Travel times of seismic waves between the New Hebrides and Fiji Islands: A zone of low velocity beneath the Fiji Plateau

Abstract

Shallow earthquakes that occurred during 10 years between the New Hebrides and Fiji islands are relocated by using a digital computer. The spatial distribution of the earthquakes may outline plate boundaries in the Fiji plateau; these boundaries, however, are diffuse and could be broad zones of deformation. In the center of the plateau west of Fiji times of P and S waves traveling in the uppermost mantle indicate velocities of 7.70 and 4.30 km/sec, respectively. Along the seismically active margins of the plateau P velocities are 7.30–7.40 km/sec. These velocities are considerably lower than P and S velocities of about 8.45 and 4.75 km/sec, respectively, of the normal oceanic basins of the Pacific plate to the north and east of the plateau. The zone of low velocity beneath the Fiji plateau and its boundaries seems to coincide with a high seismic wave attenuation zone that exists in the uppermost mantle between the Fiji and New Hebrides islands. These observations and other geophysical and geological aspects of the Fiji plateau clearly imply that the different lithospheric plates between the two opposite-facing lithospheric consumption zones of Tonga and New Hebrides arcs were recently generated and are not part of the oceanic Pacific plate.