Seismic attenuation in Indian shield and Himalayan regions and its implications

Seismic attenuation is a key parameter to elucidate the properties of the Earth's internal structure and is defined in terms of Quality factor Q, a dimensionless quantity which is the relative energy loss per oscillation cycle. Various studies have been carried out in the Indian shield and Himalayan regions to understand the risks and hazards due to the earthquake, using attenuation studies. Here, we provide a bird's eye view on the attenuation studies carried out using Qp, Qs and Qc, which are the attenuation values of P, S and coda wave respectively in the Indian shield and Himalayan regions by combining the available published works by various researchers. Based on the available data set, four broad classifications have been suggested i.e. shield and cratonic regions, rift zones, orogenic belts, and sedimentary basins. Further, an attempt has been made to correlate these values with the geological provinces. It is found that the attenuation value, which has a large variation, agree very well with the geology of the region. The entire Indian shield has in general Qs/Qp > 1, implying that the scattering is a predominant factor which causes the attenuation. Interestingly, in sedimentary basins and rifted regions of India, Q values have been found less, indicating more attenuation of seismic waves. Qs/Qp < 1, would also imply that the underlying rocks are partially fluid-saturated. In addition, we also found that the attenuation values have an anti-correlation with the surface heat flow values, suggesting that the attenuation of seismic waves in the Indian shield and Himalayan region, is not only controlled by the temperature, but also influenced by the other factors such as composition to a large extent.