Under the glare of portable floodlights and the flashlights that some held, men in sweat-soaked jumpsuits dug into a hillock of rubble in this city, the night after the largest earthquake to hit Mexico in a century flattened buildings here and across southern Mexico.

Atop the mound of debris, the men at times moved like archaeologists, sifting with bare, dirt-encrusted hands, chunk by concrete chunk. At other times, they powered up an excavator, which, with its own brutish precision, moved the workers closer to their goal.