The relationship between the propagation characteristics of Lg and
lateral heterogeneity within the continental crust has been observed for
many years. We present a study of the propagation of Lg within the Tibetan
Plateau in two parts. The first is a simple, qualitative analysis of Lg
amplitude. The second is a quantitative attempt to estimate the frequency
dependence of Lg Q within the Tibetan Plateau. From July, 1991 through June,
1992, 11 broadband digital seismic stations were deployed across the east-central
Tibetan Plateau (seven along the Qinghai-Tibet highway from Golmud to Lhasa).
During this period, 185 local and regional earthquakes were recorded at distances
from 150-2000 km allowing us to analyze over 1200 seismograms for Lg propagation.
The propagation of Lg has been observed across most of Asia, however, Lg
has not been observed for paths crossing the Tibetan Plateau. We are able
to analyze paths that both cross the boundaries and are contained within
the Tibetan Plateau. The most significant observation from our dataset is
that Lg is generated within the Tibetan Plateau and can propagate efficiently
to epicentral distances of at least 600 km. For events from outside of the
Tibetan Plateau Lg is absent for paths that cross both the Himalayan and
Kunlun ranges confirming that the margins of the plateau effectively block
Lg transmission. We invert Lg amplitudes for paths from 52 events that are
confined to the Tibetan Plateau for the quality factor, Q. This yields the
frequency dependent Q function: Q(f)= 366 f exp(0.45) (0.5~<=~f~<=~16
Hz). Similar observations in other areas indicate that frequency dependent
apparent Q within the Tibetan Plateau is well below that expected for a typical
continental interior. Instead it is similar to an area with active tectonics,
such as the Basin and Range Province of North America.