Array Analysis of Precursors to PKIKP in the Distance Range 128 to 142

Abstract

PKP precursor wave trains from twelve large earthquakes, in the distance range 128° to 142° from the Warramunga seismic array in northern Australia, have been processed digitally. Direct measurements of average slowness of sections of the wave trains are presented and compared with calculations based on a theoretical analysis of seismic wave scattering by Haddon. On the basis of this comparison, we interpret our results as offering strong support to the suggestion that the precursors result from the scattering of PKP waves by random irregularities near the base of the mantle. This conclusion is supported by evidence from analyses of reverse azimuth arrivals from Novaya Zemlya explosions (Δ= 106°) and amplitude characteristics along precursor wave trains. Unconventional array processing techniques were used in the analysis of amplitude variations.