Frequency-Dependent Attenuation of Body and Coda Waves in the Andaman Sea Basin

We estimated frequency-dependent attenuation of coda waves (Q(C)(-1)) and body waves (Q(P)(-1) and Q(S)(-1)) 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 (M-w 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, Q(0)(Q(C) at 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 Q(C)(-1) 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 Q(P)(-1) and Q(S)(-1) 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 Q(P)(-1)/Q(S)(-1) is found to be larger than unity for the whole frequency range. We separated intrinsic absorption (Q(i)(-1)) and scattering attenuation (Q(sc)(-1)) using the independent estimates of Q(C)(1) and Q(S)(-1). and The results show that Q(C)(-1) is close to Q(i)(-1) and both of them are larger than Q(sc)(-1), suggesting that coda decay is predominantly caused by intrinsic attenuation.