Decoupled faulting

As described in a classic paper by Fitch (1972), the Sumatra subduction zone is characterized by decoupled faulting, as in (b) above. In this case, nearly pure thrust faulting occurs along the interplate thrust and strike-slip faulting occurs in the overriding plate, most notably along the Great Sumatran fault. 

An example of oblique faulting, as in (a) above, occurs in the northern Puerto Rico subduction zone.