There have been a M6.6, 6.4, and 6.6 within 3 1/2 hours of each other. The first 2 are over 2 deg of longitude apart, and the 3rd if over 2 deg of latitude north of the other. Unless a coincidence, which I doubt, they are triggering each other at distance. The first 6.6 is east of the trench and its focal mechanism is mostly normal. This is consistent with stretching, and the unlocking of the subduction zone during the M9 might allow stretching of the ocean lithosphere. The 2nd and 3rd quake don't have focal mechanisms yet, but they are located west of the trench.

The link is to a quake animation, which I already posted on a thread below. I posted below that the M4.5+ aftershocks seem to occur in bursts, usually close to each other, which is non-surprising.
Chris

• Japan quake animation