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 arc, directly above a large gap in intermediate depth seismicity and between two seismic boundaries. Ambient seismicity occurs mostly in the upper part of the SDSZ, while earthquakes in the lower part occur in clusters (swarms or aftershocks of large earthquakes). The distance between 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 faulting, despite their being aftershocks of large thrust events. We suggest that with the absence of a pull from the detached lithosphere the upper part of the Australian plate in the region of the SDSZ is resistant to subduction, and thus the downward displacements caused by large earthquakes in the adjoining regions result in a localized rebound. The location of the aftershocks within the plate suggests that a new plate boundary is forming, which will eventually replace that outlined by the residual seismicity in the upper level. Thus the leading edge is decoupling, and the boundary will eventually shift back to the lower level of the SDSZ.