1 ANNALS OF GEOPHYSICS, 61, 6, SE668, 2018; doi: 10.4401/ag-7917 “ P- AND S-WAVE ATTENUATION IN THE KOPILI REGION OF NORTHEASTERN INDIA„ Nilutpal Bora 1 and Rajib Biswas *,1 (1) Geophysical Lab, Department of Physics, Tezpur University, Tezpur, Assam, India 1. INTRODUCTION Seismic waves are in general influenced by path ef- fect, source effect and site effect. While propagating, there occurs diminution of amplitudes as well as energy of these seismic waves. This is referred to as seismic at- tenuation, designated by the term Q -1 . Seismic attenu- ation arises basically from two constituents, viz, body and coda waves. Between them, attenuation from body waves bears signature of the formation and material state of the interior part of the Earth [Sato, 1992]. Again, attenuation from body waves has got paramount importance in the prediction of ground mo- tion parameters. The causes for attenuation can be var- ious factors. Geometrical spreading and redistribution of seismic energy due to its passage through diverse medium need special mention [Tsujiura, 1966; Aki, 1969,1980; Aki and Chouet, 1975; Sato and Fehler, 1998; Mukhopadhyay et al., 2010]. The quality factor Q attempts to quantify the diminu- tion of progressive seismic amplitudes from source to re- ceiver site. Q also relates to the energy disipation in ac- cordance with Knopoff and Hudson [1964], Aki and Chouet [1975]. Sato and Fehler [1998] reported that seismic activity of a region can be classified with mag- nitude of Q where lower Q characterizes higher activity and lower activity is implied by greater magnitude of Q. Q also bears direct correspondence with total har- monic wave energy. The harmonic wave energy is math- ematically expressed as: 2π/Q=-DE/E [Knopoff and Hud- son, 1964], where E denotes total energy available in the harmonic wave and DE is the energy lost per cycle. Since P and S wave are manifested by varying seismic ener- gies, body wave quality factors also exhibit different characteristics. Throughout the world, researchers have adopted diverse techniques to study quality factors [Aki, Article history Receveid September 6, 2018; accepted February 5, 2019. Subject classification: Attenuation; Seismic wave; Propagation; Quality factor; Heterogeneity. ABSTRACT We comprehensively analyze attenuation factor in Kopili region and its neighboring area. Corresponding to influence of frequency on at- tenuation, Q P and Q S are estimated from direct P and S waves. The results establishes higher values of Q for S waves (Q S =(101±4)f(1.4±0.05)) than for P waves (Q P =(45±3)f(1.3±0.07))in the 1.5-12 Hz frequency band. It is seen that our findings agree with other published results on tectonically active regions that is characterized by high degree of heterogeneity. Hence, it can be useful for further studies in ground mo- tion prediction in Kopili region. The low value of quality factors corresponding to P and S waves refers to seismically actives areas with complex heterogeneity. The estimated value of Q S /Q P and their subsequent association with frequency affirm the dominance of scattering as a major contribution to diminution of seismic amplitudes. This is further corroborated by local geological settings.