Abstract

We have obtained tomographic maps of Lg coda Q and its frequency dependence at 1 Hz for all of North America assuming that all of our coda Q data can be described by a power law. Our map of Q0 displays highest values (700–1000) throughout a large region that coincides with the Canadian Shield and exhibits sharp changes in Q0 across much of the boundary separating the high values of the shield from lower values outside it. A second high Q region appears as a small oval of high Q0 values (650–700) overlying much of the Ozark uplift in the central United States. A band of low values (200–350) covers most of Central America and Mexico and extends northward from California into western Canada before broadening in Alaska. A north–south‐trending band of high η values, extending from northern Canada to the gulf coast, separates the North American Cordillera from stable portions of central North America.

We compared our Q0 and η determinations with those found in five earlier studies in two regions of the United States. In the present study, in each region these values agree with values found in the earlier studies when earthquake–station distances are greater than 300 km but differ when distances are less than that. The similarity of our Q0 measurements to previous direct Lg and Lg coda measurements and the inverse correlation we find of Q0 with upper mantle temperatures suggest that we may predominantly be measuring intrinsic Q0.

We propose a mechanism for explaining why scattering dominates over intrinsic attenuation for Lg and Lg coda propagation at short distances but can be secondary to intrinsic attenuation at long distances. It can be nicely described using a ray characterization for Lg and understanding that at short distances many near‐vertical traveling rays are multiply reflected in the crust and much energy is lost to the mantle. At greater distances (say 300 km), virtually all of the reflected waves exceed the critical angle and so cannot easily be lost to the mantle.

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