Physical Sciences: Variations in Australian Upper Mantle Structure, from Observations of the Cannikin Explosion

IT has been established that P and S times to stations in shield areas are significantly earlier than times to other continental regions at similar epicentral distances 1–4 . The observed differences, called “station anomalies”, are largely produced by variations in the properties of the low velocity layer within the upper mantle 5,6 , thus providing one of the few means available for estimating such variations. Except for some studies based on the dense seismic network in California 7–9 , however, detailed investigations of upper mantle structure by this method have been precluded by the relatively sparse distribution of permanent stations.