Methodologies for estimating t*(f) from short-period body waves and regional variations of t*(f) in the United States

Der, Zoltan A.; Lees, Alison C.

doi:10.1111/j.1365-246x.1985.tb05131.x

Cited by 45 publications

(41 citation statements)

References 23 publications

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“…Across a broad range of surface-and body-wave frequencies from *0.01 to *100 Hz (e.g., DER et al, 1982DER et al, , 1986LEES et al, 1986;ABERCROMBIE, 1998), Q values consistently increase with frequency in a vast majority of observations. However, attenuation is also always measured indirectly and in the background of strong amplitude variations, and the difficulty of differentiating its apparent attributes from the true in situ properties is well known (DER and LEES, 1985;WHITE, 1992). Predominant observations of positive Q(f) dependencies often reaching and exceeding the linear Q µ f rate still suggest serious concerns about the measurement and interpretation methodology.…”

Section: Introductionmentioning

confidence: 99%

On the Causes of Frequency-Dependent Apparent Seismological Q

Morozov

2010

Pure Appl. Geophys.

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Variability of the Earth's structure makes a firstorder impact on attenuation measurements which often does not receive adequate attention. Geometrical spreading (GS) can be used as a simple measure of the effects of such structure. The traditional simplified GS compensation is insufficiently accurate for attenuation measurements, and the residual GS appears as biases in both Q 0 and g parameters in the frequency-dependent attenuation law Q(f) = Q 0 f g . A new interpretation approach bypassing Q(f) and using the attenuation coefficient v(f) = c ? pf/Q e (f) resolves this problem by directly measuring the residual GS, denoted c, and effective attenuation, Q e . The approach is illustrated by re-interpreting several published datasets, including nuclear-explosion and local-earthquake codas, Pn, and synthetic 50-300-s surface waves. Some of these examples were key to establishing the Q(f) concept.In all examples considered, v(f) shows a linear dependence on the frequency, c = 0, and Q e can be considered frequency-independent. Short-period crustal body waves are characterized by positive c SP values of (0.6-2.0) 9 10 -2 s -1 interpreted as related to the downward upper-crustal reflectivity. Long-period surface waves show negative c LP & -1.9 9 10 -5 s -1 , which could be caused by insufficient modeling accuracy at long periods. The above c values also provide a simple explanation for the absorption band observed within the Earth. The band is interpreted as apparent and formed by levels of Q e & 1,100 within the crust decreasing to Q e & 120 within the uppermost mantle, with frequencies of its flanks corresponding to c LP and c SP . Therefore, the observed absorption band could be purely geometrical in nature, and relaxation or scattering models may not be necessary for explaining the observed apparent Q(f). Linearity of the attenuation coefficient suggests that at all periods, the attenuation of both Rayleigh and Love waves should be principally accumulated at the sub-crustal depths (*38-100 km).

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Section: Introductionmentioning

confidence: 99%

On the Causes of Frequency-Dependent Apparent Seismological Q

Morozov

2010

Pure Appl. Geophys.

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“…1982a;Der & Lees 1985). In the western Pacific, Sipkin &Jordan (1979) have found that multiple ScS waves required a frequency dependence of Q.…”

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confidence: 96%

Frequency dependence of Q in the mantle underlying the shield areas of Eurasia, Part III: The Q model

