Attenuative Dispersion of P Waves in and near the New Madrid Seismic Zone

Abstract A procedure for measuring the attenuative dispersion of body-wave phases has been developed and applied to PANDA recordings of direct P waves in the New Madrid seismic zone. We have determined relative group delays of spectral components of those waves for 152 paths and related that observed dispersion to seismic-wave attenuation using an expression we have derived for a continuous re-laxation model. The group delays are sensitive to, the value of Q1(x) over the1Qm flat portion of the attenuation spectrum, and 1/s2, the frequency of the half-amplitude point at the high-frequency end of the spectrum. Qm increases with epicentral distance from about 45 to 145 between 10 and 60 km, suggesting an increase of crustal P-wave Q with depth. In addition, Qm values for paths that are entirely, or mostly, confined to the active portion of the New Madrid seismic zone are lower than for paths that traverse a significant portion of crust outside the active region. The reduced Q in the source zone of New Madrid earthquakes can be explained by fluids in permeable rock, a mechanism previously proposed by Al-Shukri and Mitchell (1988) to explain reduced velocities there. A comparison of P-wave Q values reported in this study of attenuative dispersion with those previously deduced from the decay of spectral amplitudes shows that mean values obtained by the two methods are about the same, but that Q values from attenuative-dispersion measurements exhibit less scatter