SENDAI, Japan — A big aftershock rocked quake-weary Japan late Thursday, rattling nerves as it knocked out power to the northern part of the country and prompted tsunami warnings that were later canceled.

The quake was initially measured at magnitude-7.4, though the U.S. Geological Survey in Golden, Colo., later downgraded it to 7.1. Either way, it was the strongest aftershock since several were recorded on March 11 — the day of the magnitude-9.0 earthquake and tsunami that killed as many as 25,000 people and touched off a nuclear crisis last month.

There were no immediate reports of serious injuries or major damage, and the operator of the tsunami-ravaged Fukushima Dai-ichi nuclear plant said there was no sign the aftershock had caused new problems there. Workers retreated to a quake-resistant shelter in the complex, with no injuries.

The aftershock around 11:30 p.m. was strong enough to knock items off store shelves and move a large automated teller machine at a FamilyMart convenience store in the major northern industrial center of Sendai.