A devastating earthquake in Verny. How did this happen?

On the 28th of May, according to the old style, at 4:35 AM, a strong underground rumble was heard, and the townspeople felt a sharp jolt. At that moment, no one paid a slightest attention to this. Residents were feeling minor earthquakes all the time starting from 1885, and over time, they got used to them. However, the tremors repeated and then significantly intensified. Plaster started to crack, stoves and walls were crumbling, ceilings were falling. Underground shocks could be felt all day long. The city was almost gone. For more information, see the next Flashback release.