The catastrophic 2004 Sumatra-Andaman Mw9.1 earthquake associated with destructive tsunamis has characterized Sumatra as one of the most dangerous convergence zones. Nevertheless, the effects of the thermohydrous state on the strongly coupled megathrust of the incoming plate remain enigmatic. By using a 3-D thermomechanical model to compute the temperature variation and the complicated phase transition process of the water-bearing descending plate which generates unstable thrust slips, we find that Sumatran earthquakes at varying depths are likely under the control of the inter- or intraplate hydrothermal regime, which occurs in or close to the petrological metamorphism transition area. The slab dehydration of the water-rich mid-ocean-ridge basalts (MORB) and the ultramafic rocks in the oceanic lithosphere releases a large amount of fluid to the continental wedge and further facilitates arc magmatism. The fluids are prone to upwelling following the subduction channel along the plate interface and thus contribute to the clustering of earthquakes updip of the dehydration front beneath offshore Sumatra. Brittle failure and dehydration embrittlement at depth, along with the temperature differences caused by variant slab geometry, are conjectured to greatly influence the occurrence of fast and slow earthquakes in Sumatra.