A SHALLOW DOUBLE SEISMIC ZONE BENEATH THE CENTRAL NEW HEBRIDES (VANUATU) - EVIDENCE FOR FRAGMENTATION AND ACCRETION OF THE DESCENDING PLATE

A shallow double seismic zone (SDSZ) has been found in the descending Australian plate beneath the central part of the New Hebrides island a rc, directly above a large gap in intermediate depth seismicity and be tween two seismic boundaries. Ambient seismicity occurs mostly in the upper part of the SDSZ, while earthquakes in the lower part occur in c lusters (swarms or aftershocks of large earthquakes). The distance bet ween the upper and lower levels of the SDSZ is 50-70 km, and they are joined at 80 km depth by a near-horizontal band of seismicity. Thrust- faulting mechanisms predominate for earthquakes in the upper level of the SDSZ. Those in the lower level, however, appear to be normal fault ing, despite their being aftershocks of large thrust events. We sugges t that with the absence of a pull from the detached lithosphere the up per part of the Australian plate in the region of the SDSZ is resistan t to subduction, and thus the downward displacements caused by large e arthquakes in the adjoining regions result in a localized rebound. The location of the aftershocks within the plate suggests that a new plat e boundary is forming, which will eventually replace that outlined by the residual seismicity in the upper level. Thus the leading edge is d ecoupling, and the boundary will eventually shift back to the lower le vel of the SDSZ.