The quake with a preliminary magnitude of 6.7 hit about 12:30 p.m. (0330 GMT) Thursday, near the Urakawa town on the southern tip of Hokkaido, according to the Japan Meteorological Agency. It said the quake's center was located at a depth of 50 kilometers (30 miles) below the sea surface.
There were no immediate reports of injuries or damage.
Hiroyuki Kenai, a disaster prevention official at the Urakawa town told Japan's NHK national television in a telephone interview that he was caught by surprise when having lunch at his office and that officials were still assessing the extent of damage.
A newly launched Hokkaido "bullet train" service was temporarily suspended but has since resumed, according to NHK.
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