Depth-wise attenuation mechanism of seismic waves in the Andaman region

This study evaluates the depth-dependent seismic wave attenuation characteristics in the Andaman region which is a part of the Andaman-Nicobar Subduction zone (ANSZ), and possible causes. We have analyzed seismic waveforms of 1402 earthquakes recorded at seven broadband seismic stations located over the Andaman Island, to estimate the direct S-wave (Q(s)), coda wave (Q(c)), intrinsic (Q(i)) and scattering (Q(sc)) attenuation parameters. The coda normalization method is used to calculate Q(s). Additionally, the variation of Q(c) with frequency and coda window length is studied using the single back-scattering method. The separation of Q(i) and Q(sc) is achieved using the Wennerberg's method. Results show a strong frequency dependence of Q(c) and Q(s). At 1 Hz, the values of Q(c) are 136, 137, 137 and 146 for coda window length of 30, 40, 50 and 60 s respectively, with the corresponding frequency-dependent coefficient (n) of 0.82, 0.83, 0.85 and 0.82. At 1 Hz the observed Q(i) values show a close proximity to Q(c) values, which indicates a strong dominance of intrinsic attenuation. Also, it is observed that Q(0i) shows low value and remains almost the same between 115 and 132 km depth, with a slight increase in between 132 - 140 km. The observed results imply high intrinsic attenuation, possibly due to partial melting or fluid filled rocks at these depths. Whereas Q(0sc) shows a different trend. It gradually increases up to a depth of 132 km, with a slight decrease beyond that. This indicates a decrease in heterogeneity up to 132 km and a slight increase in the same below that. Further, the estimates of pressure (P) and temperature (T) conditions of the subducting slab with depth show an increase in temperature from 996 degrees to 1015 degrees C and an increase in pressure from 33.8 to 41.2 kbar, as the depth increases from 115 to 140 km. The phase relationship for the basaltic rocks with 5 6 wt% bound H2O in eclogite facies reveals that the P - T condition observed in this study lies above the solidus temperature. This promotes the formation of interstitial liquid (5-10%) at these depths, probably causing high intrinsic attenuation.