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 of M 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/R-1.3 to a transition distance of 50 km, with a geometric spreading of 1/R-0.5 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 = 525f(0.45). This model provides a match, on average, 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.