epistle. As is frequently the case in similar circumstances, the bishops were divided in opinion on the subject, and Basil repaired to the emperor, who was then at Sirmium. He found several bishops at that city who had gone thither on their own private affairs, and among them were Mark, bishop of Arethusa, and George, who had been appointed to preside over the church of Alexandria. When at length it was decided that the council should be held in Seleucia, a city of Isauria, by Valens and his adherents, for Valens was then sojourning in Sirmium; since they favored the heresy of the Anomians, they urged the bishops who were present at the military court, to subscribe to a formulary of the faith which had been prepared, and in which there was no mention of the term "substance." But while preparations were being zealously made for convening the council, Eudoxius and Acacius, Ursacius and Valens, with their followers, reflected that, while many of the bishops were attached to the Nicene faith, and others favored the formulary drawn up at the consecration of the church of Antioch, yet that both parties retained the use of the term "substance," and maintained that the Son was, in every respect, like unto the Father; and being aware that if both parties assembled together in one place they would readily condemn the doctrines of Aetius, as being contrary to their respective creeds, they so contrived matters that the bishops of the West were convened at Ariminum, and those of the East at Seleucia, a city of Isauria. As it is easier to convince a few than a great many individuals, they conceived that they might possibly lead both parties to favor their sentiments by dealing with them separately, or that they might, at any rate, succeed with one, so that their heresy might not incur universal condemnation. They accomplished this through Eusebius, a eunuch who was superintendent of the imperial house: he was on terms of friendship with Eudoxius, and upheld the same doctrines, and many of those in power were seeking to conciliate this very Eusebius.
Sozemus, Ecclesiastical History, Book IV, Chapter xvi.