An earthquake measuring 5.1 on the Richter scale on Sunday jolted off the east coast of Honshu, Japan, the U.S. Geological Survey said, Xinhua reported.
The quake, one of the aftershocks following a 8.8-magnitude quake on Friday, struck at 04:11 local time (1911 GMT on Saturday). The epicenter, with a depth of 13.70 km, was initially determined to be at 37.7297 degrees north latitude and 142.5865 degrees east longitude.
Friday's catastrophic earthquake in Japan and the following devastating tsunami have ravaged the country, and the death toll exceeded 620 as of Saturday evening.
An explosion occurred at the Fukushima No. 1 nuclear power plant earlier on Saturday triggered public panic, but the Chief Cabinet Secretary Yukio Edano said late Saturday that owner and operator of the nuclear facility, Tokyo Electric Power Co., has confirmed that the steel container housing the reactor is intact.
The trouble at a Fukushima nuclear plant prompted calls for electricity savings in Japan.
Tokyo Electric Power referred Saturday to the possibility of limiting overall electricity supplies from Monday by carrying out intentional power outages. Among the options being considered by the company is one to divide its service area into several blocks and stop supplying power on a rotation basis, possibly for three hours a day for one area.
The lights were turned off Saturday night at some of the best known landmarks in major Japanese cities due to concern for electricity shortage.
Many countries across the world have expressed condolence and offered a helping hand to Japan.
China's Premier Wen Jiabao expressed "deep sympathy and solicitudes to the Japanese government and people" on Friday, and said China is willing to offer necessary assistance to Japan.