Residents of southern Japanese islands scrambled to community centers early Wednesday when a strong offshore quake briefly triggered a tsunami alert, but the 7.4-magnitude temblor prompted only a mild swelling of waves.
There were no immediate reports of damage from the quake, which struck at 2:20 a.m. (1720 GMT Tuesday) about 80 miles (130 kilometers) off the southern coast of Chichi Island in the Pacific Ocean and was felt as far away as Tokyo.
"It shook quite violently. I'm sure everyone was scared," said Kenji Komura, principal at a high school on Chichi Island.
About 30 temblors above magnitude 5.0 were reported over nine hours after the larger quake hit the Bonin Islands region.
Japan's Meteorological Agency issued an alert for a tsunami of up to 6 feet (2 meters) for Chichi and nearby islands and warned of a milder tsunami for the southern coasts of the main Japanese island. It later lifted all warnings and said a minor swelling of about 1 foot (30 centimeters) was observed on Chichi's shorelines about 40 minutes after the quake.
Scores of residents of Chichi, which is about 600 miles (1,000 kilometers) south of Tokyo, and nearby Haha islands rushed to community centers and school buildings before the warnings were lifted. Island fisheries official Tomoo Yamawaki said fishermen moved boats from the coast "to protect them from the tsunami."
The quake took place at a depth of 6 miles (10 kilometers), the Meteorological Agency said. The U.S. Geological Survey put the quake's magnitude at 7.4.
Japan is one of the world's most earthquake-prone countries. In 1995, a magnitude-7.2 quake in the western port city of Kobe killed 6,400 people.