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Elastic Character of Seismic Coda Envelopes Within East Indian Shield

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Abstract

Seismic codas are usually characterized by the values of coda Q (Qc). However, interpretation of this quantity is often tricky because of its frequency dependence and acute sensitivity to subjective theoretical assumptions and processing parameters. Here, a simpler and physically more consistent parameterization of coda envelopes is proposed by noting that their temporal decay rates are often nearly frequency-independent. This weak frequency dependence shows that codas mostly consist of elastic reverberations and scattering on larger-scale structures, and the subwavelength-scale scattering and Q-type wave attenuation are weak. A recent study of the eastern Indian Shield by Singh et al. (in this journal) gives an illustration of such elastic coda. From that study, the inferred Qc steeply increases with frequency, lapse times, window lengths, and distances from the seismic station. However, we show that all of these dependencies of Qc represent a common artifact of the acquisition geometry and inversion procedure. In an alternate interpretation, we explain the same coda envelopes by two frequency-independent properties of the Earth’s subsurface: geometrical attenuation denoted γc,Earth and effective Q denoted Qc,Earth. Based on these parameters, the model becomes independent of theoretical assumptions and comparable to other areas, and the acquisition/inversion artifact is reduced. The estimated γc,Earth is above 0.01 s−1, which is also found in other areas of active tectonics. The effective attenuation is weak (Qc,Earth > 5700, likely below the measurable level), which is typical for stable tectonic areas. The data indicate near-surface resonances beneath the recording station. Effects of these resonances on coda envelopes also exceed those of Q-type attenuation. Thus, in the eastern Indian Shield and likely many other areas, coda envelopes are principally controlled by elastic structures such as crustal and near-surface layering, and not necessarily by the S-wave Q and uniformly-distributed random, small-scale scattering as commonly thought.

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Fig. 1

Source waveforms are schematically modeled by Gabor wavelets. Note that the scattering points are not necessarily located on a straight line

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Notes

  1. Singh et al. (2019) provide a resolution estimate that appears comparable to the footprint area (their Fig. 8). However, this estimate refers only to their inverse approach and does not account for the non-uniform weighting of the forward model (Safarshahi and Morozov 2020).

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Acknowledgements

Although the authors of Singh et al. (2019) did not wish to participate in revisiting their results, we highly commend the quality and detail of presentation in their paper. This detail has facilitated the additional analysis carried out in the present paper. Comments and discussions by two anonymous reviewers have contributed to improving this manuscript. This study was funded by the Natural Sciences and Engineering, Natural Sciences and Engineering Research Council of Canada.

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  1. Department of Geological Sciences, University of Saskatchewan, 114 Science Place, Saskatoon, Canada

    Igor B. Morozov & Maryam Safarshahi

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  1. Igor B. Morozov
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Correspondence to Igor B. Morozov.

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Morozov, I.B., Safarshahi, M. Elastic Character of Seismic Coda Envelopes Within East Indian Shield. Pure Appl. Geophys. 177, 5799–5818 (2020). https://doi.org/10.1007/s00024-020-02600-2

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