TOKYO—An earthquake with a magnitude of 6.8, revised up from an earlier estimate of 6.6, jolted northeast Japan on Saturday, shaking buildings 400 kilometers away in Tokyo and raising the possibility of landslides closer to the epicenter. No tsunami alert was issued after the quake, which struck at 10:27 a.m. Japan time (0127 GMT) off the coast of Miyagi prefecture at a depth of 51 kilometers (32 miles), the Japan Meteorological Agency (JMA) said. The JMA earlier estimated its depth at 60 kilometers. There were no reports of serious injuries caused by the quake, but the JMA warned of the potential for strong aftershocks for about a week, as well as the heightened risk of landslides. Tokyo Electric Power reported there were no issues at the devastated Fukushima Daiichi nuclear plant, wrecked by a massive earthquake and tsunami in northeast Japan in March 2011, and sited some 105 kilometers from …