High levels also in adjoining Chiba Prefecture.
Levels and causes being investigated.
Some was in rainwater. The article adds a bit to the Guardian article linked by Peter Labuan.
Other articles in Mainichi Daily News, October 14, 2011-

-Radioactive strontium found in Yokohama, 250 km from Fukushima plant.
-Japan's food radiation limits set too high: Belarussian scientist.
-Editorial- Prioritization key to effective nuclear decontamination plan.--
The basic plan has been decided on. The central government will designate areas with radioactive emissions of 1 millisievert per year or more "important contamination condition survey sites," and will task local governments with cleanup operations.
.... One millisievert per year is the upper exposure limit set by the International Commission on Radiological Protection (ICRP) for regular citizens.
The editorial says decontamination of the most dangerous sites should be first. However, the effects of low doses of radiation on the human body are uncertain. Areas with lower concentrations will continue to affect the population, to an unknown extent, while high concentration areas are being worked on.
Mainichi Daily News