New observations of the differential attenuation of long period S waves, measured using a spectral ratio technique, and a re-examination of reported attenuation measurements for P waves indicate significant lateral variations in attenuation beneath the United States. High attentuation is observed at stations between the Rocky Mountains and the Sierra Nevada-Cascade ranges and in the northeast United States. Low attenuation is seen at stations in the central and eastern parts of the country and along the Pacific coast. Relative magnitudes of S- and P-differential attenuation are consistent with P-wave absorption being due entirely to losses in shear. A large azimuthal dependence for near-source attenuation is suggested for an earthquake on a mid-ocean ridge.

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