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Attenuation of multiphase surface waves in the Basin and Range Province-III. Inversion for crustal anelasticity

1994; Oxford University Press; Volume: 116; Issue: 2 Linguagem: Inglês

10.1111/j.1365-246x.1994.tb01809.x

1365-246X

Brian J. Mitchell, Jiakang Xie,

Seismic Imaging and Inversion Techniques

A combined inversion/forward modelling procedure, in which the frequency dependence of shear-wave internal friction (Qμ−1) is allowed to vary with depth, was developed and applied to selected Rayleigh wave and Lg attenuation data in the Basin and Range Province of the south-western United States. Both Q and the frequency dependence of Lg waves were used to constrain the models. Many models can explain Rayleigh wave and Lg data sets within their uncertainties, but at 1 Hz most have low values of Qμ (50–80) in the upper 8 km of the crust, rapidly increasing values to about 1000 at mid-crustal depths, and decreasing values at greater depths. Models which include a layer of higher Q values (80–150) in the upper few kilometres of the crust, overlying a region of lower Q values, cannot be precluded by the attenuation data of this study. Assuming that Qμ varies with frequency as fζ, models for which the frequency dependence is low (ζ=0.0−0.1) in the upper crust best explain the data of this study. In the lower crust that frequency dependence is not well determined, but the models which best explain both the fundamental-mode and Lg data and produce realistic models of Qμ are characterized at 1 Hz by high values of both Qμ and its frequency dependence. Because of that frequency dependence Qμ may be an order of magnitude lower at a period of 100 s (≈ 100) than it is at 1 s (≈ 1000). Investigations of the effects of changing crustal velocity on values for Lg Q and its frequency dependence indicate that realistic velocity changes cause only small changes in those values and thus are inconsequential to our results.