The magnitude-6.8 earthquake struck on Saturday near Nagano city shortly after 10 pm (1300 GMT) at a depth of 10 kilometres (6 miles), the Japan Meteorological Agency said.

The US Geological Survey measured the quake's magnitude at 6.2. Since the quake occurred inland, there was no possibility of a tsunami.

Japan's Nuclear Regulation Authority said no abnormalities were reported at three nuclear power plants in the affected areas. All of Japan's nuclear plants are offline following a magnitude-9.0 earthquake and massive tsunami in 2011 that sent three reactors at the Fukushima Dai-ichi plant into meltdown.

Fukushima is about 250 kilometres (155 miles) northeast of where yesterday's earthquake occurred.

At least 22 people were injured, three of them seriously, in Nagano city, the Hakuba ski resort and elsewhere, the National Polilce Agency told Japan's Kyodo news agency.

"We are trying to assess the situation as quickly as possible, and we'll do our utmost for the rescue of the
injured people," Japan's top government spokesman, Yoshihide Suga, told reporters.

One of the hardest-hit areas appeared to be Hakuba, a ski town west of Nagano that hosted events in the 1998 games. At least 10 homes collapsed there, said Shigeharu Fujimura, a Nagano prefecture disaster management official.

The National Police Agency told Kyodo that 21 people were trapped underneath the collapsed houses, but they all were rescued, with two of them injured.

Many houses also lost water, apparently because of a ruptured pipe, and landslides on two major roads blocked
access to some areas. "We are afraid there could be some areas that may have been isolated, so we need to conduct a thorough assessment of damage after sunrise," Fujimura said.

The earthquake was felt across much of northern Japan and in Tokyo, about 200 kilometres (125 miles) southeast of Hakuba.

NHK reported that a landslide blocked a road after the quake struck. Kyodo said all shinkansen bullet train systems in the region were temporarily suspended. The quake caused scattered blackouts, including about 1,600 homes in Hakuba and Omachi city, the Chubu Electric Power Co. said.

The quake was followed by 21 aftershocks, said Yohei Hasegawa of the Meteorological Agency's earthquake and tsunami division. He warned of further aftershocks and urged residents to watch out for landslides. The area was struck by a magnitude-6.7 earthquake the day after the massive March 2011 quake.

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