The Attenuation of Fourier Amplitudes for Rock Sites in Eastern North America

We develop an empirical model of the decay of Fourier amplitudes for earthquakes ofM 3–6 recorded on rock sites in eastern North America and discuss its implications for source parameters. Attenuation at distances from 10 to 500 km may be adequately described using a bilinear model with a geometric spreading of 1=R1:3 to a transition distance of 50 km, with a geometric spreading of 1=R0:5 at greater distances. For low frequencies and distances less than 50 km, the effective geometric spreading given by the model is perturbed using a frequency- and hypocentral depth-dependent factor defined in such a way as to increase amplitudes at lower frequencies near the epicenter but leave the 1 km source amplitudes unchanged. The associated anelastic attenuation is determined for each event, with an average value being given by a regional quality factor of Q 525f0:45. This model provides a match, on aver-age, between the known seismic moment of events and the inferred low-frequency spectral amplitudes at R 1 km (obtained by correcting for the attenuation model). The inferred Brune stress parameters from the high-frequency source terms are about 600 bars (60 MPa), on average, for events of M>4:5.