Elastic Character of Seismic Coda Envelopes Within East Indian Shield
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Pure and Applied Geophysics
Elastic Character of Seismic Coda Envelopes Within East Indian Shield IGOR B. MOROZOV1 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 cc,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 cc,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. Keywords: Crust, measurement, seismic coda, frequency dependence of Q, scattering, seismic attenuation.
1 Department of Geological Sciences, University of Saskatchewan, 114 Science Place, Saskatoon, Canada. E-mail: [email protected]
and MARYAM SAFARSHAHI1 1. Introduction In a continuously growing number of studies of seismic codas from local and regional earthquakes, frequency-dependent coda Q (denoted Qc(f) here) is measured for numerous areas. The observed Qc is usually explained by a similar ‘‘quality-factor’’ property of the Earth, which is further subdivided into S-wave and P-wave, intrinsic, scattering, and other types of Q. However, despite the long history and routine use of seismic Q-factors, their physical meanings and particularly frequency dependencies are still poorly understood. Apart from the general trust in the notation of ‘‘Q’’, phy
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