The darker the red hue, the worse the earthquake risk. It's pretty easy to see where the Philippine tectonic plate, the Asian plate, the Okhotsk plate, and the Pacific plates all come together. Just east of Tokyo! If you'd like to see a geologic map, check out the image here. The dark red spur that cuts through the main island of Honshu at its widest is a really interesting bit of geology. For some time it was below sea level, meaning that Honshu was cut in half and that it was deposited upon with sedements. It was then thrust upward again, presumably as the Okhotsk plate was pushed up between the Asian and Pacific plates. So it's a strip of sedementary rock cutting through Japan's otherwise tortured volcanic rocks.