Distinctly different rupture patterns of December 2004 and March 2005 megathrust earthquakes occurred in the Andaman-Sumatra region suggest strong influence of tectonic and structural elements. We have analysed the shipboard gravity, bathymetry and seismic data across the West Andaman Fault (WAF), a major tectonic feature in the Andaman Sea, to infer the crustal structure and to examine its influence in controlling the aftershock pattern. Our results support the idea of Singh et al. (2005) that WAF forms a lithospheric scale boundary and together with other tectonic elements modulate the occurrence of large earthquakes and their rupture pattern. The active strike-slip motion along the WAF, presence of backarc spreading coupled with increased obliquity of subduction in the Andaman Sector reduce the probability of occurrence of major or great earthquakes north of 100N.