Barry....like Don, I think your question is too vague to answer. Try again. It is pretty well understood why different earthquakes are at different depths. Are you talking about the difference in one area like where the 9 occurred between 10 km and 30 km, or between this range and quakes at 500 or 600 km depth? (the latter are considered to be within the subducting slab, while the former are at the top of the subducting slab.....where it is in contact with the over-riding crust.
Chris