Attenuation of coda waves in New England

Seismic wave attenuation was measured in New England from the time domain decay of coda wave amplitudes on narrow bandpass-filtered seismograms of local earthquakes. The frequency band of interest was 0.75 to 10 Hz. Q c ( f ) was found to increase with frequency across this band, but there was also a difference between the Q c ( f ) values for short ( t t > 100 sec) lapse times of coda wave propagation. For the short-lapse time data, corresponding to wave paths primarily in the upper crust, Q c ( f ) increases from 400 at 3 Hz to 1300 at 10 Hz. For the large-lapse time data, corresponding to wave paths in the lower crust and upper mantle, Q c ( f ) was found to vary from 660 at 1 Hz to 1500 at 10 Hz. If we interpret this dataset in terms of a model incorporating both scattering and anelastic attenuation, we find that the mean free path of seismic waves over this frequency band is 80 km for short-lapse times, and 400 km for long-lapse times. If we assume that scattering is entirely responsible for the observed attenuation, we find that the minimum mean free path for short-lapse times is about 75 km over all frequencies, whereas for long-lapse times, the minimum mean free path decreases from 400 km at 0.75 Hz to 90 km at 10 Hz.