Attenuation of Seismic Waves in the New Madrid Seismic Zone

Average frequency-independent Q values have been determined for the upper crust in a 105 × 105 km region centered around the active portion of the New Madrid seismic zone using the spectral decay of P waves from local earthquakes. Q values generally increase with increasing path length, both inside and outside the active region, a result which suggests that Q increases with depth in the upper crust. Q values for paths which are largely confined to the active region are reduced by 1/2 to 1/3 compared to those values for paths outside the active region. The low Q values correlate spatially with regions of low seismic velocity found previously for the New Madrid region (Al-Shukri and Mitchell, 1988), and can be explained by a greater density of fluid-filled cracks in the seismically active region than in the surrounding regions.