Observations of regional phase propagation across the Tibetan Plateau

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Abstract
We present observations of regional phase velocity and propagation characteristics using data recorded during a 1‐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 8.16±0.07 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±0.18 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.

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