UPPER MANTLE Q-STRUCTURE BENEATH THE EAST PACIFIC RISE

A model of Q(beta) as a function of depth has been determined for the upper mantle beneath the East Pacific Rise. The model is appropriate for shear waves with periods from roughly 15 to 30 s. The amplitudes of multibounce SH phases which propagate within the upper mantle were used to constrain the Q structure. The multibounce phase amplitudes were measured with respect to the S wave amplitude for each seismogram. The data consist of S, SS, SSS, and SSSS waves for distances from 30-degrees to 80-degrees. The sources are earthquakes which occurred on transform faults associated with the East Pacific Rise and the seismograms were recorded by World-Wide Standard Seismograph Network and Canadian Seismic Network stations in western North America. Mantle beneath oceanic crust less than 15 Ma and tectonically active continental crust is sampled. Since the S wave data primarily sample the deep mantle, the resulting upper mantle Q model depends on the Q structure assumed for the lower mantle. We assumed a constant Q of 312 for the mantle beneath 650 km depth. With this assumption, the upper mantle phase amplitudes require a Q of about 70 in the upper 150 km of the mantle. From 150 to 400 km depth, Q increases to 180. The Q in the transition zone, from 400 to 650 km depth, was found to be 180, significantly lower than the lower mantle Q. The resulting Q model is consistent with several surface wave studies of mantle attenuation beneath the Pacific However, our results predict a Q(ScS)(210) which is significantly higher than many direct measurements of ScS attenuation indicate. Furthermore, the large range in measured Q(ScS) is inconsistent with lateral Q variations being confined to the upper mantle if the region we have studied represents the low Q extreme of upper mantle. We conclude that large lateral variations in Q exist in the lower mantle.