Data from five new seismograph stations in the Fiji-Tonga region of the southwest Pacific and from teleseismic stations were used to study the spatial distribution of deep and shallow earthquakes in the Tonga island arc. More than about 70 per cent of the World's very deep earthquakes occur in this region. About two or three deep shocks and several shallow earthquakes were detected each day by the new network; some of the deep events were as small as magnitude 3.
Since more precise hypocentral locations are now available, the zone of activity that dips under the Fiji-Tonga region to depths of about 650 km could be examined in considerable detail. This seismic zone is extremely thin and at some depths is less than about 20 km thick. Nearly all of the small earthquakes in the Fiji-Tonga region are confined to the same narrow zones that are defined by events of larger magnitude. This distribution may be contrasted with the greater scatter of the foci of small shocks in several continental areas.
A very active zone of shallow earthquakes north of the Fijian island of Vanua Levu may mark the location of a major submarine fault zone. This zone is part of a larger system of major fault zones that extend from the Tonga arc to Fiji and thence to the New Hebrides arc. Four mechanism solutions for earthquakes located along a seismic zone between Fiji and the southern end of the New Hebrides trench indicate that this zone is typified by strike-slip faulting and thrust faulting.