Abstract
Upper mantle velocity structure in the New Guinea-New Britain-Solomon Islands regions of the Southwest Pacific has been studied to a depth of about 550 km from the analysis of P and S wave travel times data of 128 mantle earthquakes. Wave velocities were obtained at the depths of foci of earthquakes in the inclined seismic zones in these regions, by using KAILA'S (1969) analytical method. By a linear fitting of velocity variation with depth, it is found that the P velocity in the New Guinea region increases from 7.92 km/s at 40 km depth to 8.26 km/s at 230 km depth. On the other hand, the P velocity function for the Solomon Islands region reveals a velocity of 7.80 km/s at 40 km depth which increases, with a velocity gradient of 0.280±0.003 km/s per 100 km, to 8.81 km/s at a depth of 400 km. At this transition depth of 400 km, there is a sharp first-order velocity discontinuity-the velocity increasing from 8.81 to 9.44 km/s which is about a 7.2 % velocity increase across this discontinuity. Below this disco tinuity at 400 km, P velocity increases again from 9.44 km/s, with a gradient of 0.230±0.035 km/s per 100 km, to 9.77 km/s at a depth of 545 km. There is no evidence, from the present study, for the presence of a significant low-velocity layer in the inclined seismic zones beneath the New Guinea-New Britain-Solomon Islands regions. The velocities obtained in the Solomon Islands region are 3 to 6 % (on an average) higher and lower, respectively, than those in the Japan and the Tonga-Kermadec regions.
