A series of strong earthquakes rattled northeast Japan on Tuesday, keeping residents on edge more than a week after a devastating tremor and tsunami battered the region.
Two 6.6-magnitude tremors and one 6.4 quake struck within two and a half hours off Japan's tsunami-stricken northeast coast, starting from 0718 GMT, the US Geological Survey reported.
There were no reports of casualties or damage and no tsunami warnings.
A record 9.0 magnitude which spawned a towering tsunami battered Japan's northeast coast on March 11, leaving more than 21,000 people dead or missing and triggering a crisis at a nuclear power plant in Fukushima.
Around 20 percent of the world's most powerful earthquakes strike Japan, which sits on the "Ring of Fire" surrounding the Pacific Ocean.
An executive of Tokyo Electric Power Company, the operator of Japan's crippled nuclear plant, Tuesday bowed deeply and apologised to evacuees forced from their homes by the crisis.
TEPCO vice president Norio Tsuzumi visited evacuation centres in Fukushima prefecture and bowed to each of their residents who had been forced to flee the high-radiation zone around the nuclear plant.
"I apologise deeply because the company has caused anxiety and nuisance to the local residents around the plants, in the prefecture and in the wider society," he later told reporters.
Tsuzumi said he would stay in the TEPCO office in Fukushima prefecture while engineers, workers, troops and fire fighters struggle to douse the plant and repair reactor cooling systems damaged by explosions and fires.
"Since I have tried to manage this problem hand-in-hand with the government, my visit here to directly meet you was belated," he said. "For this I also apologise from the bottom of my heart."
It was the first time a senior TEPCO official had visited evacuee shelters for people forced to flee a 20 kilometre exclusion zone.
At some shelters, people were so angry that the TEPCO officials could not enter the buildings, the Sankei Shimbun daily reported.
An old man tearfully told Tsuzumi: "I want to ask you to let us go home as early as possible. I have left behind my livestock and everything else."
The twin quake and tsunami disaster, Japan's worst crisis since World War II, has now left at least 9,079 people dead and 12,645 missing, with entire communities along the northeast coast swept away.