Seismicity at intersections of spreading centers and transform faults

Ocean‐bottom seismographs were used in a microearthquake monitoring experiment at the eastern junction of the Oceanographer transform with the mid‐Atlantic ridge at 35°N. Microearthquake activity at the junction occurred over a broad area (>7 km). These microearthquakes ‘cut across’ the corner between the transform and median valley and are associated with fault scarps that form the inner walls on the west and north sides of the median and transform valleys. At intersections of other major fracture zones (>100‐km offset) and slow to moderate spreading centers microearthquake activity is also diffuse and cuts across the corner between the spreading center and transform fault. The narrow zone of decoupling (∼1 km) observed between spreading center and transform boundaries by detailed geological studies at the Tamayo/East Pacific Rise and Vema/mid‐Atlantic Ridge intersections suggest that the diffuse seismicity (20 to 30 km in width) does not reflect a diffuse plate boundary at the transition from rift to transform valley. Instead, the faulting probably reflects internal deformation of the corner by secondary faults off of the plate boundary.