Attenuation and dispersion ofGwaves

Unusually well-developed G waves up to the eighth order (G8) from the Peru earthquake of January 13, 1960, were recorded by the long-period Press–Ewing instruments at Uppsala. A Fourier analysis of amplitude readings enables us to construct amplitude spectra, which display a clear effect of selective attenuation in the shift of their maxima toward lower frequencies with increasing order of G. Absorption coefficients are derived and internal friction is computed for the period range 75–300 sec and compared with earlier results. Phase-velocity dispersion curves are computed and compared with results of other authors. Group-velocity dispersion is derived both from phase-velocity curves and directly from the record. They agree well and give an essentially constant group velocity of 4.45 km/sec for the period range 40–300 sec. The impulsive large-amplitude G1 and G2 are preceded by waves of smaller amplitude and shorter period, interpreted as Sa waves. It is emphasized that continued detailed investigations of G waves will be of great importance for elucidating the properties of the upper mantle.