Since Hurricane Maria lashed Puerto Rico this week, Carol Mitchell has not heard a word from her mother.

Ms. Mitchell, who is in Tampa, Fla., said her mother lives in Utuado, a mountainous Puerto Rican municipality where there have been reports of fatal landslides. Unable to reach the area directly — power is out across the island, and communication is severely constrained — Ms. Mitchell is using phone messaging apps like Zello and WhatsApp to reach people in San Juan, the capital.

“People in Puerto Rico are getting pictures, and then they’re sending them to us so we know what’s going on as much as we can,” she said, adding that the snippets of information she received on her phone were not enough, but were better than nothing.

“It’s the only way.”

Seeking help, from anyone

During hurricanes in the Caribbean, Texas and Florida in recent weeks, and after the earthquake that killed hundreds in Mexico on Tuesday, many people turned their smartphones into lifelines.