Frequency-dependent attenuation of S and coda waves in Erzincan region (Turkey)

Abstract

Abstract The attenuation structure of the Erzincan region is studied using the single scattering model of the coda wave generation and coda normalization method for S waves. We have determined the seismic quality factors Q s ( f ) (for S waves) and Q c ( f ) (for coda waves) as a function of frequency for the frequency range 1.5–24 Hz. The quality factors were derived for 161 seismograms that were registered in a 7-day period after the 13 March 1992 Erzincan earthquake ( M s = 6.8). Digital recordings from six stations at epicentral distances ranging from 5 to 40 km were used. Frequency-dependent attenuation of S waves according to Q s ( f ) = Q o fn in the crust beneath Erzincan was obtained as Q s ( f ) = 35 f 0.83 by analysing seismograms of 161 local earthquakes which were selected on the basis of good signal to noise ratios. Coda Q values were calculated from the amplitude decay rate of the S-wave coda in seven frequency bands from 1.5 to 24 Hz. In order to investigate a possible lapse time dependence of the estimated coda Q , we have carried out the analysis of each seismogram using four coda lengths which correspond to four different lapse times 20, 30, 40, 50 s measured from the origin time of the earthquake. Q c averages about 46 and 766 at 1.5 Hz and 24 Hz, respectively, at 20 s lapse time and has a frequency dependence of the form Q c = 29 f 1.03 . For large lapse time data, t c = 50 s, Q c ( f ) was found to vary from 84 at 1.5 Hz to 783 at 24 Hz with the degree of the frequency dependence, n = 0.81.