NEW : It happened exactly one month after the country 's devastating 9.0-magnitude quake and tsunami . Six people were killed and 20 injured in Monday 's quake . It was centered about 100 miles east - southeast of Tokyo . Monday 's 6.6-magnitude quake triggers several earthquakes in Iwaki . The earlier landslide prompted workers at the nearby nuclear plant to evacuate overnight .
NEW : Six people were killed and 20 injured in Monday 's quake . It was centered about 100 miles east - southeast of Tokyo . Monday 's 6.6-magnitude quake triggers several earthquakes in Iwaki . The earlier landslide prompted workers at the nearby nuclear plant to evacuate overnight .