OBSERVATIONS OF REGIONAL PHASE PROPAGATION ACROSS THE TIBETAN PLATEAU

We present observations of regional phase velocity and propagation cha racteristics using data recorded during a 1-year deployment of broadba nd digital seismic stations across the central Tibetan Plateau along t he Qinghai-Tibet highway from Golmud to Lhasa. Previous seismological studies within this region have had to rely on earthquakes recorded al most exclusively at stations outside of the plateau. We have the oppor tunity to study numerous source-receiver paths confined entirely withi n the Tibetan Plateau. Our analysis concentrates on travel time, ampli tude, 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 8.16+/-0.07 km/s within the Tibetan Plateau, Sn,ho wever, shows dramatic variations in propagation efficiency across the Tibetan Plateau that is strongly dependent on frequency, We observe th at Sn rapidly decreases in frequency and amplitude as it passes throug h the northern portion of the plateau. We show that in general, Sn pro pagation efficiency decreases with increasing frequency content. We us e 122 events from outside of the plateau and 61 from within to refine the boundaries of a region of inefficient high-frequency Sn propagatio n. Specifically, we show that a larger portion of the northern Tibetan Plateau attenuates Sn energy than was previously suggested. In the so uthern plateau, where high-frequency Sn is observed, we computed an av erage velocity of 4.59+/-0.18 km/s. We also observed that the Pn veloc ity within this region of inefficient high-frequency Sn propagation is nearly 4% slower than the Pn velocity computed for paths restricted t o the southern plateau and that the crust is about 10 lan thinner than in the south. The coincident locations of inefficient Sn propagation and slow Pn velocity is commonly observed in regions of active tectoni cs. Our results add constraints to the velocity structure of the litho sphere beneath the Tibetan Plateau and require first-order lateral var iations in the uppermost mantle structure, despite the relatively unif orm topography of the plateau.