CHRISTCHURCH, New Zealand—Rescue workers and volunteers intensified the search for an estimated 300 people who remain unaccounted for in the rubble of buildings destroyed in an earthquake, as police enforced a curfew to stop looting.

Aftershocks from the quake, which killed more than 70 people, continued to rattle nerves as emergency services worked to free survivors from the wreckage. With hundreds still missing, the death toll was expected to climb.

Early...