I've heard in past that seamounts clog the subduction zone and are asperities, so the opposite of what you said.

What about it being an aftershock that relieves the differential stress enough that the aftershocks are smaller than 4.5? Meaning, after 2 years there are patches that can slip but that stresses are not enough around the area after 2 years of aftershocks to trigger large ones?

I made that up (speculative hypothesis). Anyone have better explanation?

Chris