UNKNOWN (BNO NEWS) — A massive earthquake with a preliminary magnitude of 7.6 struck off India’s Andaman Islands on Tuesday morning. A second earthquake struck Japan only minutes later.

The first earthquake, with a magnitude of 7.6, struck 160 miles north of Port Blair on the Andaman Islands of India, the United States Geological Survey reported. It happened at 1.55 a.m. local time and struck about 20 miles deep. It was initially reported as a 7.7 magnitude earthquake, but was quickly revised to a 7.6. The magnitude remains subject to change, although a large revision is not expected.

A seismologist at the United States Geological Survey told BNO News that the earthquake appears to be an aftershock to the major earthquake in December 2004, which sparked a deadly tsunami that killed hundreds of thousands of people. He said aftershocks can continue for years.

After the earthquake, the Pacific Tsunami Warning Center issued a tsunami watch for India, Myanmar, Indonesia, Thailand and Bangladesh. “Earthquakes of this size have the potential to generate a destructive local tsunami and sometimes a destructive regional tsunami along coasts usually no more than a thousand kilometers from the earthquake epicenter,” the agency said in a bulletin. The agency did not issue a tsunami warning. It advised authorities in the region to take appropriate action and said that areas further away can experience small sea level changes in the hours after the earthquake.

It is not yet known if a tsunami was generated. A tsunami, if generated, would reach the coasts of the Andaman and Nicobar Islands of India within one to three hours.

A separate earthquake, with a preliminary magnitude of 6.4 to 6.7, struck Japan. It struck at 5.07 a.m. local time, about 20 miles off Shizuoka in Japan. A tsunami warning was immediately issued for the Izu Islands and the Shizuoka Prefecture. “Tsunami height is expected to be about 0.5 meters, pay attention to tsunamis,” the Japan Meteorological Agency said.

Strong shaking was felt in Tokyo. NHK, Japan’s government broadcaster, reported that the Chubu Electric Power nuclear power plant at Hamaoka automatically shut down two units for safety checks. The number 4 and 5 reactors were shut down, but no environmental changes were detected at nearby posts. A highway was also closed, said Reuters, without providing a location.

There was no immediate word on casualties or damage from the earthquakes, which do not appear to be related. The West Coast and Alaska Tsunami Warning Center said there was no danger to the U.S. coast.

UPDATE: Authorities say a tsunami was generated after the earthquake in the Indian Ocean; size unclear.