Twenty years after a 6.7 earthquake hit Northridge, Calif., a lot has changed.

Public buildings, overpasses and hospitals have been retrofitted to better withstand another disaster. New construction must meet new standards. After all, 40,000 structures—some put the number at more than 100,000—were either damaged or destroyed in the 10-second jolt at 4:31 a.m. on Jan. 17, 1994, or in the aftershocks that followed.