P-wave residuals at Fiji from deep earthquakes in the Tonga subduction zone

Abstract

A total of 47 deep earthquakes occurring between 1982 and 1985 in the Tonga subduction zone were used to derive station corrections for the Fiji Seismograph Network (FSN ). The Joint Hypocenter Determination (JHD) method was employed to relocate hypocenters using P -wave arrival times at selected teleseismic and FSN stations. P -wave residuals for Fiji show variations of more than 1 sec between southern Viti Levu on the one hand and northern Viti Levu and Vanua Levu on the other hand. The smallest residuals are observed in southern Viti Levu where the oldest volcanic rocks in Fiji are mapped and on the island of Koro. Station residuals in northern Viti Levu, Vanua Levu, and the smaller Fijian islands except for Koro are on average 0.4 to 0.7 sec higher than in southern Viti Levu. The effect of hypocentral mislocation on the observed residual variations is negligible at stations in Viti Levu, Ovalau, and Beqa, but it may be important for stations in Vanua Levu, Koro, Taveuni, and Yasawa. Large P -wave delays observed at some stations are probably caused by low velocity sedimentary surface layers. The variations in crustal thickness that are probably not larger than 5 km would explain only a minor part of the residual differences. Without a better knowledge of crustal structure the contribution of lateral variations in upper mantle P velocities is difficult to assess.