Abstract

Variations in attenuation and travel time of multiple ScS whole-mantle reverberations indicate very strong lateral gradients in upper-mantle properties beneath the tectonically active province of western North America. Whole-mantle attenuation estimates (QScS) obtained by spectral stacking of phases sampling discrete subregions span the entire range previously determined using a similar procedure for a global set of continental and oceanic QScS measurements. The relationship between travel time and attenuation anomalies, which has been used to argue for predominantly thermal control on the variations in oceanic environments, is poorly defined within the continental regime. However, a correlation between surface heat flow and QScS variations in the United States indicates that thermal variations do control attenuation heterogeneity. For paths beneath Mexico, stacking 13 pairs of ScSn phases from the 24 October 1980 Huajuapan, Mexico and the 19 June 1982 El Salvador events yields QScS = 142 ± 10 in the frequency band 7.8 to 46.9 mHz. Time-domain measurements for the same phases yield a similar estimate of QScS = 148 ± 9. The average ScSn+1 - ScSn JB (Jeffreys-Bullen) travel-time residual (ΔtScS) is +0.9 sec (slower than JB). All of the differential travel-time estimates are corrected for dispersion effects in the passband of the observations, and are appropriate for a reference period of about 30 sec. Multiple ScS arrivals from the 29 April 1965 Puget Sound earthquake recorded by WWSSN, CSN, LRSM and Pasadena stations provide the first determination of whole-mantle attenuation beneath the westernmost United States. Pasadena recordings from three instruments provide ScSn paths traversing the Cascade and Sierra Nevada ranges. Stacking 9 ScSn pairs yields QScS = 208 ± 39 (7.8 to 54.7 mHz) and the same phases give ΔtScS = +2.5 sec. Six pairs of ScSn reverberations under the Pacific Northwest give QScS = 344 ± 88 (15.6 to 54.7 mHz) and ΔtScS = +5.5 sec. A combined estimate for the relatively low heat flow region of the Pacific borderlands gives QScS = 257 ± 20 (7.8 to 46.9 mHz), comparable to the high QScS estimates previously determined for South America. On the other hand, 12 ScSn pairs sampling the northern Basin and Range, which has high heat flow, yield a very low QScS = 95 ± 4 (7.8 to 31.3 mHz) with ΔtScS = +5.1 sec. The latter QScS value is even lower than estimates for young oceanic regions, suggesting extensive partial melting in the low-velocity zone under the northern Basin and Range.

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