An array study of P-wave velocities in the upper mantle transition zone beneath northeastern Australia

Abstract

The use of a seismometer array in determining slowness and travel time greatly enhances the identification of later-arrival branches of the P-wave travel-time curve between 12degrees and 30degrees. An adaptive processing method has been used to determine travel times and slownesses for 494 P-wave arrivals, at the WRA medium aperture array in northern Australia, from earthquakes in the New Guinea-Solomon Islands region. Pronounced later arrivals are observed from 12degrees to 18degrees and from 21degrees to 24degrees. Abrupt changes in slowness are found near crossover distances of 19degrees and 24.5degrees. A velocity model, SMAK I, based on a combined interpretation of the travel-time and slowness data has a low gradient in velocity above 300 km, a gradual increase in velocity between 300 and 400 km, a minor zone of high-velocity gradient near 520 km, a low gradient in velocity from 550 to 680 km and an abrupt increase in velocity near 680 km. The main features in this model are in substantial agreement with the changes in physical properties expected from mineralogical variations in a pyrolite upper mantle. A comparison of the WRA data with the results of similar studies in North America suggests that significant differences in velocity between the western United States and northeastern Australia may exist to depths of 400 km, whereas below that depth the structure beneath the two regions may be similar.