Der

Lees

Cormier

1986

Geophysical Journal International

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A large set of teleseismic body wave data covering a broad range in frequency was analysed t o determine the frequency arid depth dependence of Q for P-and S-waves under the northern shield areas of Eurasia. Based o n numerous f * estimates for P-and S-waves covering the seismic band between 0.02 and 8 Hz, a Q model of the Eurasian shield was constructed. The data require a model in which Q increases with frequency and which is characterized by Q values in the upper mantle that are generally higher than those of global average models. The model with the best fit includes a minimum in Q between about 100 and 200 k m depth and high Q values on the order of thousands throughout most of the deeper mantle. These results are generally consistent with proposed models of attenuation as a thermally activated process which is influenced by the temperature and pressure in the earth. Preliminary results suggest that t* under tectonic regions is higher than t* under shield regions for all frequencies over the 0.02 to 8 Hz frequency range suggesting that Q varies regionally as well as with frequency and depth. lntroduction Average Q values for the Earth are well defined from free oscillation measurements (Anderson & Hart 1978) in the long-period band. Translating these measurements into tF and I ; estimates for an average spherically symmetric Earth gives values close to I and 4 s respectively for mantle P-and S-waves in the 30 to 90" distance range. However, with the advent o f digital recording of seismic signals over a broad frequency range, it became apparent that the t* values derived from long period, free oscillation observations cannot be applied to short-period signals. It was found that teleseismic short-period P-waves often

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“…Spectral domain measures of intrinsic attenuation such as t * ( f ) (see Der & Lees 1985) that use average spectral shapes should systematically yield lower intrinsic attenuation estimates than time domain estimates of attenuation from the initial P-wave rise time. The spectral estimate of the high-frequency content of the waveform contains the delayed highfrequency energy while the P-wave rise time relies only upon the earliest arriving P-wave energy.…”

Section: Discussionmentioning

confidence: 99%

Stochastic dispersion of short-period P-waves due to scattering and multipathing

McLaughlin

Anderson

1987

Geophys J Int

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The delay of high frequencies in the teleseismic short-period explosion P-wave due to scattering is observed at a number of high Q sites.Recordings for the arrays NORSAR, YKA, GBA, and EKA have been examined as well as the individual sites RSNT, RSON, RSNY, RSSD, ANTO, CTAO, and KONO. P-waves from Eastern Kazakh explosions observed at NORSAR, and several high Q stations show a systematic delay of high frequencies (4-5 Hz) relative to lower frequencies (1 -2 Hz) by as much as 0.5 s. This apparent dispersion is random and variable from station to station at a given array and from event to event at any single station. Hence it is referred to as 'stochastic dispersion' of the P-wave. The stochastic dispersion of the 4-5 Hz energy correlates with the decay of spatial coherency of the teleseismic P-wave.Attempts to model this stochastic dispersion have shown that simple ray theoretical multipathing due to large-scale length scatterers is not sufficient to delay high frequencies with respect to low frequencies as observed. Also, high-frequency scalar scattering theory based on the parabolic wave equation does not correctly model the decline of spatial coherency with increasing frequency. Neither do Gaussian beam synthetics mimic the delays of high frequency seen in the first few seconds of the P-wave. All of the above methods are high frequency approximations to the propagation of seismic waves and do not include wide-angle scattering and conversion.Linear elastodynamic finite difference calculations of 1 -D and 2-D heterogeneous media confirm that high frequencies can be delayed with respect to low frequencies with moderate amounts of heterogeneity in the crust and lithosphere. Simulations of 2-D media demonstrate that significant smallscale heterogeneity in the lithosphere could delay high frequencies with respect to lower frequencies if the spectrum of heterogeneity in the lithosphere were characterized by multiple scale lengths. The long wavelength 934 K. L. McLaughlin and L. M. Anderson heterogeneities in the lithosphere are still required to produce the broadband focusing-defocusing that is observed at arrays.As a result of the delay of 4-5 Hz P-wave energy by an average of 0.5 s as averaged over a 3-6 s window, it is expected that time domain and frequency domain measures of attenuation will show systematic differences. Spectral domain measures of attenuation such as t * that use average spectral shapes should systematically yield lower attenuation estimates than time-domain estimates of attenuation based on initial P-wave rise time. RAY DENSITY DlSTRfBUTlONu = 1.3% (run 61 J -6 k n

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Methodologies for estimating t(f) from short-period body waves and regional variations of t(f) in the United States

Cited by 45 publications

References 23 publications

On the Causes of Frequency-Dependent Apparent Seismological Q

On the Causes of Frequency-Dependent Apparent Seismological Q

Frequency dependence of Q in the mantle underlying the shield areas of Eurasia, Part III: The Q model

Stochastic dispersion of short-period P-waves due to scattering and multipathing

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