Brian, the 7.2 foreshock only broke a small piece as you know, and it is possible or probably likely that the M8.9 or 9.1 had additional slip on that same part. But, the Great quake would have broken the entire fault area and the aftershock are mostly within that same fault zone that broke. Many could be not on the same megathrust, but likely close to it? Subduction zones have the main plate boundary megathrust, then a stack of thrust faults issuing off of it, mainly above it (I'm thinking of 3D seismic I've seen of other subduction zones, and this includes the Nankai trough which is south of Tokyo.

I don't actually know from data what I say above. It would be good to have a 3D animation or series of views of all the hypocenters of all the aftershocks. These likely exist. I could do one (I have the software) but it would take me too long because I have not done this for too long.

A seismologist here at "heartland U" said he thought that the subduction zone got steeper, towards 45 deg dip, downdip. But, I'm looking now, all the aftershocks are above about 40 km. The USGS WPhase Moment solution has the dip as 14 degrees. So, if the seismologist is correct, the steepening of the subduction zone must be farther west; I think he said west of Honshu.

Chris