A strong earthquake measuring an estimated 6.1 in magnitude jolted eastern Japan early Friday, triggering emergency measures to check the state of railways, highways and nuclear plants.

There were no immediate reports of casualties and no tsunami warnings were issued, the Meteorological Agency said.

The temblor did not immediately affect the operation of nuclear power plants in Ibaraki and Fukushima prefectures, two of the most seriously affected areas, according to the Agency of Natural Resources and Energy.

The quake, which struck at 3:39 a.m., was felt from Aomori and Iwate prefectures in the north to Aichi and Shiga prefectures in central Japan. The strongest tremors were felt in Ibaraki Prefecture.

Elevators in buildings throughout Ibaraki Prefecture stopped operating, local police said.

Seismic instruments registered a lower 5 on the Japanese scale of 7 in the cities of Mito, Hitachi-Ota and Takahagi, all in northern Ibaraki Prefecture, and in Kasama in central Ibaraki, the Meteorological Agency said.

The temblor registered 4 in Koriyama and Shirakawa in Fukushima Prefecture and various parts of Ibaraki Prefecture, as well as Imaichi and Nasu in Tochigi Prefecture.

It registered 3 on the Japanese scale in Sendai, Takasaki in Gunma Prefecture, Kumagaya in Saitama Prefecture, Suwa in Nagano Prefecture and Yokohama, it said.

East Japan Railway Co. said the first train on the Tohoku Shinkansen Line traveled at a slower speed to make sure the rails and other equipment on the line were in proper order.

JR East said the quake temporarily halted train runs Friday morning on its Joban Line, which links Ueno and Sendai, and the Suigun Line, which links Mito in Ibaraki Prefecture and Koriyama in Fukushima Prefecture.

Parts of the Tohoku, Joban and Kita-Kanto expressways were closed to traffic following the quake, but reopened in time for the morning rush hour, the Japan Highway Public Corp. said.

According to the Meteorological Agency's seismic gauging system, an intensity-5 earthquake is strong enough to produce cracks in walls but otherwise would not cause major damage.

The agency traced the epicenter to some 40 km off Ibaraki and placed the focus at about 50 km underground.

Reactor is shut down

SENDAI (Kyodo) Tokyo Electric Power Co. announced on Friday that it has started to manually shut down a reactor at its nuclear power plant in Fukushima Prefecture due to an increase in waste gas, possibly caused by an earthquake that jolted extensive areas of eastern Japan early in the day.

Tepco started to shut down the No. 6 reactor at the Fukushima No. 1 plant on the Pacific coast at 3 p.m. Friday.

The shutdown is necessary to determine the cause of a rapid increase in radioactive exhaust emitted by the reactor, detected shortly after the magnitude 6.1 quake struck at 3:39 a.m., it said.

No radioactivity was leaked outside the plant and operations at the reactor were otherwise normal, it said.

The 1.1 million-kw reactor, the largest of the six reactors at the plant, was scheduled to be completely shut down by around 10 p.m. Friday, the company said.

Tepco suspects the earthquake loosened pipe joints, sending air into the system that removes radioactive substances from waste gas generated at the No. 6 reactor, it said.

The six reactors at the Fukushima No. 1 plant have a combined generating capacity of 4.69 million kw.