DEEP EARTHQUAKES BENEATH THE FIJI BASIN, SW PACIFIC - EARTH MOST INTENSE DEEP SEISMICITY IN STAGNANT SLABS

Previous work has suggested that many of the deep earthquakes beneath the Fiji Basin occur in slab material that has been detached and found ered to the bottom of the transition zone or has been laid down by tre nch migration in a similar recumbent position. Since nowhere else in t he Earth do so many earthquakes occur in slabs stagnated in the transi tion zone, these earthquakes merit closer study. Accordingly, we have assembled from historical and modern data a comprehensive catalogue of the relocated hypocenters and focal mechanisms of well-located deep e vents in the geographic area between the bottoms of the main Vanuatu a nd Tonga Wadati-Benioff zones. Two regions of deep seismogenesis are r ecognized there: (i) 163 deep shocks have occurred north of 15 degrees S in the Vityaz Group from 1949 to 1996. These seismological observat ions and the absence of other features characteristic of active subduc tion suggest that the Vityaz group represents deep Failure in a detach ed slab that has foundered to a horizontal orientation near the bottom of the transition zone. (ii) Another group of nearly 50 'outboard' de ep shocks occur between about 450 and 660 km depth, west of the comple xly buckled and offset western edge of the Tonga Wadati-Benioff zone. Their geometry is in the form of two or possibly three small-circle ar cs that roughly parallel the inferred motion of Tonga trench migration . Earthquakes in the southernmost of these arcs occur in a recumbent h igh-seismic-wavespeed slab anomaly that connects both to the main incl ined Tonga anomaly to the east and a lower mantle anomaly to the west [Van der Hilst, R., 1995. Complex morphology of subducted lithosphere in the mantle beneath the Tonga trench. Nature, Vol. 374, pp. 154-157. ]. Both groups show complexity in their focal mechanisms. The major qu estion raised by these observations is the cause of this apparent temp orary arrest in the descent of the Tonga slab into the lower mantle. W e approach these questions by considering the effects of buoyant metas table peridotite in cold slab material that was detached and rapidly f oundered, or was buckled, segmented and laid out in the transition zon e. (C) 1998 Published by Elsevier Science B.V. All rights reserved.