TOKYO - A magnitude 7.3 earthquake struck southern Japan early on Saturday, killing at least three people, injuring many more and bringing down buildings, media reported, just over a day after a quake killed nine people in the same region.
The authorities warned of damage over a wide area, as reports came in of scores of people trapped in collapsed buildings, fires and power outages.
Residents living near a dam were told to leave because of fears it might crumble, broadcaster NHK said.
Saturday's tremblor triggered a tsunami advisory, although it was later lifted and no irregularities were reported at three nuclear power plants in the area, a senior government official said.
People still reeling from Thursday's shock poured onto the streets after the Saturday quake struck at 1:25 a.m. (1625 GMT).
A fire erupted in a what appeared to be an apartment building in Yatsushiro city, while some people were trapped in a nursing home in the town of Mashiki, according to NHK.
NHK reported three deaths and nearly 400 people treated in hospitals, but that figure included "people who don't feel well", so it was not clear how many serious injuries there were.
Chief Cabinet Secretary Yoshihide Suga said nearly 80 people were believed trapped or buried in rubble. Extra troops would be sent to help, with up to 15,000 due on Saturday, as well as more police, firefighters and medics, he said.
"We are making every effort to respond," Suga said.
Troops fanned out to search ruined houses as dawn broke.
The epicenter of the quake was near the city of Kumamoto and measured at a shallow depth of 10 km, the U.S. Geological Survey said. The entire city of 730,000 was without power.
Prime Minister Shinzo Abe, arriving at his office, told reporters the government was making every effort to determine the extent of the damage, carry out rescue and recovery, and to get accurate information to citizens.
"It's possible that there may be damage over a wide area," Abe said.
The region's transport network suffered considerable damage with one tunnel caved in, a highway bridge damaged, roads blocked by landslips and train services halted, local media reported.
The earthquake on Thursday evening in the same region was of 6.4 magnitude and experts said the two tectonic events could be linked.
"Thursday's quake might have been a foreshock of this one," Shinji Toda, a professor at Tohoku University, told NHK.
Several aftershocks rattled the region later on Saturday, including two of nearly 6 magnitude, and experts warned of more.
"We would not be surprised to see more earthquakes of this size," said John Bellini, a geophysicist with the U.S. Geological Survey.
Television footage showed many frightened people wrapped in blankets sitting outside their homes.
The Japan Meteorological Agency initially said the Saturday quake was 7.1 magnitude but later revised it up to 7.3.
Soon after the quake struck, the agency issued a tsunami advisory, which identifies the presence of a marine threat and asks people to leave coastal regions, for the Ariake and Yatsushiro seas.
NHK said the advisory suggested a possible wave of one meter in height. The advisory was later lifted.
A magnitude 9 quake in March 2011, to the north of Tokyo, touched off a massive tsunami and nuclear meltdowns at Fukushima. Nearly 20,000 people were killed in the tsunami.
Japan is on the seismically active "ring of fire" around the Pacific Ocean and has building codes aimed at helping structures withstand earthquakes.
The 2011 quake temporarily crippled part of Japan's auto supply chain, but some companies have since adjusted the industry's "Just in Time" production philosophy in a bid to limit any repeat of the costly disruption.