A study of P travel-times from some Australian earthquakes

Abstract

The epicentres of 11 Australian earthquakes have been determined using two different sets of travel-times, namely, the Jeffreys-Bullen times and the linear travel-time relation obtained by Bolt, Doyle, and Sutton from the 1956 nuclear explosions at Maralinga. The latter gave the better fit for earthquakes on the mainland. The residuals from the revised epicentres have been used to draw up a tentative average P travel-time curve. This is linear to 18½0 , beyond which the slope decreases sharply from 13 • 54 sec/deg to 10 • 5 sec/deg. The intersection and the change in slope probably vary from the north-west to the south-east of Australia. Beyond 25° the Jeffreys-Bullen times apply. At distances less than 5°, slopes between 13 • 7 and 13 • 8 sec/deg have been found for P n• There are indications that the P n velocity decreases considerably towards the edge of the continental shelf. A model for the upper mantle is suggested.