TOKYO A powerful earthquake struck off the northern Japanese coast early today, injuring at least 91 people, triggering landslides and cutting power to thousands of people, officials said. Japan's Meteorological Agency said there was no danger of a tsunami from the temblor, which had a preliminary magnitude of 6.8. It struck shortly after midnight about 65 miles underground and centered just off the coast of Iwate, 280 miles northeast of Tokyo. The 91 people injured were mainly suffering from cuts and bruises from broken glass and falling objects, and none of the injuries were life-threatening, said National Police Agency official on condition of anonymity, citing department policy.