Observations of regional phase propagation across the Tibetan Plateau

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.

Frequency dependent Lg attenuation in south‐central Alaska

“…Direct phase and coda amplitude measurements, using modern digital seismic instruments, have confirmed that attenuation in the western United States is significantly higher than in the eastern United States [Mitchell, 1975[Mitchell, , 1981Frankel et al, 1990;Benz et al, 1997]. In fact, global observations confirm that S-wave coda and Lg attenuation is higher for regions with active tectonism than for stable continental interiors [Aki, 1980a[Aki, , 1980bBenz et al, 1997;McNamara et al, 1996]. This study focuses on the frequencydependence of the Lg wave quality-factor in south-central Alaska.…”

“…The frequency-dependent quality factor, Q(/), is commonly modeled using a power law of the form: [Frankel and Wennerberg, 1987]; however, good correlation between coda Q and Lg Q estimates have been observed in regions such as the Tibetan Plateau [Xie and Mitchell, 1991;McNamara et al, 1996]. This study presents the first estimate of Lg frequency dependent Q in south-central Alaska using broadband instrumentation.…”

“…In stable continental regions, such as northern Africa, Lg is observed at distances as great as 6000km [McNamara and Walter, 2000]. In contrast, when propagating through active tectonic regions, such as the Himalaya and Tibetan Plateau [McNamara et al, 1996], or ocean basins [Zhang and Lay, 1995], Lg is completely attenuated due to scattering along tectonic faults and variations in the thickness of the crustal waveguide [Kennett, 1986].…”

About the lithospheric structure of central Tibet, based on seismic data from the INDEPTH III profile

“…As most of the path of P n for the shot records is within the 'No-S n -zone' previously mapped (Barazangi and Ni, 1982;McNamara et al, 1995), one can speculate that the absence of P n on the INDEPTH III shot records is related to the absence of S npropagation in the northern part of the INDEPTH III array. By introducing unusually strong attenuation into the sub-Moho layers of the synthetic models, we found that a P-wave quality factor Q P of f 180 -340 (corresponding to Q A f 80 -150) could explain the absence of high frequency P n , while leaving the lower-frequency P n phase from the earthquakes strong enough to be detected over the ranges considered in this paper.…”

“…Rapine et al (2003) find a low lower-crustal S-wave velocities and the absence of an S velocity gradient in the mantle beneath northern Tibet from dispersion studies. McNamara et al (1995), studying S n regionally, map a large elliptically formed attenuation area in central Tibet, mainly in the Changtang Block. Quaternary volcanism and our unusually low P-velocities in the lower crust and upper mantle from wide-angle and refraction studies, and P n tomography (McNamara et al, 1997;Haines et al, 2003) fit perfectly into the concept of a hot, attenuating and weak, possibly anisotropic and easterly escaping, lithosphere.…”