Japan Earthquakes, Triple junction seismic activity.

10-degree Map Centered at 40°N,140°E

This is from the U.S. geological Survey, the larger the square, the bigger the earthquake. Tokyo is located near the Boso triple junction, where are known areas of seismic activity. Triple junctions occur at tectonic plate margins. These areas are described as zones where any combination of transforms faults, subduction zones, or ocean ridges meet, sometimes with disatrous consequenses.