Indian Ocean subduction zone is one of the most active plate margins of the globe as evident from its vast record of great magnitude earthquake and tsunami events.We use Bouguer admittance (Morlet isostatic response function)in Sumatra –Java subduction zones comprising both the subduction and over-riding plates to determine the lithospheric mechanical strength variations. We determine effective elastic thickness $(T_e)$ for five oceanic windows (size 990 × 990 km²) by analyzing the admittance using Bouguer gravity and bathymetry data. The results show bimodal $T_e$ values > 20 km for Sumatra and 20 –40 km for Java.The lower bimodal values obtained for Sumatra appears to correlate well with the zones of historical seismicity.This is in sharp contrast with Java subduction zone,which shows higher $T_e$ values (20 –40 km)and apparently associated with low magnitude earthquakes.We suggest a strong and wide interseismic coupling for Sumatra between the subducting and over-riding plates,and deeper mantle contributing to low strength,shallow focus –high magnitude seismicity and vice versa for Java,leading to their seismogenic zonation.