Study of lapse time dependence coda Q in the Andaman Islands using the aftershocks of the 2002 earthquake (Mw 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 similar to 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.