The high observed frequency dependence of Q for 1-Hz Lg waves (f0.4--0.6) in the Basin and Range province can be explained by a frequency-independent layered model of intrinsic Q for shear waves (Q&mgr;) which has very low values in the upper crust and rapidly increasing values at greater depths. By contrast, earlier work in the eastern United States indicated Q&mgr; values in the upper crust which are high and which vary with frequency. The Q&mgr; model which best explains reported values of QLg and its frequency dependence in the Basin-and-range consiRts of an 8-km thick low-Q layer (60 or less) overlying Q values as high as 1000 or more at mid-crustal depths. This Q&mgr; distribution is best explained by an upper crust which contains fluids in interconnected cracks overlying a lower crust where higher lithostatic pressure has closed those cracks.

These results indicate that whereas the frequency dependencce of Q&mgr; is an intrinsic property of crustal material, the frequency dependence of QLg arises from both the intrinsic frequency dependence of crustal material and its layered structure. A notation is suggested which distinguishes between the frequency dependence of QLg and Q&mgr;. ¿ American Geophysical Union 1991