Diffracted ScS and the Shear Velocity at the Core Boundary

SH waves from the surface-focus Iranian earthquake of 1968 August 31, have been analysed to study the structure at the base of the mantle. The long-period E-W component records from western North American stations (91° < Δ < 114°) recorded essentially only the SH polarized wave as a pulse; high frequency but less energetic precursors (perhaps SKS) were also present. The gradient of the arrival-times was estimated to be 8.68±0⋅13 s deg−1. The result is close to a similar estimate of J. Cleary. For a radius of 3479 km, assuming the waves are S and diffracted ScS, the S velocity at the boundary is 6⋅99±0⋅10 km s−1 which suggests a slight velocity decrease downwards at the mantle bottom. Values of log A/T, used in magnitude determination from S waves, show a monotonic decrease with distance for 100° < Δ < 114°. The unadjusted SH amplitude attenuation (Δ + 105°) for period 35 s is −0⋅065 (In A versus Δ°). The frequency spectra of the waves do not seem to vary with distance at the low frequencies in the manner predicted by simple diffraction theory for a homogeneous mantle.