Shan, I'm not checking this, but recall that the Kuril quakes broke adjoining parts of the subduction zone (or maybe one of them was not on subduction zone). They did not break the same part of the same fault. In the discussion that John V and I got into here a couple weeks ago, John V and I were not communicating well: I have been specifically talking about major quakes on the same part of the same fault, while he would chime in with stuff about adjoining faults. Concerning what you have to say: I have no problem with M 6s or even low M7s on the same part of the same fault zone that failed in the M9.3, 8.7, 8.4, and 7.9 quakes the last couple of years. And, the occurence of these subduction quakes does not make it any less likely that there could be a major quake on nearby but different fault zones...like the right-lateral Great Sumatra fault. What I don't believe is that the southern part of the subduction quake of Dec 2004, that slipped 10 to 20 m, could repeat that without the passage of decades. There are seismologists and earthquake geologists who have a different opinion. The question that interests me is whether the northern part of that M9.3, where the slip was low (a couple of meters), could break again in a M8-8.5 (for example) within the next few years. I doubt it. I think it would take decades to build up the stress. Yes, I understand the concept that only part of the differential stress was released, but the remaining differential stress (or shear stress on the fault) is unlikely in my opinion to be enough to allow that part of the fault to slip. Sure, it might be able to have a M7 or 7.5 (magnitudes given as examples of what I am trying to say only). One other question would be whether an earthquake that fired off, say, north of the north end of the M9.3 could rupture south into the previously broken fault. Hmmm...maybe...but I still doubt it would rupture very far into the previously broken part.

As for whether the recent M8.4 and 7.9 were aftershocks of the M9.3 and 8.7: I don't really care what they are called and there was a discussion on this page about the definition of aftershock a couple of months ago.Sure, it might be able to have a M7 or 7.5 (magnitudes given as examples of what I am trying to say only).

As for whether the recent M8.4 and 7.9 were aftershocks of the M9.3 and 8.7: I don't really care what they are called and there was a discussion on this page about the definition of aftershock a couple of months ago. I think the classic definition would say these are too far (the 8.5 started 500 km or more away from the southern end of the M8.7), but I do agree that the 2004-2005 quakes contributed to the timing of the 2007 M8.4, so there is some relation.

I think no one who works with faults would be surprised if the part of the subduction zone between the equator and 1.6 or so south of the equator could break in a M8 or so, any time (days to decades). This part did not rupture in the other Great quakes.

These are my opinions as a structural geologist of sorts: I'm not a seismologist. And, as is discussed in this post, there does not seem to be agreement among the professionals on these topics.
Chris