Abstract

Using data from earthquakes in the 1982 Miramichi earthquake source zone, spectral excitation and attenuation of the Lg phase are studied. With data in the distance range of 135 to 994 km, interpretation is complicated by the presence of high-frequency Sn and Pn waves which interfere with the Lg wave. At the larger distances, the signal at frequencies above 7 Hz is completely dominated by the non-Lg arrivals. A frequency-dependent Lg-Q is determined which rises from 300 at 0.5 Hz to about 1400 at 10 Hz, at which it flattens at higher frequency. The Sn coda apparent Q rises above 3000 at frequencies higher than 10 Hz. Seismic moment and corner frequency estimates are made using Lg-Q corrected spectra. The moment estimates compare well with those obtained from long-period surface waves and short distance spectral estimates. The Lg corner frequency estimates are substantially lower than the short distance estimates. This discrepancy is the subject of discussion, but the Lg moment-corner frequency estimates do model observed data well using a Brune (1970) source model and the derived attenuation relation.

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