A 6.6-magnitude earthquake rocked Japan's northwest coast on Monday, injuring at least 20 people, knocking over homes and triggering a series of small tsunamis. Several smaller aftershocks jolted the region. National broadcaster NHK said one person had been injured and taken to hospital, but there were no more detailed reports on the severity of injuries. Kyodo News agency said "dozens" of homes were toppled in a town outside Niigata city. Black smoke was seen pouring from the nearby Kashiwazaki nuclear plant, which was automatically shut down in the quake. NHK reported the smoke was caused by a fire in an electrical transformer, and that no radioactivity had been released. The Meteorological Agency issued tsunami warnings along the coasts of Niigata Prefecture, warning of small tsunamis in other coasts, but no significant damage was expected. The warning was later lifted.