Spatial variations in mechanical properties of the interplate thrust faults along the Japan and Middle America subduction zones are examined using teleseismic broadband earthquake recordings. Moment-normalized source duration is used to probe rigidity variations along the interface. We invert body waves to estimate source depth and source duration for 40 events in the Japan subduction zone and 38 events in the Middle America subduction zone. For both areas, there is a systematic decrease in source duration with increasing depth along the subduction zone interface. This is most likely a result of variation in properties of sediments on the plate contact. Variations in source duration are greatly reduced at depths greater than 18 km in both regions. Enhanced spatial heterogeneity at shallow depth may reflect variations in plate roughness, sediment distribution, permeability of the fault zone, and stress.