An earthquake as powerful as last month's San Francisco temblor rocked Japan's Pacific coast early Thursday, sending thousands of seaside residents scurrying to higher ground for fear of tidal waves.

There were no reports of damage or injuries from the quake, which officials said measured 7.1 on the open-ended Richter scale and was centered 40 miles off the coast of Sanriku, a small fishing village that has been plagued by more than 250 smaller temblors in the past week.At least six aftershocks followed the main quake, one of the biggest in the region since a catastrophic 8.5-magnitude quake in 1896 killed 27,000 people, most the victims of huge tidal waves, the national Meteorological Agency said.

"It's difficult to speculate whether another big earthquake will occur or not," an agency spokesman said. "We have to keep watching carefully."

U.S. Geological Survey earthquake monitors in Golden, Colo., measured the Japanese quake at 7.3 on the Richter scale and registered a 5.8-magnitude aftershock 10 minutes after the main quake, spokesman Don Finley said in Washington.