Study of lapse time dependence coda Q in the Andaman Islands using the aftershocks of the 2002 earthquake (M w 6.5)

The attenuation of seismic wave energy in and around the source area of the Andaman earthquake of 13 September 2002 is estimated using high-quality aftershock data. Thirteen aftershocks, duration magnitude (M d ) ranging between 2.9 and 4.1, depth >25 km, which were well recorded by a temporary digital seismic network established in the Andaman Islands to monitor the aftershocks are used to estimate coda Q (Q c ) values in this interesting and complex tectonic environment. We have used single backscattering model to calculate frequency-dependent values of coda Q c . The estimation of Q c is made at central frequencies 1.5, 2, 3, 4, 6, 8, 12 and 16 Hz through five lapse time windows from 10 to 50 s starting at double the travel time of the S-wave. The observed Q c is found to be strongly frequency dependent, which indicates that the region is tectonically active with high heterogeneities. The trend of variation of Q c with lapse times and the corresponding apparent depths is also studied. It is observed that between ~142 and 150 km apparent depth there is a change in attenuation property. At lesser depths, Q c increases faster and at greater depths and it increases at a lower rate for lower frequency ranges. At higher frequencies, within this depth range, Q c remains more or less constant, or becomes slightly lower. This may be indicative of change in the mechanical property of the subducted slab at such depth ranges. Copyright Springer Science+Business Media Dordrecht 2015