The Effect of Attenuation on Seismic Body Waves

Both spectral and time domain studies indicate that the frequency dependence and regional variation of the attenuation of P waves parallels that of S waves with t * β = 4 t * α . Scattering Q cannot be generally separated from intrinsic Q in the mantle. Forward scattering can generate time-dependent variations in the frequency content and complexity of body waves that affect the measurement of t * . Assuming that lateral heterogeneity biases the apparent Q of surface waves, the frequency dependence of Q can be explained by a relaxation model of intrinsic Q . In this model, Q is constant with frequency up to a cutoff frequency 1/(2πτ m ) Hz, where τ m = 0.1 to 0.2 sec. Regional variations in mantle attenuation are consistent with radiometric or magnetic age and tectonic activity, regions of higher relative attenuation coincident with younger, tectonically active crust. Continental cratons are underlain by mantle having small attenuation at all depths. High attenuation usually correlates with slow travel times, lower Pn velocity, and inefficient Pn and Sn propagation. Measures of differential frequency content (δt * and δ Q ) generally correlate better with differential travel time than measures of differential amplitude and m b . The regional pattern and intensity of both travel-time anomalies and t * measurements suggest that both share a common origin due to the regional variation of the thermal structure of the upper 200 to 400 km of the mantle.