Source scaling of Pn and Lg spectra and their ratios from explosions in central Asia: Implications for the identification of small seismic events at regional distances

Abstract

Multiple station recordings of Pn and Lg spectra from an mb = 6.6 nuclear explosion and several mb ≤ 3.7 chemical explosions in central Asia are used to invert for source seismic moments (Mo), corner frequencies (fc), and path-variable Q models. A modified Mueller-Murphy source model fits the nuclear explosion well and fits the chemical explosions reasonably well, although Pn from chemical explosions are complicated by event-variable spectral fluctuations. New source spectral scalings are derived for underground explosions in a wide mb range between ∼ 3.7 and 6.6; they preserve features of the previous scalings, such as (1) larger Pn Mo than Lg Mo, (2) quarter-root scaling between fc and Mo, and (3) higher Pn fc than Lg fc by factors of ∼4. A necessary and important consequence of the scalings is that the Pn/Lg spectral ratio grows rapidly in an intermediate frequency range between the fc of Pn and Lg. Since fc scales with event size, this frequency range shifts higher for smaller events. This magnitude dependence is directly confirmed using observed Pn/Lg ratios from the above explosions. It is also confirmed using Pn/Lg ratios observed at station WMQ using many ∼ 6 and mb ∼ 5 Soviet explosions. A procedure is proposed to account for then magnitude dependence of Pn/Lg ratios in the explosion identification.