Estimation of Coda Q for northeast India using nonlinear regression
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Ó Indian Academy of Sciences (0123456789().,-volV)(0123456789( ).,-volV)
Estimation of Coda Q for northeast India using nonlinear regression PARDEEP SANGWAN1,* and DINESH KUMAR2 1 Paradigm 2
Geophysical, Mumbai 400 076, India. Department of Geophysics, Kurukshetra University, Kurukshetra, India. *Corresponding author. e-mail: [email protected] MS received 1 October 2019; revised 13 July 2020; accepted 15 July 2020
Classical logarithmic linearization of the single backscattering model to estimate the coda Q (Qc Þ leads to biased results in the presence of low signal-to-noise ratio. Non-linear regression using the Levenberg–Marquardt (L–M) method has been proposed to estimate the Qc in the frequency range of 3–24 Hz on local earthquakes recorded in northeast region of India. Results of both classical log-linear and nonlinear approaches to the single backscattering model are compared. On datasets with good signal-to-noise ratio both the approaches lead to almost the same results. However, for datasets having low signal-tonoise ratio, we found that the log-linear technique estimates are biased. Results demonstrate that the loglinear approach overestimates the Qc in comparison to the non-linear approach. Frequency dependence parameter ‘g’ for the L–M method is slightly higher than the conventional log-linear approach. The Qc variation with lapse-time is also studied with both the approaches. The biased results of log-linear approach were observed at different frequencies at all lapse times. Significantly lower Q0 (1 Hz) estimates of non-linear approach indicated more heterogeneous lithosphere than the log-linear approach. Keywords. Coda wave; lapse time; northeast India; nonlinear regression; quality factor; scattering; single backscattering model.
1. Introduction Seismic quality factor is a very essential parameter to understand the medium both in earthquake as well in exploration seismology. The seismic waves suffer differential spectral amplitude losses while propagating through the Earth, which is described as the energy loss per cycle (KnopoA and Hudson 1964). In exploration seismic, the quality factor of P waves is generally estimated to improve the seismic resolution and to infer the petro-physical properties of the subsurface. However, in earthquake seismology attenuation of the different phases P, S, coda-wave, Lg, surface waves, etc., is studied for earthquake hazard assessment (Aki
1969; Aki and Chouet 1975; Sato 1977; Mitchell 1981a, b; Yoshimoto et al. 1993; Mitchell 1995; McNamara et al. 1996; Xie 2002; Sharma et al. 2007; Ma’hood and Hamzehloo 2009; Hazarika et al. 2013), and to decipher the tectonic activity of the region and the earth’s structure from the crust to core (Anderson et al. 1965; Romanowicz and Mitchell 2007). Coda of local earthquakes is often studied to understand the seismic wave attenuation in the lithosphere (Aki 1969; Aki and Chouet 1975; Sato 1977). Coda is basically backscattered S waves from the heterogeneities in the subsurface and provide average attenuation of the p
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