A powerful earthquake hit Tokyo and nearby areas shortly after dawn today, halting trains and forcing two nuclear reactors to shut down for safety checks.
More than 30 people suffered minor injuries.
The US Geological Survey said another, unrelated quake with a 7.6 magnitude hit the Indian Ocean about 160 miles north of Port Blair in India's Andaman Islands.
A tsunami watch was called for India, Burma, Indonesia, Thailand and Bangladesh. The caution was later lifted without any tsunami being recorded.
On December 26, 2004, about 230,000 people were killed in a dozen countries around the Indian Ocean when a magnitude 9.0 earthquake triggered a tsunami.
Today's Andaman Islands' quake was 20.6 miles deep, the US Geological Survey said.
In Japan's magnitude-6.5 tremor, at least seven people were slightly hurt, the National Police Agency said. Public broadcaster NHK reported 43 were injured.
Japan's Meteorological Agency - which downgraded the quake from 6.6 - also issued a tsunami warning, but that was later lifted. The quake was centred off Suruga Bay, southwest of Tokyo, at a depth of 12 miles.
A small increase of waves of up to 23 inches was observed along the coastline of Yaizu city, the agency said.
More than 30 people were hurt but none seriously, said officials in the two hardest-hit prefectures (states), Shizuoka and Aichi.
"The shaking was quite strong, even here. It wasn't the usual little ones," said Mitsuharu Wakamori, a police official in Aichi, about 86 miles west of the quake's centre.
"I was nervous at the beginning, but now I'm relieved that the damage seems to be minor."
Two reactors at the Hamaoka nuclear power plant were temporarily halted for safety checks. No damage was reported, however. Reactors are automatically shut down whenever a quake of a certain strength is registered.
Koki Saguchi, of Chubu Electric Power Co, said no problems or damage were initially detected at the reactors. Plant workers were to inspect the reactors fully before resuming operation. The inspection was expected to take at least half a day.
The quake prompted Central Japan Railway Co to stop operations of Shinkansen bullet trains briefly, and some local trains were still out of service hours later, the company said.
Chief cabinet secretary Takeo Kawamura told reporters the government set up a task force at the prime minister's office.
"We are trying to assess the extent of damage as quickly as possible," Mr Kawamura said. "We'll do our best to rescue those who were affected."
A magnitude 6.9 quake rattled the region on Sunday, but caused no damage or casualties. The US Geological Survey measured it at magnitude 7.1.
Japan is one of the world's most earthquake-prone countries, and experts believe Tokyo has a 90% chance of being hit by a major quake over the next 50 years.
In 1995, a magnitude-7.2 quake in the western port city of Kobe killed 6,400 people.
Katsuyuki Abe, a Tokyo University seismology professor, said experts were studying whether today's quake could foreshadow a major quake that experts have warned is imminent in the region. Tokyo was devastated by an earthquake in 1923, and experts say it is overdue for another major jolt.
"We are closely monitoring the developments and examining any changes in the region," he said.