(Adds no effect on nuclear plant, tsunami small)

By Isabel Reynolds

TOKYO, Sept 11 (Reuters) - An earthquake with a preliminary magnitude of 7.0 struck off the coast of Japan's northern island of Hokkaido on Thursday, the Japanese meteorological agency said, but there were no immediate reports of injuries or damage.

A tsunami warning was issued for the area after the tremor struck at 0021 GMT (09:21 JST) but only a 10 cm (4 inch) tsunami was observed on the Japanese coast, broadcaster NHK said.

"I felt a shake from side to side," Hiroshi Sasahara, an town official at Hiro on the southeast coast of Hokkaido told broadcaster NHK. "Nothing fell off from bookshelves but we found a crack running 5-6 cm (2-2.4 inches) along the wall of the town office."

The focus of the tremor was 20 km (12 miles) below the surface of the earth, off the coast of Tokachi on the sparsely populated island of Hokkaido, about 800 km (500 miles) north of Tokyo, the Japan Meteorological Agency said. It was followed by less powerful aftershocks.

The runway at Shin Chitose Airport on Hokkaido was closed for safety checks, as were some roads, domestic media said. Ten thousand people were advised to evacuate

Hokkaido Electric Power <9509.T> said its Tomari nuclear plant was operating normally after the quake, and Nippon Oil <5001.T> said its Muroran refinery was also operating as normally.

Earthquakes are common in Japan, one of the world's most seismically active areas. The country accounts for about 20 percent of the world's earthquakes of magnitude 6 or greater.

In October 2004, an earthquake with a magnitude of 6.8 struck the Niigata region in northern Japan, killing 65 people and injuring more than 3,000.

That was the deadliest quake since a magnitude 7.3 tremor hit the city of Kobe in 1995, killing more than 6,400. (Additional reporting by Tetsushi Kajimoto and Yoko Nishikawa)

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