Magnitude corrections for attenuation in the upper mantle

The mb:Ms relation for explosions at the Nevada Test Site (NTS) differs from those for explosions in other parts of the world. There is considerable evidence that this results mostly from high body-wave attenuation in the upper mantle beneath the western US. The authors have developed an empirical magnitude correction for body-wave attenuation and applied it to both source and receiver ends of the teleseismic body-wave path. The results imply that mb values are lower for NTS explosions than for Soviet explosions of comparable yield and seismic coupling. The authors have also developed and applied a source-depth correction to account for pP-P interference in the P-wave arrival. The body-wave magnitude resulting from these corrections is designated mQ to distinguish it from other definitions of mb. Values of mQ determined for a world-wide set of large explosions show that a single mQ: yield relation is a fair fit to the data for the explosions with high seismic coupling. However, grouping the explosions under two mQ:yield relations gives a better fit to the data. All the studied explosions in salt or granite or below the water table fit a common Ms:yield relation. Explosions from North America, Eurasia and Africa have a common mQ: Ms relation.