1 Hz Lg Q tomography in central Asia

Phillips, W. S.; Hartse, Hans E.; Taylor, Steven R.; Randall, George E.

doi:10.1029/2000gl011482

Cited by 39 publications

(28 citation statements)

References 18 publications

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“…Comparisons with other distance‐ and magnitude‐matched pairs indicate that there is nothing anomalous about the Lg signals from events on the northern Tibetan Plateau margin relative to events elsewhere in central Eurasia. This indicates that Lg propagation across the eastern Tarim Basin is similar to that elsewhere in the China platform, consistent with results from Lg attenuation tomography by Phillips et al [2000]. It also appears that there is no abrupt effect on Lg associated with the rapid increase in topography and presumably rapid crustal thickening along the northern margin of the plateau, as similar waveforms and spectra are observed for events located beneath topography ranging from 2000 to 4000 m. This is consistent with prior observations.…”

Section: Distance Dependence Of Lg Spectra In Tibetsupporting

confidence: 88%

Characteristics of Lg attenuation in the Tibetan Plateau

Fan

Lay

2002

J. Geophys. Res.

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[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 ...

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Section: Distance Dependence Of Lg Spectra In Tibetsupporting

confidence: 88%

Characteristics of Lg attenuation in the Tibetan Plateau

Fan

Lay

2002

J. Geophys. Res.

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“…Lg waves and their coda have been used successfully for the past three decades to infer and measure the attenuation structure of the crust (Nuttli, 1973;Kadinsky-Cade et al, 1981;Xie and Nuttli, 1988;McNamara et al, 1996;Mitchell et al, 1997;Rodgers et al, 1997;Cong and Mitchell, 1998;Mellors et al, 1999;Philips et al, 2000;Sandvol et al, 2001). Lg, the most prominent phase over stable continental paths, is usually found in a group-velocity window between 3.6 and 3.0 km/sec at a frequency band of 0.2 to 5 Hz.…”

Section: Introductionmentioning

confidence: 98%

“…Single-station methods have also been used by applying a correction for source size based on independent measures such as teleseismic m b to account for the source contribution to Lg amplitudes. Using m b directly to calculate seismic moment in regional studies may cause upper mantle bias in the Q-measurements (Marshall et al, 1979;Philips et al, 2000;Taylor et al, 2003). Taylor et al (2003) specified the source terms to absorb some of the problem with the m b bias associated with source scaling.…”

Section: Introductionmentioning

confidence: 99%

“…Because of the complexity of coda generation and propagation and other types of nonuniqueness in modeling coda such as the unclear role of modal conversions, several studies have used direct Lg waveforms processed with a two-station method (e.g., McNamara et al, 1996;Xie et al, 2004). Alternatively, Philips et al (2000) and Sandvol et al (2001) applied tomography to Lg/ Pg-amplitude ratios. These methods assume that the source effects are canceled in the spectral ratio of the Lg and Pg phases.…”

Section: Introductionmentioning

confidence: 99%

“…These methods assume that the source effects are canceled in the spectral ratio of the Lg and Pg phases. This approach has some difficulties in interpretation because of complexities in Pg propagation and the respective corrections (Philips et al, 2000). Single-station methods have also been used by applying a correction for source size based on independent measures such as teleseismic m b to account for the source contribution to Lg amplitudes.…”

Section: Introductionmentioning

confidence: 99%

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Crustal Attenuation within the Turkish Plateau and Surrounding Regions

Zor

Sandvol

Xie

et al. 2007

Bulletin of the Seismological Society of America

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The lateral variations of seismic attenuation in the crust of Turkey and surrounding regions have been imaged from the inversion of interstation Lg Q-measurements.This study develops the first tomographic model for Lg Q 0 in this region. That model is consistent with previous more qualitative Lg-attenuation models that showed inefficient or blocked Lg across the Eurasian-Arabian plate boundary. In general, the northern Arabian platform has low to normal Lg Q 0 -values (ϳ250-350), whereas high Q 0 -values (ϳ670-800) occur in the southern Arabian Plate. Additionally, we have found a dramatic decrease in Lg Q 0 across the Bitlis suture, the plate boundary between the Arabian and Eurasian plates. Beneath the Turkish Plateau, our high to moderate Lg-attenuation values (Q 0 ϳ100-200) probably originates from both scattering and intrinsic attenuation due to the tectonic complexity and the widespread young volcanics in the region. We conclude that the lowest Q 0 -values for the East Anatolian plateau (ϳ70-100) and the portion of western Turkey around the Menderes Massif (ϳ60-150) are probably caused by intrinsic attenuation. We find low to normal Lg Q 0 -values (ϳ150-300) beneath the part of the Taurus Mountains in western Anatolia. These higher values may be related to the nature of the crust in the Tauride mountain belt. We also mapped the lateral variations in Lg g and observed a fairly consistent negative correlation between g and Lg Q 0 in the northern Middle East.

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