A strong earthquake with a preliminary magnitude of 7.4 has struck in the Pacific Ocean, triggering a tsunami warning off the southern coast of Japan.
Japan's Meteorological Agency said the quake occurred about 80 miles off the southern coast of Chichi Island in the Pacific Ocean. The offshore quake struck at around 2:20 a.m. local time.
The U.S. Geological Survey also put the quake's magnitude at 7.4.
The Japanese agency issued a tsunami alert of up to 6 feet for nearby islands and warnings of milder tsunami for the southern coasts on the main Japanese island.
A minor swelling of waves of about 1 foot was observed on the island's shorelines about 40 minutes after the quake, the agency said. The island is about 600 miles south of Tokyo.
The agency later downgraded the tsunami alert on the island to a warning and said only smaller waves of up to 1.5 feet were predicted. But the agency's seismology official Hirofumi Yokoyama said island residents should use caution for "several more hours'' given the size of the quake's magnitude.
There was no immediate report of any damage or injuries.
"It shook quite violently. I'm sure everyone was scared,'' said Kenji Komura, principal at a high school on the island. He rushed to school, where about 20 students gathered to take refuge. Despite the shaking, nothing fell on the floor or got damaged at school, Komura said.
About 170 people evacuated to several community centers and school buildings on the Chichi and nearby Haha islands, public broadcaster NHK said. Tomoo Yamawaki, a fisheries cooperative official on the Chichi island, said he has observed no significant swelling of the waves so far.
"We've taken all fishing boats on the island off coast to protect them from the tsunami,'' said Yamawaki, who is in charge of community tsunami broadcast, told NHK. "We haven't observed any significant change in the waves, but we urge all residents to immediately evacuate to a safer place.''
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.