The Quarantine Zone was a section of Fairport that was cordoned off and evacuated by the Metro Police Department.
The zone was originally around the South River Wastewater Treatment Plant and the Armacham Technology Corporation's headquarters which were the first areas taken by Replica Forces. The zone continued to expand as the Replica Forces expanded their search for the Vault. The Metro Police Department (possibly overpowered by Hind D Gunships and Replica Armored Trucks) likely saw how the situation was spiraling out of control, and were forced to fall back and increase the zone to include the Auburn District and began to evacuate civilians from the nearby residential areas as more and more military units were sent in to combat the Replica Forces.
After the Origin Facility Explosion occurred, the explosion devastated both the Quarantine Zone and a significant portion of the city. It's probable that many residents and a considerable number of Metro Police personnel were killed or faced even worse outcomes, being transformed into Remnants or Specters by Alma's influence. While it's plausible that the Metro Police were successful in evacuating a significant portion of the population near the quarantine zone, most of Fairport continued with their usual routines. Unfortunately, this ultimately resulted in the majority of the city's inhabitants being either killed or transformed due to the impact of the blast.