A direct measurement of the Earth's short-period attenuation along a teleseismic ray path

Abstract

Measurements of ground motion taken near large underground explosions, when they are compared with teleseismic recordings of the same event, allow a direct estimation of the earth's short-period attenuation. Spectra from ground motion measurements made within 15 km of the explosion Boxcar in Nevada are compared with those computed from recordings from Norway at Δ ≃ 72°. The effective quality parameter QE is estimated over the band 0.6–3.0 Hz. From three near-source stations, QE is found to range between 1400 and 2300 with a mean value of 1700. From time-domain calculations a comparable value of QE is obtained by scaling up Haskell's explosion source model for granite and finding the attenuation operator that produces the recorded P wave at Norway. A similar experiment by Trembly and Berg (1968) yielded a QE ∼ 450 km from the Nevada Test Site to Mould Bay, Canada, for Δ ≃ 37°. These two values of QE verify that under western North America the upper mantle attenuates seismic waves much more severely than the lower mantle.