A loud bang startled Anna P. Timofeyeva awake. She reached for the light, but the electricity had gone out. In the dark, she and her husband quickly dressed their 2-year-old son and prepared to flee. “We understood something was wrong,” she said. But when they opened the front door of their apartment they stopped short. From the doorstep of the family’s seventh-floor apartment, she said, they could look directly down on a heap of rubble far below, all that was left of 25 neighboring apartments.