Data from a telemetered seismic network in Fiji provide new constraints on the crustal structure and active deformation within and around the Fiji Platform, an unusual fragmented arc complex situated within the Pacific/Indo-Australian plate boundary. Seismic refraction data indicate that crustal thickness in Fiji is typical of island arcs, averaging about 15--20 km, with relatively low midcrustal seismic velocities (Vp=6.0 km/s), characteristic of felsic arc-related rocks. The anticipated higher velocity lower crustal layer is either thin or absent. Mantle velocity beneath the platform is low (7.6 km/s), in the same range as that observed beneath the neighboring marginal basins and island arcs. The record of crustal seismicity, based on historical, teleseismic, and microearthquake catalogs, indicates significant activity both within and along the boundaries of the Fiji Platform. Earthquakes located along the edges of the platform are associated with major zones of back arc deformation of the North Fiji and Lau basins: the Fiji Fracture Zone to the north, the Hunter Fracture Zone to the south, and a deformed zone in the eastern portion of the North Fiji Basin to the west.

Earthquake within the platform (''interplate events'') occur more sporadically than those along its margins; areas of significant intraplate activity include the Taveuni/northeastern Vanau Levu area, Koro Sea, the Baravi basin, and central and south-eastern Viti Levu. Earthquake depths within the platform are concentrated in the upper 13--16 km of lithosphere, as in other areas of extensional/strike-slip tectonics. In Fiji, however, the seismicity apparently extends through the entire crustal section. The large 1953 intraplate Suva earthquake (MS=6.75) was caused by strike-slip faulting, under the same east-west extensional stress field that is deforming the North Fiji and Lau basins. Microearthquakes within and around the island of Viti Levu appear to occur as a result of the same stresses. These earthquakes occur in discrete nests that are tightly clustered in space and time; they are related to late Cenozoic faults that cut the Mio-Pliocene rocks of Viti Levu. The characteristics of its crucial structure, the anomalous mantle velocity, and the active deformation within the Fiji Platform distinguish it from microcontinental blocks that have been observed elsewhere. The seismicity and focal mechanisms within the Fiji Platform demonstrate that it is not tectonically isolated from the active deformational processes that pervade the neighboring arcs and back arc basins. ¿ American Geophysical Union 1990