Propagation of Lg and lateral variations in crustal structure in Asia

Abstract

Lg propagates efficiently with predominant periods of about 1 to 1 1/2 s across the stable regions of Asia: the Indian shield, the Eurasian platform, the Tarim basin, and (less efficiently) eastern China. This suggests that the structure is relatively homogeneous and Q must be high in the crust in these regions. For paths across the Tien Shan, where the crust is abnormally thick, Lg is observed with somewhat less sharp arrivals, but nevertheless, frequencies are high, and amplitudes large. For paths along the Tien Shan, signals are weaker, and arrivals are still less sharp. Lg was not observed for paths crossing the Tibetan Plateau. We infer that the change in crustal structure on the margins of the Tibetan Plateau and at the Tien Shan may disrupt the wave guide for Lg and may scatter energy out of the relatively homogeneous wave guide of the more stable crust. Alternatively or in addition, Lg may not propagate across the Tibetan Plateau because of an unusual velocity structure or because of high attenuation in the crust.

A Study of Seismic Wave Propagation at Regional Distances in Five Areas of the World

“…-78- which L propagates very efficiently. These findings are similar to g those of Ruzaikin et al (1977).…”

Lateral variation of Lg wave propagation in southern Mexico

“…The Lg waves have been modeled as higher-mode surface waves [Herrmann and Kijko, 1983] or as multiple reflected S waves [Campillo et al, 1984;Campillo, 1990], which propagate in the crust. The characteristics of the propagation of Lg waves have been discussed for many regions [e.g., Ruzaikin et al, 1977;Campillo and Plantet, 1991;Campillo et al, 1993;Chazalon et al, 1993;McNamara et al, 1996;Shi et al, 1996;Benz et al, 1997;Rial and Ritzwoller, 1997;Rodgers et al, 1997;Mellors et al, 1999;Calvert et al, 2000] and have been studied using numerical simulations [e.g., Kennett, 1986;Maupin, 1989;Regan and Harkrider, 1989;Cao and Muirhead, 1993;Gibson and Campillo, 1994;Kennett, 1997, 1998;Vaccari and Gregersen, 1998]. It has been observed that Lg propagation becomes inefficient or is blocked if the crustal structure undergoes fast lateral changes [Ruzaikin et al, 1977;Baumgardt, 1990;Campillo et al, 1993;Chazalon et al, 1993] or if the travel path crosses oceanic crust [Press and Ewing, 1952;Knopoff et al, 1979;Kennett et al, 1985;Gregersen and Vaccari, 1993;Shapiro et al, 1996].…”

Contemporary Tectonics of China

“…Lg show inefficient propagation along paths that cross Tibet [Molnar and Oliver, 1969;Ruzaikin et al, 1977]. [Shih et al, 1973;York et al, 1976].…”