OCEANIC TRANSFORM EARTHQUAKES WITH UNUSUAL MECHANISMS OR LOCATIONS - RELATION TO FAULT GEOMETRY AND STATE OF STRESS IN THE ADJACENT LITHOSPHERE

On oceanic transforms, most earthquakes are expected to occur on the p rincipal transform displacement zone (PTDZ) and to have strike-slip me chanisms consistent with transform-parallel motion. We conducted a sea rch for transform earthquakes departing from this pattern on the basis of source mechanisms and locations taken from the Harvard centroid mo ment tensor catalogue and the bulletin of the International Seismologi cal Centre, respectively. Events with unusual mechanisms occur on seve ral transforms. We have determined the source mechanisms and centroid depths of 10 such earthquakes on the St. Paul's, Marathon, Owen, Heeze n, Tharp, Menard, and Rivera transforms from inversions of long-period body waveforms. Relative locations of earthquakes along these transfo rms have been determined with a multiple-event relocation technique. M uch of the anomalous earthquake activity on oceanic transforms is asso ciated with complexities in the geometry of the PTDZ or the presence o f large structural features that may influence slip on the fault. Reve rse-faulting earthquakes occur at a compressional bend in the Owen tra nsform in the area of Mount Error and at the St. Paul's transform near St. Peter's and St. Paul's Rocks. A normal-faulting earthquake on the Heezen transform is located at the edge of a pull-apart basin marking an extensional offset of the fault. Normal-faulting earthquakes along the Tharp, Menard, and Rivera transforms may also be related to exten sional offsets. Some events with unusual mechanisms occur outside of t he transform fault zone, however, and do not appear to be related to f ault zone geometry. For instance, earthquakes with mechanisms indicati ng reverse-faulting on ridge-parallel fault planes are located near th e ridge-transform intersections of the St. Paul's and the Marathon tra nsforms. Possible additional contributors to the occurrence of anomalo us earthquakes include recent changes in plate motion, differential li thospheric cooling, and the development of a zone of weakness along th e fault zone, but we do not find strong evidence to confirm the influe nce of these processes.