Seismic Source Descriptions of Underground Explosions and a Depth Discriminate

Abstract

Synthetic seismograms of both body waves and Rayleigh waves are used to determine the radiation field of a few large contained underground explosions. A number of possible source descriptions are investigated. A reduced displacement potential of the form, ø(t) = ø_0t^ξ exp(-ηt), fits the long- and short-period data. The source parameters appropriate for the Boxcar event are ξ = 0·5 and η = 0·15. Synthetic PL and Rayleigh waves are compared with observations from a number of different size events to determine the dependence of η on yield. The amplitude of the long period synthetic body wave responses at ranges greater than about 12° increases rapidly as the source depth is increased. Thus the difference in spectral properties of explosions and earthquakes can be largely explained by the depth effect. The theoretical ratio SP/LP, that is the short period divided by the long-period amplitude, is computed from 12 to 25° for the Johnson upper mantle model and the Boxcar source. A study of an earthquake which cannot be distinguished from an explosion using the m_b vs. M_s criterion is investigated by the SP/LP discriminate.