A powerful aftershock rocked northeastern Japan late Thursday, killing two people and injuring at least 130, cutting electricity to 4 million homes and disrupting the primary power supply at two nuclear facilities.

The 7.1-magnitude quake, the strongest since the March 11 earthquake and tsunami that devastated much of the same coastal region, again highlighted the vulnerability of Japan’s nuclear network. But Japan’s nuclear agency said none of the region’s plants reported immediate damage, including the troubled Fukushima Daiichi facility, about 70 miles southwest of the aftershock’s epicenter.

Authorities said that electrical troubles at the Onagawa nuclear power station, north of Sendai, the region’s biggest city, and at a nuclear reprocessing plant farther north, near a U.S. air base in Aomori prefecture, did not appear likely to cause a catastrophic failure, as happened after last month’s tsunami slammed into the Daiichi plant.

Seeking to calm a public traumatized by the earlier debacle, Tetsuro Fukuyama, the deputy chief cabinet secretary, told a hastily convened late-night news conference that adequate power had been restored to the Onagawa plant and the reprocessing facility, either from generators or from undamaged power lines, and that they pose no danger.

The quake came just hours after a rare bit of good news — a successful operation to avert a possible explosion at a crippled reactor at Fukushima Daiichi by injecting nitrogen. Earlier blasts at the six-reactor plant had led to the world’s worst nuclear disaster since Chernobyl. Nuclear experts had also expressed concern that damage caused by the explosions had left the plant’s containment buildings more vulnerable to strong aftershocks.

A spokesman for Tokyo Electric Power Co. said at a news conference that the company had no information of any further damage to the Daiichi plant and that Thursday’s quake would not interrupt efforts to bring the facility under control.

Across much of eastern Japan, including Tokyo, 205 miles southeast of the epicenter, this latest aftershock served as a late-night flashback to the trauma of four weeks ago. The aftershock struck at 11:32 p.m. In Tokyo, buildings swayed for almost a minute. Trains stopped, as designed. In Sendai, windows shattered at a gym being used as a shelter for evacuees. Japan’s meteorological agency issued a tsunami warning, and television broadcaster NHK urged coastal residents to head to higher ground. Roughly an hour later, the warning was lifted with no reports of high waves reaching land.

(Source: U.S.G.S., Japan Meteorological Agency)

The earthquake damage was nowhere near as severe as that caused by the March quake and tsunami, which killed more than 12,500 people, with nearly 15,000 still listed as missing. But it plunged the country back into a state of deep anxiety that, at least in areas undamaged last month, had been slowly beginning to lift.