No tsunami warning was issued and officials reported no immediate casualties or damage, although buildings in Tokyo rattled, officials said.
The earthquake -- which struck the hilly, rice-farming Chiba Prefecture at 10:37 p.m. local time (8:37 a.m. EDT) -- was also felt in Japan's Miyagi, Fukushima, Tochigi, Gunma, Saitama, Kanagawa and Yamanashi prefectures, Kyodo News reported.
The quake released its energy 26.7 miles below the ground, the USGS said.
Tens of thousands of people had evacuated from the area after March 11's magnitude 9 undersea megathrust earthquake, although some had gone back to collect belongings.
The earlier quake, known as the Great East Japan Earthquake, triggered extremely destructive tsunami waves of up to 124 feet that traveled in some cases 6 miles inland.
That quake and tsunami left more than 28,000 people dead or missing and 5,000 injured, along with a nuclear power-plant disaster and more than 125,000 buildings damaged or destroyed and heavy damage to roads and railways.
The nuclear power-plant disaster included explosions and leaks of radioactive gas in three reactors at the Fukushima Daiichi Nuclear Power Station, which suffered partial meltdowns, and fires from overheating in spent fuel rods at another reactor, releasing radioactive material into the atmosphere.
Radioactive water 20,000 times the allowable limit leaked into the Pacific Ocean from the Fukushima plant, Tokyo Electric Power Co. said earlier Thursday.
The plant operator estimated the early April leakage lasted for six days through April 6 and amounted to 520 tons, Kyodo said.
In a related development, Prime Minister Naoto Kan said Japan would enforce a no-entry zone within a 12.4-mile radius of the crippled nuclear plant, prohibiting evacuees from returning to their homes without government permission.
The restriction has been in force since the early days of the disaster but has often been ignored, CNN reported.
Many of the 78,000 residents who have homes in the evacuation zone have gone back in recent weeks to retrieve their belongings and check on businesses.
Chief Cabinet Secretary Yukio Edano told reporters entry into the area will now be highly regulated.
People who temporarily return will have to wear protective suits and ride into the restricted zone on a designated bus.
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