TOKYO (Reuters) - A moderately strong earthquake hit northern Japan on Friday, causing buildings to shake in Tokyo, but there were no immediate reports of damage or injuries.National broadcaster NHK said the quake, which struck at around 8:06 p.m. EST Thursday, measured 6.8 on the open-ended Richter scale.
It said the focus of the earthquake was 19 miles below the surface off the coast of Fukushima prefecture in northeastern Japan.
NHK said the earthquake measured four on Japan's seismic intensity scale of seven in some areas of Miyagi prefecture, which is just north of Fukushima, and registered at lower levels in various other locations in northern Japan.
The Japan Meteorological Agency later issued a tsunami warning for Miyagi prefecture, and the agency said a tsunami measuring 20 inches in height was already thought to have reached the coast.
Kyodo news agency said the quake caused no damage to the Onagawa nuclear power plant in Miyagi Prefecture or the Fukushima No. 1 and No. 2 plants in Fukushima Prefecture.
An earthquake with a Japanese scale of four is considered strong enough to make hanging objects swing considerably and cause dishes in cupboards to rattle.
Japan sits atop the junction of at least three tectonic plates, immense slabs of the earth's crust whose gradual movements are thought to cause earthquakes, making it one of the world's most earthquake-prone regions.
A powerful earthquake registering 8.0 on the open-ended Richter scale rocked Japan's northern island of Hokkaido late last month, injuring nearly 600 people and causing a fire to break out at an oil refinery.
