Lg propagates efficiently with predominant periods of about 1 to 1 1/2 s across the stable regions of Asia: the Indian shield, the Eurasian platform, the Tarim basin, and (less efficiently) eastern China. This suggests that the structure is relatively homogeneous and Q must be high in the crust in these regions. For paths across the Tien Shan, where the crust is abnormally thick, Lg is observed with somewhat less sharp arrivals, but nevertheless, frequencies are high, and amplitudes large. For paths along the TienShan, signals ae weaker, and arrivals are still less sharp. Lg was not observed for paths crossing the Tibetan Plateau. We infer that the change in crustal structure on the margins of the Tibetan Plateau and at the Tien Shan may disrupt the wave guide for Lg and may scatter energy out of the relatively homogeneous wave guide of the more stable crust. Alternatively or in addition, Lg may not propagate across the Tibetan Plateau becuase of an unusual velocity structure or because of high attenuation in the crust. |