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

Abstract

We estimated frequency-dependent attenuation of coda waves (![Graphic][1] ) and body waves (![Graphic][2] and ![Graphic][3] ) 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 ( Q C 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 ![Graphic][4] vary from 0.02 f -1.1 to 0.01 f -0.94 with an increase in lapse time window from 10 s to 40 s, respectively. The values of ![Graphic][5] and ![Graphic][6] corresponding to spectral amplitude decays show strong frequency dependence and are expressed as 0.02 f -1.01 and 0.01 f -1.0, respectively. Our results are consistent with those of other seismically active regions. The ratio ![Graphic][7] is found to be larger than unity for the whole frequency range. We separated intrinsic absorption (![Graphic][8] ) and scattering attenuation (![Graphic][9] ) using the independent estimates of ![Graphic][10] and ![Graphic][11] . The results show that ![Graphic][12] is close to ![Graphic][13] and both of them are larger than ![Graphic][14] , suggesting that coda decay is predominantly caused by intrinsic attenuation. [1]: /embed/inline-graphic-1.gif [2]: /embed/inline-graphic-2.gif [3]: /embed/inline-graphic-3.gif [4]: /embed/inline-graphic-4.gif [5]: /embed/inline-graphic-5.gif [6]: /embed/inline-graphic-6.gif [7]: /embed/inline-graphic-7.gif [8]: /embed/inline-graphic-8.gif [9]: /embed/inline-graphic-9.gif [10]: /embed/inline-graphic-10.gif [11]: /embed/inline-graphic-11.gif [12]: /embed/inline-graphic-12.gif [13]: /embed/inline-graphic-13.gif [14]: /embed/inline-graphic-14.gif