A Comparison of Pn Attenuation in Stable And Active Continental Regions

There have been a few studies on Pn attenuation on continents, including those in the shield-like regions of Scandinavia, Canada, central Asia, and active regions of the Tibetan plateau and Basin and Range province. Results of these studies are used to calculate Pn attenuations at a common reference distance of 1200 km, taking into account the different parameterizations used. In the three shield regions the attenuations are found to be similar, within a factor of 2, between frequencies of 0.1 and 5 Hz. In a region in and around north central Tibet (N.C.T.), where mantle lid may be partially molten, Pn attenuation differs from those in shield regions in a strongly frequency-dependent manner: at low frequencies near 0.1 Hz N.C.T. is only marginably more attenuative, by less than a factor of 2, than the shield regions. The differences increase with increasing frequency such that at 4-5 Hz, N.C.T. is about 100 times more attenuative then the shield regions. Pn attenuations in southern and eastern Tibet exhibit a puzzling behavior of being similar to those in shield regions at low frequencies, while being similar to that in N.C.T. at high frequencies. A possible explanation of this behavior is that Pn attenuation beneath Tibet is more affected by intrinsic Q at lower frequencies, and by scattering and multiple bouncing ("whispering gallery") mechanisms at higher frequencies.