Variations of coda Q in the crust of Southern Granulite Terrain (SGT), India

Abstract

In this study, the seismic wave attenuation beneath the Southern Granulite Terrain (SGT), India has been investigated using the coda waves from 22 local earthquakes (2.0 ≤ M_L ≤ 3.6) recorded by 21 broadband seismic stations. The dependence of the attenuation ‘Qc’ on frequency was extracted using the single backscattering model at central frequencies 1.5, 2, 2.5, 4, 5, 8, 10, 12 and 16 Hz. Different lapse time windows, from 10 to 90 s with an interval of 10 s, were used to test the lapse time dependency. The Qc value usually ranges from 150 to 1000 within the frequency range of 2–8 Hz. However, at higher frequencies (12–16 Hz), Qc value variations range from 1500 to 2500. Estimated Qc values are comparatively lower at stations ELP and MVT across frequencies and these lower values are aligned with the previous seismic activity in the surrounding area. This correlation is further supported by the presence of shear zones, lineament, and other northwest-oriented faults, indicating a pronounced level of heterogeneity and complexity in the crust, ultimately contributing to the observed lower Qc values. The estimated Qo values range from 150 to 350, and N values range from 0.70 to 0.95. The observed Qo and N values are slightly lower than those of the other stable continental regions. The significant spatial variation in Qo values observed within the study region may be attributed to the potential existence of pore fluids, as supported by the reported Vp/Vs ratio and shear wave velocity models, which introduce heterogeneities within the crust. Taking into account all other existing studies, along with the current findings, it can be suggested that both scattering (g) and intrinsic attenuation (Qi) factors contribute to the observed attenuation of the study region. The estimated intrinsic and scattering factors align well with the global attenuation model. In the absence of detailed body wave attenuation studies in this region, the frequency-dependent Q relationships developed here are useful for the estimation of earthquake source parameters of the region. These relations may be used for the simulation of earthquake-strong ground motions, which are required for the estimation of seismic hazards and geotechnical and retrofitting analysis of critical structures in the region.