Oceanic transform earthquakes with unusual mechanisms or locations: Relation to fault geometry and state of stress in the adjacent lithosphere

Abstract

On oceanic transforms, most earthquakes are expected to occur on the principal transform displacement zone (PTDZ) and to have strike‐slip mechanisms consistent with transform‐parallel motion. We conducted a search for transform earthquakes departing from this pattern on the basis of source mechanisms and locations taken from the Harvard centroid moment tensor catalogue and the bulletin of the International Seismological Centre, respectively. Events with unusual mechanisms occur on several transforms. We have determined the source mechanisms and centroid depths of 10 such earthquakes on the St. Paul's, Marathon, Owen, Heezen, Tharp, Menard, and Rivera transforms from inversions of long‐period body waveforms. Relative locations of earthquakes along these transforms have been determined with a multiple‐event relocation technique. Much of the anomalous earthquake activity on oceanic transforms is associated with complexities in the geometry of the PTDZ or the presence of large structural features that may influence slip on the fault. Reverse‐faulting earthquakes occur at a compressional bend in the Owen transform 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 extensional offsets. Some events with unusual mechanisms occur outside of the transform fault zone, however, and do not appear to be related to fault zone geometry. For instance, earthquakes with mechanisms indicating reverse‐faulting on ridge‐parallel fault planes are located near the ridge‐transform intersections of the St. Paul's and the Marathon transforms. Possible additional contributors to the occurrence of anomalous earthquakes include recent changes in plate motion, differential lithospheric cooling, and the development of a zone of weakness along the fault zone, but we do not find strong evidence to confirm the influence of these processes.