We present observations of regional phase velocity and propagation
characteristics using data recorded during a one year deployment of broadband
digital seismic stations across the central Tibetan Plateau along the Qinghai-Tibet
highway from Golmud to Lhasa. Previous seismological studies within this
region have had to rely on earthquakes recorded almost exclusively at stations
outside of the plateau. We have the opportunity to study numerous source-receiver
paths confined entirely within the Tibetan Plateau. Our analysis concentrates
on travel-time, amplitude and frequency content measurements of the Pg, Pn
and Sn phases. Pn can be clearly picked for all observed paths and propagates
at an average velocity of 8.3 km/s throughout
the entire region and is diminished to 8.16 km/s within the Tibetan Plateau.
Sn however, shows dramatic variations in propagation efficiency across the
Tibetan Plateau that is strongly dependent on frequency. We observe that
Sn rapidly decreases in frequency and amplitude as it passes through the
northern portion of the plateau. We show that in general, Sn propagation
efficiency decreases with increasing frequency content. We use 122 events
from outside of the plateau and 61 from within to refine the boundaries of
a region of inefficient high frequency Sn propagation. Specifically, we show that a larger portion of
the northern Tibetan Plateau attenuates Sn energy than was previously suggested.
In the southern plateau, where high frequency Sn is observed, we computed
an average velocity of 4.59 km/s. We also observed that the Pn velocity within this region of inefficient
high frequency Sn propagation is nearly 4% slower than the Pn velocity computed
for paths restricted to the southern plateau and that the crust is about
10 km thinner than in the south. The coincident locations of inefficient
Sn propagation and slow Pn velocity is commonly observed in regions of active
tectonics. Our results add constraints to the velocity structure of the lithosphere
beneath the Tibetan Plateau, and require first-order lateral variations in
the uppermost mantle structure, despite the relatively uniform topography
of the plateau.
