TOKYO — An earthquake with a preliminary magnitude of 7.1 struck Japan's southern Kumamoto prefecture on Tuesday, knocking out power to thousands of homes, stopping rail services and triggering warnings for tsunamis and aftershocks. Companies including Sony and TSMC, the world's largest contract chipmaker, have plants in the area of the quake. A Sony spokesperson said the company was checking the situation. TSMC did not immediately respond to a request for comment. The Japanese government issued emergency earthquake warnings for Kumamoto, Nagasaki, Kagoshima, Fukuoka, Saga, Oita, and Miyazaki prefectures, all on Japan's southern Kyushu island. A tsunami warning for a wave measuring 1 m (3.28 feet) was issued after the earthquake, the Japan Meteorological Agency said. Japan is one of the world's most earthquake-prone countries, with a tremor occurring at least every five minutes. Located along the "Ring of Fire" of volcanoes and oceanic trenches partly encircling the Pacific Basin, Japan accounts for about 20% of the world's earthquakes of magnitude 6.0 or more. A massive earthquake in