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.

Global estimates of the elastic thickness (Te) of the structure of passive continental margins show wide and varying results owing to the use of different methodologies. Earlier estimates of the elastic thickness of the North Atlantic passive continental margins that used flexural modelling yielded a Te value of ∼20–100 km. Here, we compare these estimates with the Te value obtained using orthonormalized Hermite multitaper recovered isostatic coherence functions. We discuss how Te is correlated with heat flow distribution and depth of necking. The E–W segment in the southern study region comprising Nova Scotia and the Southern Grand Banks show low Te values, while the zones comprising the NE–SW zones, viz., Western Greenland, Labrador, Orphan Basin and the Northern Grand Bank show comparatively high Te values. As expected, Te broadly reflects the depth of the 200$–$400°C isotherm below the weak surface sediment layer at the time of loading, and at the margins most of the loading occurred during rifting. We infer that these low Te measurements indicate Te frozen into the lithosphere. This could be due to the passive nature of the margin when the loads were emplaced during the continental break-up process at high temperature gradients.