Lois had not really noticed that Clark seemed to running for cover. As soon as she registered the source of the attack, she turned and ran for the stairs. The elevators were probably on emergency shutdown, and they were not exactly a safe bet when the building structure was being compromised.
The stairs were a longer haul though, and Lois raced down them as fast as she could. About five stories up, the rumbling ceased for a moment, and she traveled even faster, hoping to make it to the ground before she missed when was happening. One flight from the bottom, though, it started shaking much more violently, and she slipped, hitting the cement landing hard, but immediately leaped up and continued traveling.