ALBERT

Description: We estimated frequency-dependent attenuation of coda waves ([IMG]/medium/109eq1.gif" ALT="Formula "〉) and body waves ([IMG]/medium/109eq2.gif" ALT="Formula "〉 and [IMG]/medium/109eq3.gif" ALT="Formula "〉) in 1.5-24 Hz by applying the single isotropic scattering theory and the extended coda-normalization method, respectively, in the crust beneath the Andaman Sea. We used 43 aftershocks of the 13 September 2002 earthquake (Mw 6.5) in the Andaman Sea recorded by three stations installed in the Andaman Islands. The coda Q factors calculated from the amplitude decay rate of the S-wave coda show a dependence on frequency and lapse time. We found that with the increase in lapse time window from 10 to 40 s, Q0 (QC at 1 Hz) increases from 55 to 153, while the frequency-dependent coefficient n decreases from 1.1 to 0.94. The average frequency-dependent relations of [IMG]/medium/109eq4.gif" ALT="Formula "〉 vary from 0.02f-1.1 to 0.01f-0.94 with an increase in lapse time window from 10 s to 40 s, respectively. The values of [IMG]/medium/109eq5.gif" ALT="Formula "〉 and [IMG]/medium/109eq6.gif" ALT="Formula "〉 corresponding to spectral amplitude decays show strong frequency dependence and are expressed as 0.02f-1.01 and 0.01f-1.0, respectively. Our results are consistent with those of other seismically active regions. The ratio [IMG]/medium/109eq7.gif" ALT="Formula "〉 is found to be larger than unity for the whole frequency range. We separated intrinsic absorption ([IMG]/medium/109eq8.gif" ALT="Formula "〉) and scattering attenuation ([IMG]/medium/109eq9.gif" ALT="Formula "〉) using the independent estimates of [IMG]/medium/109eq10.gif" ALT="Formula "〉 and [IMG]/medium/109eq11.gif" ALT="Formula "〉. The results show that [IMG]/medium/109eq12.gif" ALT="Formula "〉 is close to [IMG]/medium/109eq13.gif" ALT="Formula "〉 and both of them are larger than [IMG]/medium/109eq14.gif" ALT="Formula "〉, suggesting that coda decay is predominantly caused by intrinsic attenuation.