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Pn Attenuation beneath the Tibetan Plateau

Air Force Research Laboratory, Hanscom Air Force Base, Massachusetts 01731 afrl.rvb.pa{at}hanscom.af.mil

Pn spectra are collected from three PASSCAL seismic experiments in the Tibetan Plateau (TP) over four path groups to study Pn attenuation. The first and second path groups run southward from the Lop Nor Test Site (LTS) to stations in north- and south-central Tibet. The third and fourth path groups form two north-northeast-oriented profiles in the eastern TP. Events recorded by the first two path groups are also recorded over central Asian paths running westerly from the LTS to the Khyrghistan network (KNET). A comparison of Pn spectra from these events reveals that the mantle lid under the north-central TP attenuates Pn more severely than the lid under central Asia. Apparent Q₀ and (Pn Q at 1 Hz and its frequency dependence) are estimated with a simplified geometrical spreading (GST) of ^-1.3. Over path group 1 that heavily samples the northern TP, Q₀ and are estimated to be 183±33 and 0.3±0.1, respectively. Over path groups 2 and 3 that sample either a mixture of northern and southern Tibet or eastern Tibet, Q₀ and are between about 250–270 and 0.0–0.1, respectively. Over the fourth path group that samples the easternmost TP, the respective estimates are 374±51 and 0.3±0.1, respectively, similar to the estimates of 364 and 0.5 for central Asian paths from LTS to KNET. A comparison of the total Pn attenuations that include the effects of both GST and Q in continental regions shows that they are similar in relatively stable regions of central Asia, Scandinavia, and the Canadian shield. Within the TP, Pn attenuation is the highest under north-central Tibet and decreases toward the south and east. In easternmost Tibet Pn attenuation becomes similar to that in stable regions. Lateral variation of Pn attenuation inversely correlates with that of the Pn velocity. Possible causes of this variation include (1) a thermally driven mantle Q_p and (2) region-specific velocity structures, which may be characterized by different lid gradients, and density and aspect ratios of 3D scatterers in the vicinity of the Moho.