Scientists in Japan warn that a dangerous seismic fault has been re-activated near the site of the Fukushima Daiichi nuclear plant.
The ABC reports more than 24,000 aftershocks have recorded in the city of Iwaki, in Fukushima prefecture, since the magnitude 9.0 earthquake on 11 March last year.
Seismologists from Japan and China say the quake jolted an old fault line back into life.
In a study published this week, they called for safety measures at the nuclear plant to be strengthened so the facility can withstand more violent aftershocks.
The biggest aftershock since last year's quake was a magnitude 7.0 tremor at a shallow depth beneath Iwaki.