The distribution of the non-double couple parameter of shallow earthquakes reported in the Harvard CMT catalogue shows systematic characteristics depending on the epicentral locations and types of fault mechanisms. We suggest that they can be explained by the presence of subevents with different double couple mechanism in a single rupture sequence. The earthquakes at the ridge-transform-fault plate boundaries show a particularly interesting pattern. It is suggested that two types of faulting expected in the area (i.e., normal faults at ridges and strike slip faults at transform-faults) tend to occur almost simultaneously, although this hypothesis needs to be delineated by careful analyses using bodywave waveforms. ¿American Geophysical Union 1991