Q structure of the Basin and Range from surface waves

Regionalized Rayleigh and Love wave attenuation coefficients have been measured across the Basin and Range province of western United States in the frequency range 0.02-0.2 Hz. The measurements were made by using the methods of Tsai and Aki (1969) and Yacoub and Mitchell (1977) adapted to work on any number of events simultaneously. Rayleigh wave Q values at low frequency approach values near 40, which are significantly lower than previous measurements in the western United States. Love wave Q values, on the other hand, are quite high at low frequencies. The authors suggest that interference between fundamental and higher modes may explain the Love wave observations. The Rayleigh and Love wave attenuation measurements have been inverted simultaneously for shear-wave attenuation as a function of depth. A frequency-independent Q model is consistent with both Rayleigh wave and short-period Love wave attenuation data. The shear-wave Q (Q/sub ..beta../) model is characterized by low Q/sub ..beta../ in the lower crust (Q/sub ..beta.. approx. 100) and Q/sub ..beta../ decreasing in the upper mantle with lowest values (Q..beta.. approx. 30) beneath 60 km depth. Forward modeling shows that a high-Q lower crust or upper mantle lid is inconsistent with the data. Our interpretation of thesemore » results is that the lithosphere is poorly developed beneath the Basin and Range and that the partially molten asthenosphere may reach very shallow depths, possibly to the base of the crust. This interpretation of the Q model in conjunction with a number of geological and geophysical evidence suggests that attenuation mechanisms involving partial melt predominate in the lower crust and upper mantle of the Basin and Range.« less

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