GUÁNICA, P.R. — Not long after José Méndez Marrero, a civil engineer, arrived on Saturday to inspect the damage at a Puerto Rican town crippled by a big earthquake, the ground beneath him groaned. Again.

The woman he had been chatting with on the street began to run into her house.

“No, señora!” he hollered behind her. “To the plaza!”

It was another scary one — a 5.9-magnitude aftershock, on the 15th day since tremors large and small began terrorizing southern Puerto Rico. The quake stunned the island just as signs of life, like trucks selling fresh fruit on the side of the road, had started to return. Now there were more power outages, more cracked buildings, more feelings of dread that the worst of the shaking was, somehow, not yet over.