In 740, Constantinople was the capital of the Byzantine Empire and the largest city in the Christian world — a place of perhaps half a million people, of marble churches and imperial palaces, of crowded markets and layered aqueducts. The city had endured sieges, plagues, and political upheavals, but the earthquake of October 740 was a different kind of catastrophe: sudden, indiscriminate, and structural. The collapse of public buildings would have meant not merely damaged stone but people crushed beneath roofs and vaults they had walked under every day. The reported toll of over 1,000 dead in Constantinople itself speaks to the density of the city and the speed of the disaster. Among the structures damaged were the Walls of Constantinople — the triple land walls whose integrity was, for the Byzantines, existential. That the earthquake could crack those walls was deeply unsettling.

What the historian Theophanes the Confessor — writing in the 8th and 9th centuries — recorded next was something that would have seemed incomprehensible to those who witnessed it. The sea, he wrote, retreated from the coast in many places. It drew back, exposing what lay beneath. And then it returned, flooding towns and low-lying ground. George Kedrenos, writing in the 11th century, described the same phenomenon. Modern readers recognize it immediately: a seismic sea wave, generated by the earthquake's displacement of the seafloor beneath the Sea of Marmara. The tsunami struck communities that had no name for what was coming, no category for it, no warning. Towns in Thrace were destroyed by the earthquake itself; the sea then took what remained of others along the coast.

The earthquake did not confine its destruction to Constantinople. Across the wider region, towns in Thrace were reported destroyed. The ancient city of Nicaea — today's İznik, in northwestern Turkey — was damaged, as was Nicomedia (modern İzmit) and Praenetus. These were significant cities in the Byzantine world, places of trade, administration, and ecclesiastical importance. That all of them suffered damage in the same event speaks to the earthquake's magnitude. The sources do not give us modern seismological measurements, but the geographic footprint — from the Sea of Marmara coastline through Thrace and into Bithynia — indicates a rupture of substantial scale along one of the tectonic fault systems that still make the region seismically active today.

The 740 earthquake is known to us because Byzantine and later medieval historians took care to record it. Anastasius Bibliothecarius in the 9th century, Joannes Zonaras in the 12th, and European scholars including Carolus Sigonius and Caesar Baronius in the 16th and 17th centuries all noted and dated the event. That such a range of chroniclers across so many centuries preserved the record is a measure of how extraordinary the disaster was considered. For the people of Constantinople in 740, the earthquake was a rupture in the expected order of the world — the walls had cracked, the sea had moved, and the dead numbered in the thousands. The city rebuilt, as it always had. But the event left its mark in the written record, a reminder that beneath the foundations of one of history's great cities, the earth had never quite agreed to stay still.