The 2004 Sumatra-Andaman and 2005 Nias events provide unique geodetic and seismic data constraining the updip and downdip rupture extents of great thrust earthquakes. These limits are important for tsunami generation and earthquake shaking, respectively. There is a consistent downdip limit of rupture along strike in these great earthquakes at a depth of ∼30 km, and there is an updip limit near the trench that are interpreted to define the seismogenic limits. Temperature and downdip changes in formation compositions are controls proposed for these limits. To examine the thermal control, we developed 2-D finite element models of the Sumatra subduction zone with smoothly varying subduction dip, variable thermal properties of the rock units, frictional heating along the rupture planes, and appropriate thermal state of the incoming plate. The common updip thermal limit for seismic behavior of 100–150°C occurs close to or at the trench in agreement with the great earthquake rupture limit. Off central Sumatra the common downdip thermal limit range of 350–450°C occurs at 30–60 km depth. The 350°C location is in agreement with the earthquake limits, but 450°C is deeper. North of Sumatra, 350°C occurs ∼14 km deeper than the earthquake rupture limit. The proposed composition control for the downdip limit, the intersection of the subduction thrust with the fore-arc mantle, is at a depth of ∼30 km, 140–200 km from the trench, in good agreement with the earthquake limits. These results support the conclusion that the Sumatra updip seismogenic limit is thermally controlled but the downdip limit is governed by the intersection of the downgoing plate with the fore-arc Moho.