Shear wave attenuation characteristics over the Central India Tectonic Zone and its surroundings

摘要

The study area lies between latitude 18-26°N and longitude 73-83°E, and mainly covers the Central India Tectonic Zone (CITZ). The frequency-dependent shear wave quality factor (Q_s) has been estimated over the CITZ and its surrounding using Double Spectral Ratio (DSR) method. We have considered 25 local earthquakes with magnitude (M_L) varies from 3.0 to 4.7 recorded at 11 stations running under national seismic network. The Fast Fourier Transformed (FFT) spectra were computed from the recorded waveform having time-window from onset of S-phase to 1.0 s and for a frequency-band of 0.1-10 Hz. Three different shear wave velocities (i.e., 3.87, 3.39 and 3.96 km/s) were obtained over the study area based on a pair of earthquakes recorded at a pair of stations. The low Q_s values of 51-96 at 1 Hz (i.e., Q_s = 51f~(0.49); Q_s=90f~(0.488) and Q_s-96f~(0.49)) were found in the area covering the Son-Narmada-Tapti (SONATA) lineament, CITZ, eastern part of the Satpura fold belt, Vindhyan and Gondwana basins, Godav-ari and Mahanadi grabens, and southern part of Gangetic plain. Intermediate Q_s values of the order of 204-277 (i.e., Q_s - 204f~(0.56) and Q_s = 277/f~(0.55)) were noted in the cartonic areas, namely, Bundelkhand, Dharwar-Bhandara and Bastar. While the higher Q_s values of 391-628 at 1 Hz (i.e., Q_s = 391f~(0.49), Q_s - 409-f~(0.48), Q_s-417f~(0.48), Q_s-500f~(0.66), Q_s-585f~(0.65) and Q_s = 628f~(0.69)) were found in the eastern part of the SONATA, CITZ, and the northeastern part of the Satpura fold belt. The low Q_s values might be attributing to the more heterogeneous SONATA rift system. Low Q_s values further may presumably be associated with lower-level of seismicity and apparently account for higher tectonic stress accumulation over long duration. The long-term accumulated stress is generally released through occasional triggering of moderate magnitude earthquakes in the SONATA zone. Surrounding the SONATA region, the higher Q, values possibly accounts for a more homogeneous subsurface structure along the SONATA zone.