A magnitude 7.3 earthquake struck southern Japan early on Saturday, killing at least six people, injuring many more and bringing down buildings, media reported, just over a day after a quake killed nine people in the same region.
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.
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 six deaths and 760 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.
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. Almost 200,000 households were 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.
The Japan Meteorological Agency initially said the Saturday quake was 7.1 magnitude but later revised it up to 7.3.
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, media reported. Kumamoto airport was also closed.
This new earthquake in Kyushu was much bigger and hit a wider area than the one that struck Kumamoto on Thursday night, says the BBC's Rupert Wingfield-Hayes in Tokyo.
In one town near the coast, the city hall has been so badly damaged there are fears it could collapse. A hospital has been evacuated because it is no longer safe.