Don....60 feet (let's use meters...18 m) sounds too high for an average slip...it could have been a maximum slip. This slip could not have been measured....maybe inferred by rupture size, or maybe better than that if Sieh or someone else has uplift from corals or whatever. If in 2004 it was a uniform thrust event it should have loaded both north and south. But, we don't know enough yet about what happened near the north end of rupture...the aftershocks are strike-slip and normal...so maybe loading to north was not so great. I am guessing on the following:....but along the 2004 rupture the flex in the hanging-wall above the subduction zone is relaxed...so the pressure perpendicular to a subvertical strike-slip fault in the hanging-wall like the Great Sumatra fault could be less....this is called "normal stress". You drop the normal stress and do not drop the shear stress and a fault becomes weaker.....it is not clamped as tightly together.

But, and earthquake on the Great Sumatra Fault would be right-lateral....and would be unlikely to produce a tsunami. It is the big megathrust earthquakes that might be most dangerous in this area, in this situation (unless the Great Sumatra fault runs through or near densely populated areas...especially cities with poor construction).

Chris