Attenuation of P, S, and coda waves in Koyna region, India

Abstract

The attenuation properties of the crust in the Koyna region of the Indian shield have been investigated using 164 seismograms from 37 local earthquakes that occurred in the region. The extended coda normalization method has been used to estimate the quality factors for P waves ${\left( {Q_{\alpha } } \right)}$ and S waves ${\left( {Q_{\beta } } \right)}$, and the single back-scattering model has been used to determine the quality factor for coda waves (Q _c). The earthquakes used in the present study have the focal depth in the range of 1–9 km, and the epicentral distance vary from 11 to 55 km. The values of $Q_{\alpha } ,$ ${\left( {Q_{\beta } } \right)}$ and Q _c show a dependence on frequency in the Koyna region. The average frequency dependent relationships (Q = Q ₀ f ⁿ) estimated for the region are $Q_{\alpha } = {\left( {59 \pm 1} \right)}f^{{{\left( {1.04 \pm .04} \right)}}}$, $Q_{\beta } = {\left( {71 \pm 1} \right)}f^{{{\left( {1.32 \pm .08} \right)}}}$ and $Q_{c} = {\left( {117 \pm 2} \right)}f^{{{\left( {0.97 \pm .07} \right)}}}$. The ratio ${Q_{\beta } } \mathord{\left/ {\vphantom {{Q_{\beta } } {Q_{\alpha } }}} \right. \kern-\nulldelimiterspace} {Q_{\alpha } }$ is found to be greater than one for the frequency range considered here (1.5–18 Hz). This ratio, along with the frequency dependence of quality factors, indicates that scattering is an important factor contributing to the attenuation of body waves in the region. A comparison of Q _c and $Q_{\beta }$ in the present study shows that $Q_{\beta } < Q_{c}$ for frequencies below 4 Hz and $Q_{\beta } > Q_{c}$ for the frequencies greater than 4 Hz. This may be due to the multiple scattering effect of the medium. The outcome of this study is expected to be useful for the estimation of source parameters and near-source simulation of earthquake ground motion, which in turn are required in the seismic hazard assessment of a region.