Guangwei Fan

Characteristics of Lg attenuation in the Tibetan Plateau

[1] Lg, a regional seismic wave comprised of multiple shear wave reverberations trapped in the crustal waveguide, is important for magnitude estimation and source discrimination for monitoring nuclear testing treaties. In stable continental regions, Lg propagates with a group velocity of about 3.5 km/s and can often be observed at distances up to 4000 km. To better understand the absence of high-frequency Lg arrivals for paths traversing the northern boundary of the Tibetan Plateau, we investigate spatial variations of broadband (0.15–5.0 Hz) energy in the Lg group velocity window (3.6–3.0 km/s) using regional waveforms recorded at the Chinese Digital Seismic Network station WMQ. Vertical component seismograms are analyzed for 90 events with magnitudes of 4.4 � mb � 6.4 that occurred between 1987 and 1999 in the Tibetan Plateau and around its margins. The Lg amplitude spectra for events located near the northern margin of the plateau have apparent corner frequencies of 1–2 Hz, nearly identical to those for comparable size events at similar distances outside the plateau. High-frequency (>1 Hz) Lg energy recorded at WMQ decreases rapidly as a function of source distance into the plateau. A path length of 300–400 km within the northern Plateau suffices to eliminate 2–5 Hz Lg energy. For events in southern Tibet with paths crossing the central portion of the Tibetan Plateau, almost total Lg extinction occurs, even for energy in the low-frequency band of 0.2–1 Hz. Corresponding apparent corner frequencies of the ‘‘Lg’’ amplitude spectra range between 0.2 and 0.4 Hz. The corner frequency shift is found to vary systematically with path length across the plateau. Linear regressions demonstrate that the shift in apparent corner frequency of Lg amplitude spectra is negatively correlated with features of the Tibetan Plateau, such as mean elevation along the paths or travel distance within the plateau above specified elevation thresholds. The systematic variations in the amplitude and frequency content of energy in the Lg window as a function of path length within the plateau indicate that strong crustal attenuation plays an important role in Lg extinction for paths traversing central and northern Tibet, superimposed on any structural blockage effects associated with abrupt thinning of the crust near the northern boundary of the plateau. Spectral ratios of many event pairs along great circle paths give estimates of frequency-dependent Lg attenuation for paths crossing western, central, and eastern sectors of Tibet. The region of strong Sn attenuation in northern central Tibet also has strong Lg attenuation with QLg for 1 Hz on the order of Qo = 80–90, while in southern central Tibet, Qo increases to about 316, and in eastern and western Tibet, Qo is on the order of 120–200 for paths traversing the entire plateau. The strong Lg attenuation in northern central Tibet is responsible for the so-called Lg blockage and may be associated with partial melting in the crustal low-velocity layer in northern Tibet. Our QLg values for Tibet are significantly lower than most earlier estimates, primarily as a result of not excluding blocked observations along with allowing for lateral variations within Tibet. INDEX TERMS: 7203 Seismology: Body wave propagation; 7255 Seismology: Surface waves and free oscillations; 7205 Seismology: Continental crust (1242); KEYWORDS: Lg, Tibetan Plateau, nuclear event discrimination, crustal attenuation, Lg blockage, crustal partial melting

Strong Lg Attenuation in the Tibetan Plateau

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.