A powerful earthquake struck off southern Japan on Thursday, causing mostly minor injuries but raising the level of concern over possible major quakes stemming from an undersea trough east of the coast.

The quake prompted seismologists to hold an emergency meeting in which they raised the level of risk of major quakes associated with the Nankai Trough— the source of past devastating earthquakes— east of southern Japan.

They issued an assessment that the potential for a future quake in the area from Kyushu to central Japan is higher than previously predicted.

That does not mean there's an imminent danger of a big quake in the near future, but they urged residents on the coasts along the trough— which spans about 500 kilometers (310 miles)— to review their quake preparedness, University of Tokyo seismologist Naoshi Hirata, a member of an expert panel, told a joint news conference with JMA officials.

There is a 70-80% chance of a magnitude 8 or 9 quake stemming from the Nankai Trough within the next 30 years, Hirata said, adding that Thursday's quake raises that probability even while the timing or exact location cannot be predicted.

He urged residents to keep their caution levels high for a week for the time being.

Thursday's quake registered magnitude 7.

1 and was centered in waters off the eastern coast of Kyushu at a depth of about 30 kilometers (about 19 miles) below sea surface, the Japan Meteorological Agency said.

The quake most strongly shook Nichinan city and nearby areas in Miyazaki prefecture on Kyushu island.

The agency said tsunami waves of up to 50 centimeters (1.

6 feet) were detected along parts of Kyushu's southern coast and the nearby island of Shikoku about a half hour after the quake struck.

Tsunami advisories were issued, but lifted for most areas three hours later, and for all remaining coastlines five and a half hours later.

Officials said nine people were injured on Japan's southern main island of Kyushu, but the injuries were mostly minor.

There were no reports of serious damage.

Prime Minister Fumio Kishida told reporters that the government crisis management team was to step up disaster preparedness, and called on residents to pay close attention to information from the authorities in case of another big one and never to spread disinformation.

The agency gave Thursday's earthquake a yellow alert for estimated economic losses, meaning 'some damage is possible and the impact should be relatively localized.

' It gave the quake a green alert for estimated fatalities, meaning 'there is a low likelihood of [shaking-related] casualties.

' JMA Seismology Department official Shigeki Aoki warned that strong aftershocks could occur for about a week.

Japan's NHK public television said windows were broken at the Miyazaki airport near the epicenter.

The airport's runway was temporarily closed for safety checks.

Kyushu's shinkansen 'super-express' trains returned to normal operations after temporary safety checks, but a number of local train lines in the quake-hit areas were to be suspended through Friday, according to Kyushu Railway Co.

In Osaki in neighboring Kagoshima prefecture, concrete walls collapsed and a wooden house was damaged, but no injuries were reported.

The Nuclear Regulation Authority said all 12 nuclear reactors, including three that are currently operating, on Kyushu and Shikoku remained safe.

Earthquakes in areas with nuclear power plants have been a major concern since a massive earthquake and tsunami in March 2011 triggered the Fukushima nuclear disaster.

Japan sits on the Pacific 'Ring of Fire,' a line of seismic faults encircling the Pacific Ocean, and is one of the world's most earthquake-prone countries.