Spatial attenuation of the Lg wave in the Central United States

Abstract

abstract The spatial attenuation of the Lg wave in the Central United States is studied in the time domain in the frequency range of 1 to 10 Hz. Fitting Lg amplitudes according to the relation A = A o r − 5 / 6 exp ( − γ r ) , γ varies between 0.0007 ± 0.0001 km−1 at 1 Hz and 0.0029 ± 0.0005 km−1 at 10 Hz (standard errors given). Relating γ to Qβ by the relation γ = π f / Q β U , these results imply a frequency dependence for Qβ given by Q β ( f ) = Q β ( 1 H z ) f ( 04 ± 015 ) . This result extends Singh's (1981) estimate of 0.1 to 0.3 for the exponent of frequency dependence for the frequency range to 0.5 to 2 Hz for coda waves observed in the Central United States. Bandpassed microearthquake data were also used to estimate source spectrum shapes and seismic moments. Incorporating these results with those of Street et al. (1975), the seismic moment-magnitude scaling for the Central United States is found to be log 10 M o = 16.44 ± 0.18 + ( 1.26 ± 0.05 ) m b L g for the magnitude range 1.8 ≦ mbLg ≦ 4.9.