Lg Q in the Eastern Tibetan Plateau

by Jiakang Xie

Abstract

Lg spectra are collected from the 1991–1992 Tibetan Plateau Passive Experiment to measure Q values. Using a standard two-station method that virtually eliminates source and site effects, I obtain a model of Q₀ = (126 ± 9) and η = (0.37 ± 0.02) in a frequency range between 0.2 and 3.6 Hz, where Q₀ and η are Lg Q at 1 Hz and its power-law frequency dependence, respectively. The estimated Q₀ value is among the lowest ever reported for continental areas; it qualitatively supports the observation by McNamara et al. (1996) that Lg can not be observed inside the plateau beyond about 700 km, a limiting distance that is much shorter than those in the other low Q₀ (∼200) regions, such as Iran and the western United States. The low Q₀ value may be the cause of Lg blockage across the northern boundary of the plateau and may indicate abnormally high temperature and fluid content in the Tibetan crust.

Quantitatively, the estimated Q₀ value is lower by a factor of 3 than the value of 366 estimated by McNamara et al. (1996)>, who used data from the same experiment. Since there are several differences in the data processing and inversion procedures used in this and the previous studies, I investigated the effects of these differences on the Q estimates. I conclude that the most probable cause of the discrepancy is in the different inverse methods used. The previous inversion solved for a large number of free parameters that include the source and site terms. In this study only two free parameters (Q₀ and η) are solved for, thus avoiding the instability caused by parameter trade-offs.

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