Upper-mantle travel times in Australia — A preliminary report

This paper gives a preliminary account of an experiment to determine upper-mantle travel times in Australia. A set of fifteen 14-track tape-recording units are being used along a profile from Darwin to Alice Springs in central Australia. In addition to the 14-track recorders, eleven 6-track units are being used both on-line and off-line as monitors. The sources used are natural earthquakes occurring to the north of Australia, especially Banda Sea events.Preliminary analysis of the records from the first sub-array has confirmed the high apparent velocities for P and S previously found for the distance range 800–1,800 km in central Australia. It is inferred that the high apparent velocities are due to a sharp increase in the proportion of garnet at a depth of about 90 km.Neither the P nor S arrivals from 800 to 1,800 km show any evidence of the shadow zones or offsets in the travel-time curve which would be associated with an upper-mantle low-velocity zone. It is suggested, however, that a low-velocity zone is not precluded by the observations, but that the associated offset may not occur in the first-arrival part of the travel-time curve.