Seismic velocities and crustal structure in southern Australia

Abstract

Nuclear explosions in South Australia at Maralinga in 1956 and 1957, and at Emu in 1953, provided unique opportunities for determining accurately seismic velocities in the crust and upper mantle. Stations south and south‐east of Maralinga towards Adelaide gave P velocities of 3.4 km/sec in thin sediments, 5.8 km/sec in the upper 1–2 km of “granitic” material just below the sediments, and 6.3 km/sec (P1) for deeper granitic material which probably makes up much of the crust. S1 (Lg) velocity was 3.59 (±0–01) km/sec, Pn in the upper mantle was 8.05 (±0.04) km/sec, appreciably lower than previously found west of Maralinga, and Sn 4.61 (±0.04) km/sec. Later phases at some stations could be interpreted as produced by local velocity increases in the crust but no good evidence has been found for intermediate material except for some recent data near the coast of S.W. Australia. A simple crust of P velocity 6.3 km/sec would have a depth of 39 km. Possible reflections from the top of the mantle were recorded at three stations giving a mean depth of 37 km for a single layered crust. The short wave‐length of these possible reflections suggest a change in velocity over a depth range of half a kilometre or less.