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Strong Lg Attenuation in the Tibetan Plateau

Earth and Marine Sciences Building
University of Santa Cruz
Santa Cruz, California 95064

The widespread existence of strong Lg attenuation in the Tibetan Plateau is further demonstrated by analysis of Lg spectra on many paths within the plateau and quantitative estimates of the Lg attenuation from low-frequency Lg signals for new, localized path geometries. Strong path-length-dependent shifts of the Lg spectra to lower corner frequencies with increasing distance are observed across the plateau, consistent with a low regional average 1-Hz Lg attenuation value, Q₀, of about 125. There are clearly lateral variations within the plateau found in this and other recent studies, with localized areas having Q₀ values of 60–90, low enough to eliminate high-frequency Lg energy over path lengths of just several hundred kilometers, while some localized areas may have higher Q₀ values of up to 147 or higher. A Q₀ value of 103 is found in south-central Tibet, compatible with recent work by others for higher frequencies on very localized scales, and values from 83 to 147 are found in eastern Tibet. The lowest Q₀ estimates found in Tibet tend to be in areas for which there is evidence of volcanism and/or partial melting within the crust; however, the strong regional attenuation may have a contribution from scattering by small-scale crustal heterogeneity. The strong Lg attenuation in Tibet gives a new constraint for understanding the tectonic development of the plateau and presents challenges to seismic monitoring of the region for possible clandestine nuclear tests.

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