The U.S. Geological Survey and European-Mediterranean Seismological Center put the temblor's magnitude at at 4.3, while Italy's National Institute of Geophysics and Volcanology revised its preliminary measurement of 3.6 up to 4.0.
Ischia is in the Tyrrhenian Sea, just west of Naples.
In the small town of Casamicciola, on the island's northern coast, six buildings and a church collapsed after the quake struck just before 9 p.m. (3 p.m. ET), Reuters reported.
"I am seeing several people under the rubble," a resident there, Raimondo Giusto, told RAI NEWS 24.
A Dutch tourist, Tessa de Mönnink, told NBC News that she was at a restaurant in the village of Sant'Angelo when the room began to shake, then went dark.
It was “like something I’ve never heard before,” she said.
The quake lasted 30 seconds, she added, and after two minutes the power returned.