Lapse time dependent coda-Q (Qc) in the Kachchh, rift zone, Gujarat, India

We study the attenuation of seismic wave energy in the aftershock zone of the 2001 Bhuj earthquake using high-quality (S/N ratio ≥ 2.0) data of 181 aftershocks recorded on eleven three-component broadband seismograph stations. The moment magnitudes and focal depths of the selected aftershocks vary from 2.7 to 4.8 and 2 to 50 km, respectively. The three-component broadband data of these selected aftershocks are used to estimate a new frequency-dependent coda-Q (Qc) relation (Qc = Q0fn) in the frequency range 1.5–15 Hz, for the seismically active Kachchh rift zone, Gujarat. We have used single backscattering model to estimate frequency-dependent values of coda Qc at central frequencies 1.5, 3, 6, 9, 12 and 15 Hz through eight lapse time windows (tL) from 20 to 90 s starting at double the travel time of S-waves. The average frequency-dependent Qc relationship in the range of 1.5–15 Hz for the Kachchh region is estimated to be (82 ± 1) f(1.12±0.01), (121 ± 1) f(1.03±0.01), (154 ± 1) f(0.96±0.01), (181 ± 1) f(0.91±0.01), (207 ± 1) f(0.85±0.01), (239 ± 1) f(0.79±0.01), (271 ± 1) f(0.73±0.01) and (304 ± 1) f(0.68±0.01), for lapse time windows of 20, 30, 40, 50, 60 70, 80 and 90 s, respectively. For tL = 20–50 s, we estimate Qc = 52–163 and n = 0.87–1.18, while for tL = 60–90 s, we find Qo = 128–486 and n = 0.45–1.05. These estimates of Qo and n are found to be in good agreement with the estimates of Qo and n for highly heterogeneous and seismically active regions around the world. We also estimate actual hazard parameters, viz. the extinction distances (Le) and anelastic attenuation coefficients (γ), by assuming that attenuation is entirely intrinsic. Le estimates vary from 56 to 99 km, while the corresponding γ estimates range from 0.009 to 0.0054 km−1. The estimated Qo values at locations in the Mesozoic Kachchh rift basin are found to be low in comparison with the areas at a distance from it. This could be attributed to the fact that seismic waves being highly scattered for paths through the seismically active and fractured zone, but less scattered outside the aftershock zone. This model also gets support from the presence of 1- to 2-km-thick top low-velocity Quaternary sediments, and a large volume of crustal and upper mantle metamorphic fluids/carbonatite melts underlying the central rift basin. We also propose that the Qc relations developed in this study could be useful for predicting ground motion in the region.