S to P scattering at the 650 km discontinuity

Abstract

Summary. A search of seismograms recorded at the Warramunga seismic array (WRA) from events occurring below the Izu‐Bonin Islands shows an arrival on some, but not all, of the records, with an onset at 25–30 s after P, which is not predicted by the standard travel‐time tables. The slowness and azimuth of the phase show that it is generated almost in line with P, and the variation of arrival time with the hypocentral depth of the earthquake indicates that its origin lies on the receiver side of the source. It appears, in fact, to be an S to P conversion at a depth of 650–700 km, which is seen only when the receiver is close to a node of the P radiation pattern and an antinode for S so that its amplitude compared with that of P is at a maximum. Finally, the duration of the phase indicates that it is not simply a refracted wave, but that it has a coda of scattered arrivals from lateral heterogeneity in the neighbourhood of 650 km below the Izu‐Bonin Islands. Copyright © 1982, Wiley Blackwell. All rights reserved