TOKYO—Japanese researchers discovered high levels of radioactive material in concentrated areas in Tokyo and Yokohama, more than 150 miles away from the Fukushima Daiichi nuclear plant, as increasingly thorough tests provide a clearer picture of how far contamination has spread and accumulated after the March disaster.

In Tokyo, a sidewalk in Setagaya ward, in the western part of the city, recorded airborne radiation levels, about 50 times higher than another location in Setagaya where the ward regularly monitors radiation...