TOKYO — Two strong aftershocks rattled northern Japan Thursday, weeks after a powerful earthquake killed 39 people and injured thousands. The tremors sent residents dashing under tables and caused at least one injury.

Late Thursday, a magnitude 5.7 earthquake jolted the country's northernmost main island of Hokkaido, but there were no immediate reports of injuries. The quake, which struck at 11:03 p.m. (9:03 a.m. EST), was centered 37 miles underground, near the island of Kunashiri, about 590 miles north of Tokyo, the Meteorological Agency said.

The aftershock, about 500 miles north of the magnitude 6.8 tremor that struck rural Niigata prefecture on Oct. 23, was in an area not affected by the initial tremor. Kunashiri is one of four islands north of Hokkaido controlled by Russia but claimed by Japan. The main impact was felt in the village of Betsukai but local official Kiyomi Sato said there was no damage there.

Earlier Thursday, a magnitude 5.2 aftershock struck an area still recovering from last month's quake. It was centered about 12 miles below the earth's surface near a rural part of Niigata prefecture, about 160 miles north of Tokyo. It struck at 8:57 a.m. (6:57 p.m. EST Wednesday).

A magnitude 5 earthquake can cause considerable damage.

The Meteorological Agency warned of more strong quakes were possible over the next month.

Separately, Niigata police attributed Wednesday's death of a 71-year-old man to last month's earthquake, bringing the toll to 39, while the injury count remained more than 2,000.

The man, who had been living at an emergency shelter, suffered a stroke from the stress of the jolt and its numerous aftershocks, the police said.

The initial quake last month was the deadliest to hit Japan since 1995, when a magnitude-7.2 quake killed 6,000 people in the western city of Kobe.

Officials in Niigata said a 40-year-old man, who was cooking at a market when the aftershock struck, burned his hands. Officials received reports of burst water pipes and minor landslides.

Television showed telephone poles and trees swaying and buildings shuddering when the first aftershock struck. Inside Ojiya city hall, near the site of the worst destruction from last month's quake, officials holding a meeting grabbed their chairs or ducked below tables.

The shaking automatically triggered safety devices that shut down a nuclear reactor at a power plant and temporarily cut electricity to express "bullet" trains in the area, officials said.

Tokyo Electric Power Co. spokesman Motoyasu Tamachi said the nuclear plant in Kashiwazaki city was being inspected but that the shutdown had no impact on the outside environment.

The aftershock hit as many public schools in the Niigata area reopened for the first time since last month's quake.

Over 46,000 people remain in emergency shelters in Niigata on Wednesday because their homes were completely or partially destroyed in the quake. Many were also too afraid to go home as aftershocks continue, creating the risk their homes will be further damaged.

Japan, which rests atop several tectonic plates, is among the world's most earthquake-prone countries.