Mapping of Coda-Wave Attenuation and Its Frequency Dependency Over Eastern Indian Shield

Abstract

The study area comprises two tectonic blocks namely, the Chotanagpur microplate and the Singhbhum microplate. Waveform data of 112 local earthquakes of magnitude MW ≥ 2.5 were considered for the present analysis. Coda Q (QC) for each event at eight different central frequencies (1–14 Hz) and four different coda windows was estimated using back-projection algorithm. The average Q0 (QC at 1 Hz) and η (exponent) values were regionalized by projecting each value in selected square grid of dimension 0.1° × 0.1°. Back-projection algorithm was used for inversion of QC, and the final values were computed after ten iterations when the residual error was reduced to acceptable level. Inversion was stabilized by a nine-point spatial smoothening Gaussian filter to compute the QC values for each frequency and subsequently, combined values of Q0 and η were also computed. The final power law equation of the coda wave attenuation for the concerned region was found to be QC = (336.38 ± 16.50)f(0.83±0.036). Q0 varies from 180 to 255, 270 to 360, 297 to 386 and 340 to 420, and η vary from 0.9 to 1.02, 0.79 to 0.89, 0.79 to 0.88, and 0.79 to 0.86 for 30, 35, 40 and 45 s coda windows. Increase in Q0 and decrease in η values with increase of coda window length shows the depth dependency of seismic wave attenuation at 1 Hz frequency. The central part, surrounding the receiver, lying at the junction of the Chotanagpur and the Singhbhum tectonic provinces, documents lower Q0 and higher η values, and presumably caused by more heterogeneities present in the area. The higher concentration of lower magnitude earthquakes in this area support this observation. The mapping values of attenuation can be utilized for hazard assessment of the area.