Small tsunami hits Japan after 7.3 magnitude quake

A small tsunami hit Japan on Saturday after a powerful and shallow undersea quake, although there were no immediate signs of serious damage and no reports of injuries or deaths. -- GRAPHIC: USGS/AFP
A small tsunami hit Japan on Saturday after a powerful and shallow undersea quake, although there were no immediate signs of serious damage and no reports of injuries or deaths. -- GRAPHIC: USGS/AFP

TOKYO (AFP) - A small tsunami hit Japan on Saturday after a powerful and shallow undersea quake, although there were no immediate signs of serious damage and no reports of injuries or deaths.

People were warned to stay away from the coast as the tsunami, which was recorded as being as high as 55cm in one place, rolled ashore.

There were no new problems at the crippled Fukushima nuclear plant, NHK said, citing operator Tokyo Electric Power.

"There have been no rises at (radiation) monitoring posts at Fukushima Daiichi," the broadcaster reported.

Workers who had been patrolling wells used to measure underground water evacuated to higher ground after the tremors struck.

There were no reported problems at any other nuclear plant, including at Onagawa, the site of the largest wave - 55cm - recorded on Saturday.

All of Japan's 50 viable reactors are shut down.

The quake struck at a depth of 10km at 2.10am local time (1.10am Singapore time), 327km south-east of Ishinomaki in Miyagi Prefecture, according to the US Geological Survey.

The USGS intially said its magnitude was 7.3, and issued a green alert on its website, signalling a low probability of deaths or economic losses.

As it stood down its warnings, Japan's meteorological agency said the quake was an aftershock of the March 2011 tremor.

"We have lifted all tsunami alert but the sea level may continue to show small changes for half a day or so please be very careful when working by the sea," an official told a press conference.

The area affected largely overlapped with that hit by the March 2011 disaster when more than 18,000 people died after a towering tsunami crashed ashore following a 9.0 magnitude undersea quake.

In the town of Ofunato, a 20cm tsunami was logged just after 3am, while Ishinomaki, which was devastated in 2011, recorded a 30-centimetre wave.

Eastern Japan, a seismically active region, was struck by a 6.5 magnitude earthquake last month causing tremors that were felt 600km away in Tokyo.

The 2011 quake-tsunami knocked out cooling systems at the Fukushima nuclear plant, sending reactors into meltdown and forcing mass evacuations.

The effects of that disaster - the world's worst nuclear accident since Chernobyl 25 years earlier - are still being felt.

Tepco is battling to clean up the mess at the plant where thousands of tonnes of radiation-contaminated water are being stored in tanks after being used to cool the reactors.

Frequent mishaps, including leaks of radiation-contaminated water and a power outage caused by a rat have shaken public confidence in the huge utility.

Tepco's own estimates suggest the full decommissioning of the site could take up to four decades and that much of the trickier work is yet to be done - notably the removal of reactor cores that have probably melted beyond recognition.

According to the utility's own plan, these reactor cores - which are feared to have seeped into the containment vessels and possibly even eaten through thick concrete - will be removed around summer 2020.

Although Tepco says the reactors are now under control, critics say the plant remains in a precarious state and at the mercy of extreme weather or further earthquakes. They point out that there is still no plan for the thousands of tonnes of water being stored on site.

Tens of thousands of people remain in temporary accommodation, with some scientists warning that it could be decades before they are able to return home - if at all.