The great Earthquake of Nicomedia.
In the image you can see how the Roman/Byzantine city of Nicomedia looked like in the Roman times.
Imagine now the buildings cascading all the way down due to an earthquake, a terrifying sight. Unfortunately such a scene unfolded in the morning of August 24th, 358 when a devastating earthquake shook the city. The event was recorded by the Roman historian Ammianus Marcellinus.
Among the casualties were Nicomedia's governor Aristainetos and its bishop Aquilinus. The death toll was recorded as 15 people, but considering the extent of destruction it seems surprisingly small. Also among those killed was a Persian soldier named Arsaces, who predicted the earthquake but was laughed at by all.
Until then Nicomedia had been an important city and one of the capitals of the Roman Empire during the Tetrarchy period, as well as a temporary capital of Mediolanum when Constantine founded Constantinople nearby.