Japan issued a tsunami advisory after an earthquake struck near outlying islands in the Pacific Ocean.
The 6.6-magnitude earthquake on Thursday occurred at a depth of 10km, the Japan Meteorological Agency said.
Officials said it could cause a tsunami up to one metre in height.
The advisory, the second-lowest of a four-stage warning system, asked people on islands in the Izu chain, which stretches south from the Tokyo region on the main Japanese island of Honshu, to stay away from coasts and river mouths.
The warning came as the Fukushima nuclear plant began the second discharge of treated radioactive wastewater that has built up since its reactors were damaged in a tsunami triggered by a magnitude 9.1 earthquake in 2011.
The operator of the nuclear power plant said it began releasing a second batch of treated radioactive wastewater into the sea on Thursday.
Tokyo Electric Power Company said workers activated a pump to dilute the treated water with large amounts of seawater, slowly sending the mixture into the ocean through an underground tunnel.
The wastewater discharges, which are expected to continue for decades, have been opposed by fishing groups and neighbouring countries. Hundreds staged protests after the first discharge in August, while China banned all imports of Japanese seafood, damaging Japan's seafood producers and exporters.
Tepco said it planned to release 7,800 tonnes of wastewater from 10 storage tanks over the next 17 days, the same amount that was discharged into the Pacific Ocean between August 24 and September 11.
About 1.34 million tonnes of radioactive wastewater is stored in about 1,000 tanks at the plant, which was affected by an earthquake and tsunami in 2011.
Tepco and the government say discharging the water into the sea is unavoidable, with the tanks expected to reach capacity next year. Space at the plant is needed for its decommissioning, which is expected to take decades.
Officials say the water is treated to reduce radioactive materials to safe levels and is then diluted with seawater hundreds of times to make it safer than international standards.
The International Atomic Energy Agency has reviewed the safety of the wastewater release and concluded that if carried out as planned, it would have a negligible effect on the environment, marine life and human health.
However, some scientists say that the continuing release of low-level radioactive materials is unprecedented and needs further monitoring.
Japan’s government has set up a relief fund to help find new markets and reduce the effect of China’s seafood ban. Measures also include the temporary purchase, freezing and storage of seafood and the promotion of seafood sales at home.
Cabinet ministers have travelled to Fukushima to sample local seafood and promote its safety.
Tepco is tasked with providing compensation for reputational damage to the region's seafood caused by the wastewater release. It started accepting applications this week after it received hundreds of inquiries. Most of the damage claims are linked to China’s seafood ban and excess supply at home causing price declines, Teoco said.
Agriculture Minister Ichiro Miyashita promoted Japanese scallops at a food fair in Malaysia on Wednesday on the sidelines of a regional farm ministers’ meeting.
Government spokesman Hirokazu Matsuno told reporters on Thursday that the first release had been conducted “as planned and in a safe manner” and no abnormalities had been detected.
The government will “continue to communicate, both domestically and internationally, results of monitoring data in a highly transparent manner,” he said.
Japan will also urge China to “immediately scrap import bans on Japanese food, and act based on scientific justifications”, he added.
Russia, which has frosty relations with Japan, is reportedly considering following suit on the seafood ban.
With reporting from agencies
