This diagram from TBS illustrates the surface instability situation in Japan.
The red lines are where the plates meet. You can see Japan is bisected between east and west by a plate.
The Xes indicate the epicenters of quakes since the 9.0 one last Friday, which have continued through last night with a 6.4 one.
The quake that rocked Shizuoko and was felt in Tokyo was an entirely different system than the one that brought on the 9.0 monster quake.