Seismologists said an earthquake that struck near Japan's crippled Fukushima nuclear power plant yesterday was an aftershock of the quake that sparked 2011's deadly tsunami along the country's northeast coast, and warned of more to come.
The strong 6.8-magnitude earthquake off the Pacific coast caused a minor tsunami in the early hours yesterday, though authorities lifted all weather warnings two hours later.
Seismologist Yasuhiro Yoshida of the Japan Meteorological Agency said it was a delayed tectonic reaction to the 9.0-magnitude quake that caused reactors to melt down at the Fukushima plant after huge tsunami waves devastated the area in March 2011.
"There are fears that relatively large earthquakes will occasionally occur in the ocean area where aftershocks of the great earthquake continue," he said. "The aftershock activity has been steadily declining on a long-term basis. But aftershocks, accompanied by tsunami, will still occur."
The 2011 disaster killed more than 18,000 people.
Yesterday's quake measured up to 4 on the Japanese scale of 7 in terms of intensity, and Yoshida said there was a possibility aftershocks measuring a moderate 3 on that scale would occur in the next two weeks.
At least three people were injured in the latest quake but no more damage was done to the crippled plant, officials said.
The quake occurred at a depth of 13km off Fukushima's coast, according to the US Geological Survey. It was strong enough to be felt in Tokyo, more than 200km to the south, with many people woken by earthquake alerts sent by the Japan Meteorological Agency to their mobile phones.
The Fukushima plant's operator, Tokyo Electric Power (Tepco), said there were no reports of any abnormality early yesterday. Sea levels cannot be gauged near the nuclear plant as the tsunami monitoring system was destroyed by the 2011 disaster.
"We have not seen any damage or any change in radiation gauges after the quake," said Tepco spokesman Masahiro Asaoka.
"Our temporary breakwater that was newly built at the plant is high enough to block a one-metre tsunami," he added.
The Fukushima plant's cooling systems were swamped by the tsunami three years ago, sparking reactor meltdowns and radiation leaks in the worst atomic crisis since the 1986 Chernobyl disaster.