Abstract

We estimated frequency-dependent attenuation of coda waves (forumla) and body waves (forumla and forumla) 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 forumla 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 forumla and forumla 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 forumla is found to be larger than unity for the whole frequency range. We separated intrinsic absorption (forumla) and scattering attenuation (forumla) using the independent estimates of forumla and forumla. The results show that forumla is close to forumla and both of them are larger than forumla, suggesting that coda decay is predominantly caused by intrinsic attenuation.

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