Lynn Sykes gave a presentation at the weekly seismic record reading at Lamont yesterday on the M9.5 1960 Chile earthquake: the largest know quake (the largest in the last 100 years). It was preceded by 33 hours by a M8 foreshock in the next patch to the north...after which quakes migrated towards/or appeared within (?) the area where the M9.5 would occur. I think Lynn said that there was a M7.5 foreshock just before the mainshock....it is all a little more complicated than that, but I did not take notes. So, I suppose there is historical reason the be concerned if, for example, only a little piece of the 1833 Sumatra EQ site fails in a M7 or larger....this has already been happening during April May...there were some...what were they, 6.8. 6.7 s....

On the other hand, I have not looked into what the Kerry Sieh group has done with pre-1833 quakes...it is entirely possible that the 1833 site is not going to fail until 20 m of strain have accumulated...which would take another 200 years....although I doubt it would wait that long. While there is some record of great subduction quakes occuring within a couple years of each other on adjacent parts of the subduction zone (like the last 2 Sumatra), I am not aware of there being one after another along a subduction zone...maybe over a couple decades, but not a few years...someone could look into this is interested because I might be wrong...
Chris