Off Planet or Extinct: The Only Two Outcomes

An object of sufficient size striking the Earth would not represent the end of a civilization. It would represent the end of the species, full stop, and the only scenario in which humans survive an event of that magnitude is if some portion of the population had relocated off planet beforehand, even temporarily. The scale of the consequences is difficult to overstate. You would see seismic activity and volcanic activity triggered globally, not locally, because an impact of that size sends pressure pulses through the entire planet, and those pulses interact with the tectonic framework itself. At the epicenter, an area the size of the continental United States would be completely wiped out, and the secondary effects, the seismic and volcanic disruption radiating outward from that point, would extend the consequences to every part of the globe.