The 2004 Mw 9.0 Sumatra-Andaman earthquake initiated along the Andaman subduction zone, north of the last great Sumatra earthquake along the Sunda Trench in 1861. During the 2005 Mw 8.7 Banyak Islands earthquake, a portion of the 1861 rupture subsequently failed. The boundary between the 2004 and 2005 ruptures broadly coincides with local trench rotation and the southern edge of the Andaman microplate, which suggests structural control on fault segmentation. Aftershock relocations of the 2004 and 2005 earthquakes show little overlap, and the sharp boundary between the series locates near the 2002 Mw 7.3 Northern Sumatra earthquake. We posit that these features represent the southern extent of the stable Andaman microplate, ~50--100 km northwest of what was previously reported. Broadband analyses of the 2002 earthquake yield a bilateral rupture pattern that is used to model Coulomb stress changes near the 2004 hypocenter to assess stress interactions along adjacent fault segments.