Abstract

The central New Hebrides island arc is a morphologically complex area where the island blocks of Santo and Malekula replace the trench and override the D'Entrecasteaux Fracture Zone (DFZ), a major aseismic ridge, on the subducting plate. Earthquakes recorded by a local network have been located beneath the trench southwest of Malekula at depths of 60 to 90 km. Because these locations are unusual, and because tests with hypothetical data showed that two minima, one for shallow depths and one for deep depths, exist on the travel-time surface for events in this area, we investigated these hypocenters in more detail. Location quality was judged primarily by the fit to P and S wave arrival times calculated in a variety of velocity models. We found that while the standard deviations of the travel time residuals from some of the events were nearly the same for shallow and deep locations, they were significantly lower for several events at the deeper minimum. Moreover, the marginal a posteriori density functions for depth indicate that the deeper depths are significantly more probable. Activity in this area appears to be sporadic. Most of the events were aftershocks of a large earthquake (Mw = 7.1) in 1981, and a cluster of events was reported to have occurred in the same area over a period of one month in 1986. The existence of deep earthquakes in this area may be due to interactions of the DFZ with Santo and Malekula, because their locations follow a trend in the seismicity that parallels the western coast of the island blocks. They also occur directly north of a sharp bend in the trench and near a major seismic discontinuity, and may be a manifestation of recent, en-echelon subduction. If these earthquakes are not occurring along the boundary of a new subduction zone, the inferred existence of brittle failure at depths usually associated with a ductile rheology brings into question our understanding of the mechanics of the oceanic lithosphere.

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