SPACE-TIME DISTRIBUTION OF DEEP SEISMIC DEFORMATION IN TONGA

At a time when physical models for the occurrence of deep earthquakes are being proposed, capable of explaining their fault-like source process, it is important that seismology characterizes all elements relevant to the physical occurrence of shear at depth. To identify the characteristic modes of deformation of the deep subducted lithosphere in the Tonga region, families of events with similar faulting orientation were classified and mapped on the Benioff zone using 133 centroid-moment tensor solutions for all events with M0 greater-than-or-equal-to 3 X 10(24) dyn cm since 1977. The existence of two principal modes of deformation acting on the subducted lithosphere is confirmed and detailed in this present study: (1) The down-dip compression associated with the subducting flow is a ubiquitous factor in the deformation of the whole lithosphere, with families of focal mechanisms whose orientation follows strictly the local geometry of the Benioff zone. (2) A lateral shear involves the whole deep lithosphere, associated with the relative motion between surface plates and deep mantle. In addition, observations are made of the presence of families of events with down-dip extensional or strike-slip mechanisms, mostly located along the western margin of the seismicity, where the lithosphere has already undergone the major compressional deformation and has rotated to a more horizontal posture. The transition from the down-dip compression to the new stress state takes place over a length of no more than 100 km. which corresponds to the local geometry of the Benioff zone. The overlapping occurrence of different modes of deformation, in space and time, indicates that the physical mechanisms proposed for the generation of deep earthquakes should incorporate the concept that deep faults or their physical equivalent' at the 600-km depth conditions are stable features of a deep subducted lithosphere, whose failure behaviour sometimes obeys to a highly heterogeneous local state of stress.