TOKYO, Sept 16 - An earthquake with preliminary magnitude of 5.6 shook buildings in eastern Japan, including the capital Tokyo, on Tuesday but there were no reports of serious damage. The earthquake was centred in Ibaraki Prefecture, just northeast of Tokyo, Japan's Meteorological Agency said. The depth of the earthquake was 50 km and there was no risk of a tsunami, it added. Japan Atomic Power Co and Tokyo Electric Power Co (Tepco) reported no irregularities at their three nuclear plants in eastern Japan. A Japan Atomic official said there have been no irregularities at the 1.1-gigawatt Tokai Daini plant, which has been shut since March 2011. A Tepco spokeswoman said there has been no irregularities at the Fukushima-Daiichi nuclear plant, wrecked by the March 2011 quake and tsunami, and Fukushima-Daini plant, which has been shut since March 2011.