Separation of intrinsic and scattering seismic wave attenuation in Northeast India

Abstract

SUMMARY We have analysed the local earthquakes (2.0 ≤ ML ≤ 5.5) occurred in northeast (NE) India recorded by a temporary seismic network of 10 stations operated by National Geophysical Research Institute (NGRI), Hyderabad to evaluate the relative contributions of scattering loss (Q −1 ) and intrinsic absorption (Q −1 ) to total attenuation (Q −1 ) using the multiple lapse time window analysis assuming multiple isotropic scattering in a medium of uniformly distributed scatterers. The results show that Q −1 is greater than Q −1 at high frequencies (f > 3Hz), while the opposite is observed at low frequencies (f < 3Hz). The observed frequency dependence of Q −1 corresponds to the scale length of lithospheric heterogeneity beneath NE India, at least comparable with the wavelength for the lowest frequencies analysed, of about 1km. The observed Q −1 for the study region obtained with single scattering theory is close to Q −1 at high frequencies, in agreement with theoretical prediction for an idealized case of uniform distribution of scatterers. However, a discrepancy exists between the two at low frequencies, which can be explained by a depth-dependent velocity and attenuation structure. High value of Q −1 is correlated with the geology and tectonic settings of the region characterized by Himalayan and Burman collision zones with a strong lateral heterogeneity. The Q −1 estimates obtained in this study can be used to infer the average temperature of the lower crust with an upper limit estimate of ∼800 ◦ C assuming a lower crustal gabbroic lithology.