Frequency-dependent Shear-wave Q Models for the Crust of China and Surrounding Regions

Abstract

— Starting with fundamental-mode Rayleigh-wave attenuation coefficient values (γ _R ) predicted by previously determined frequency-independent models of shear-wave Q (Q _μ ), we have obtained frequency-dependent Q _μ models that explain measured values of γ _R as well as of Lg coda Q and its frequency dependence at 1 Hz (Q _o and η, respectively) for China and some adjacent regions. The process combines trial-and-error selection of a model for the depth distribution of the frequency dependence parameter (ζ) for Q _μ with a formal inversion for the depth distribution of Q _μ at 1 Hz. Fifteen of the derived models have depth distributions of ζ that are constant, or nearly constant, between the surface and a depth of 30 km. ζ distributions that vary with depth in the upper 30 km are necessary to explain the remaining seven models. ζ values for the depth-independent models vary between 0.4 and 0.7 everywhere except in the western portion of the Tibetan Plateau where they range between about 0.1 and 0.3 for three paths. These low ζ values lie in a region where Q_Lg and crustal Q _μ are very low and suggest that they should also be low for high-frequency propagation. The models in which ζ varies with depth all show a decrease in that value ranging between 0.55 and 0.8 in the upper 15 km of the crust and (with two exceptions where ζ=0.0) between 0.3 and 0.55 in the depth range 15–30 km. The distribution of ζ values between 0.6 and 0.8 (the higher part of the range) in the upper crust indicates that high-frequency waves will propagate most efficiently, relative to low-frequency waves, in a band that includes, and strikes north-northeastward from the path between event 212/97 and KMI to the path between event 180/95 and station HIA in the north.