Earthquakes that occur within the Nazca plate subduction zone tend to fall into two populations: the generally smaller, nearly pure double-couple events, and the usually larger, non-double-couple earthquakes. In fact, there appears to exist a direct relationship between the proportion of the non-double-couple component in the earthquake solution and the scalar moment. A similar trend is not observed on the global scale. Furthermore, most of the earthquakes with large non-doublecouple components occur in the locations where the greatest amount of structural complexity is expected. We therefore hypothesize that the non-double-couple mechanisms in this region are,in general, related to the breakdown of the assumption of slip on a planar fault-surface, one of the fundamental assumptions in obtaining moment tensor source mechanisms in accord with the doublecouple model. This failure is, in turn, related to the extreme structural complexity in this region. ¿American Geophysical Union 1987