Strong Lg Attenuation in the Tibetan Plateau

Abstract

The widespread existence of strong Lg attenuation in the Tibetan Plateau is further demonstrated by analysis of Lg spectra on many paths within the plateau and quantitative estimates of the Lg attenuation from low-frequency Lg signals for new, localized path geometries. Strong path-length-dependent shifts of the Lg spectra to lower corner frequencies with increasing distance are observed across the plateau, consistent with a low regional average 1-Hz Lg attenuation value, Q 0 , of about 125. There are clearly lateral variations within the plateau found in this and other recent studies, with localized areas having Q 0 values of 60-90, low enough to eliminate high-frequency Lg energy over path lengths of just several hundred kilometers, while some localized areas may have higher Q 0 values of up to 147 or higher. A Q 0 value of 103 is found in south-central Tibet, compatible with recent work by others for higher frequencies on very localized scales, and values from 83 to 147 are found in eastern Tibet. The lowest Q 0 estimates found in Tibet tend to be in areas for which there is evidence of volcanism and/or partial melting within the crust; however, the strong regional attenuation may have a contribution from scattering by small-scale crustal heterogeneity. The strong Lg attenuation in Tibet gives a new constraint for understanding the tectonic development of the plateau and presents challenges to seismic monitoring of the region for possible clandestine nuclear tests.

Crustal flow in Tibet: geophysical evidence for the physical state of Tibetan lithosphere, and inferred patterns of active flow

“…The region of strong Sn (upper mantle) attenuation found by Ni & Barazangi (1983) and by McNamara et al (1995, from PASSCAL) also corresponds with unusually strong crustal attenuation, detected by its effect on both Pnl phases (Rodgers & Schwartz 1998, from PASSCAL) and on Lg (trapped postcritical crustal shear-wave) (Fan & Lay 2003, from permanent broadband stations). Both Rodgers & Schwartz (1998) and Fan & Lay (2003) suggest that the high attenuation is a manifestation of widespread partial melting of the crust of northern Tibet. Regional variability in Lg (crustal) Q from 80 to 100~ across Tibet seems no larger than uncertainties in its estimation (Fan & Lay 2002).…”

On the Causes of Frequency-Dependent Apparent Seismological Q

“…To analyse this trade-off, consider the expression for the observed path factor P(t, f) from which Q(f) is usually derived (e.g., FAN and LAY, 2003):…”

Crustal strength in central Tibet determined from Holocene shoreline deflection around Siling Co

“…On one hand, it is argued that the plateau owes its very existence to widespread lateral flow of the deep crust (Bird, 1991;Royden et al, 1997Royden et al, , 2008Shen et al, 2001). Abundant geophysical data indicate that the lower crust should be quite weak at high temperatures and in the presence of fluids (Bai et al, 2010;Brown et al, 1996;Fan and Lay, 2003;Klemperer, 2006;Makovsky and Klemperer, 1999;Nelson et al, 1996;Unsworth et al, 2005;Wei et al, 2001). Geologic observations from eastern Tibet implicate the large-scale influx of lower crust to explain the growth of plateau topography in that region , and, numerous aspects of the tectonic evolution of the Himalaya can be explained by channelized flow of crustal rocks from beneath Tibet (Beaumont et al, 2001).…”