Characteristics of Lg attenuation in the Tibetan Plateau

Abstract

[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 m b 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 Q Lg for 1 Hz on the order of Q o = 80-90, while in southern central Tibet, Q o increases to about 316, and in eastern and western Tibet, Q o 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 Q Lg values for ...

Seismological evidence for shallow crustal melt beneath the Garhwal High Himalaya, India: implications for the Himalayan channel flow

“…Interestingly, the MCT zone shows a similar higher attenuation character as the High Himalaya. Low Q 0 values (60-90) are well documented in the Tibet and Ladakh region (Fan & Lay 2002Xie et al 2004;Rai et al 2009). However, low values of 30-60 are possibly being reported for the first time for any segment of the High Himalaya.…”

“…The Q 0 value is sensitive to the tectonic environment, and is found to be lower in the tectonically active regions compared to the stable continental interiors. High attenuation has been associated with regions of high temperature and partial melt in the crust (Mitchell 1995;Fan & Lay 2002.…”

“…In addition, Rai et al (2009) observed a low Q 0 (∼70) to the north of the Indus Suture and a high Q 0 (∼700) beneath the Himalaya. In the central and eastern Tibet and the Himalaya, Fan & Lay (2002 and Xie et al (2004) inferred high crustal attenuation with Q 0 varying from ∼500 beneath the Himalaya to ∼180 in the Tethys zone, ∼90 beneath the Indus suture, and to ∼65 in southern Tibet.…”

Amplitude ratio tomography for regional phase Q

“…Seismologists quantify attenuation using the quality factor (Q), which varies over several orders of magnitude for typical earth materials. Many factors affect crustal Q, including the presence of water, thus tectonic age [Mitchell et al, 1997], and temperature near the solidus [Fan and Lay, 2002]. Other phenomena can affect Q measurements, such as scattering and unmodeled geometrical spreading, and we often refer to effective Q to acknowledge such non-intrinsic mechanisms.…”

Tectonic geomorphometrics of the western United States: Speculations on the surface expression of upper mantle processes

“…The Lg Q (Figure 5b) provides additional information about tectonic processes that have an influence on the surface topography. In general, high Q values are associated with tectonically active regions which are associated with partial melt in the crust [Fan and Lay, 2002, 2003a, 2003bXie, 2002aXie, , 2002b. High Q values correlate well with the areas with both high and rough topography.…”