The U.S. Geological Survey said the aftershock had a magnitude of 6.6, while Japanese officials are saying the quake was a 7.0. It struck about 100 miles north of Tokyo.
Several cameras captured the shaking on video. The latest quake spooked people yet again in a disaster-weary northeastern Japan. Customers in a large electronics store in Sendai screamed and ran outside and mothers grabbed their children.
There were no immediate reports of any new damage from Monday's quake, though it sparked a new tsunami warning. It also caused Tokyo's main international airport to close both of its runways.
Officials at the damaged Fukushima Dai-ichi nuclear plant said it did not cause any further damage to the plant.
But a nuclear safety official said repeated strong aftershocks - another large quake hit last Thursday - were slowing work at the plant.
Radiation continues to leak, forcing tens of thousands of people to leave their homes.
Officials expanded the evacuation zone beyond the earlier 12-mile radius, citing long-term exposure risks.
The Associated Press contributed to this report.
