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

Abstract

SUMMARY 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. The low Qμ values, and their constancy with frequency, in the uppermost crust can be explained by fluid flow in a network of cracks in brittle rock. Increasing Qμ with depth to 10-15 km can be explained by the closing of those cracks due to pressure and their enhanced healing with increasing temperature. Plastic flow at greater depths may contribute further to the dissipation of cracks and to further increases in Qμ. Decreasing Qμ values at greater depths can be explained as being the result of increasing temperature, increasing content of partial melt, enhanced dislocation motion or some combination of these effects.