Double seismic zone beneath the Mariana Island Arc

A double zone of seismicity has been found beneath the northern Marianas in the depth range 80-120 km. The Benioff zone undergoes a pronounced bend, with radius of curvature about 200 km, before straightening and descending nearly vertically to a depth of more than 600 km. Thus this is an ideal area in which to search for evidence of stresses associated with unbending of the subducted oceanic plate. We relocated all teleseismic events from 1963 through 1975 by using a regional, composite master event technique, which significantly decreased the scatter of the hypocenters. The three events of the lower seismic zone are separated from the upper zone by about 30-35 km. The true thickness of the upper seismic zone is probably 20 km or less. The limited focal mechanism data indicate that there is downdip tension in the lower zone and downdip compression in the upper zone. The seismicity patterns could be generated either by thermal stresses or by unbending a plate which was originally deformed anelastically. We show that an elastic-perfectly plastic model of the rheology of the plate can account for the separation of the two seismic zones, the focal mechanisms, and the greater activity of the upper seismic zone.