4. Seismic Attenuation

Sivaram and Gupta (2022) investigated the frequency-dependent seismic attenuation characteristics of the crust beneath the Kumaun Himalaya using seismic coda waves (Qc-1) and high-frequency body waves (Qα-1 and Qβ-1). The results show that that the seismic attenuation is different for the Lesser Himalaya and the Higher Himalaya segments, which could possibly be due to the mechanism of underthrusting and deformation in the Lesser Himalaya segments, leading to a dominant scattering attenuation and the multitude of fractures and pores in the crust.
    Seismic wave attenuation study has been performed for the Kinnaur and Garhwal-Kumaun regions of the NW Himalaya using data of low magnitude- and micro-earthquake events (Kumar and Yadav, 2019; Kumari et al., 2020, 2021; Monika et al., 2020; Kumar P., et al., 2021). Attenuation characteristic divide the Kinnaur Himalaya into three zones correlating with crustal/lithosphere structure and micro-earthquake activity. Decreasing attenuation with depth indicates more heterogeneities in the upper crust which can be explained by the effect of turbidity. The attenuation largely depends on the heterogeneities developed due to the collision of the Indian and the Eurasian plates. The attenuation is larger in the Tethys Himalaya than in the High Himalayan crystallines. Small value of the resonance frequency and comparatively high attenuation to the north of the South Tibetan Detachment Zone indicate presence of low-grade meta-sedimentary rocks in the upper crust of the Tethys Himalaya (Kumar P., et al., 2021).
    A series of studies on coda, body wave, and surface wave attenuation have been carried out for the Tibet and Nepal Himalaya regions. These studies suggest dominance of intrinsic attenuation for the Nepal Himalaya and southern Tibet. The second region shows dominance of scattering attenuation at higher frequencies (> 8 Hz) (Singh C., et al., 2019a; Biswas and Singh, 2019). Similar studies have been carried out for western Tibet and Karakoram fault region (Biswas and Singh, 2020a, 2020b, 2020c; Sarkar et al., 2021; Jaiswal et al., 2022), and southeastern Tibet (Tiwari A.K., et al., 2022b).
    Singh C., et al. (2019a) investigated the spatial variations of coda wave attenuation structure using local events for Andaman-Nicobar Subduction Zone. The study shows high attenuation near Narcondum volcanic island, which also coincides with low-Vp zone, suggesting change in crustal properties. The results also reveal a good correlation between Q\(_{\circ}\) and seismicity, suggesting the presence of a highly scattered medium.
    Singh C., et al. (2019b) investigated the spatial variations of Pg attenuation structure in Nepal Himalaya by the “Two-Station method” applied to 2325 waveforms obtained from 435 events recorded at 151 stations deployed across Nepal Himalaya. Their results suggest that the areas around the existing faults and lineaments exhibit very low Q values. They inferred that intrinsic attenuation plays a major role in causing the high apparent Pg attenuation in the crust of Nepal Himalaya. This may be mainly caused by highly pressurized fluids trapped within a thin low-velocity layer (LVL) at shallow depths.
    Das and Mukhopadhyay (2020) studied the spatial variation in attenuation characteristics in Northeast India using coda Q. The average frequency dependencies of coda wave attenuation for a 30 s window length are estimated as Qc(f) = 135 ± 7f0.99± 0.03, Qc(f) = 109 ± 7f1.10±0.03 and Qc(f) = 90 ± 2f1.04±0.02 for Shillong Plateau, Mikir hills and surrounding River valley, and Indo-Burma Ranges respectively. It is observed that Q0 is greater for the Shillong Plateau than the other sub-regions. For window lengths \(\ge\) 55 s, the central part of the Indo-Burma Ranges has higher Qc values at 10 and 12 Hz compared to the Shillong plateau.
    Lg-Q for the Indian Shield derived from tomographic inversion varies from 50 to 650, while the frequency-dependent parameter varies between 0.4 and 1.1 with an average value of 0.7. Structural features such as rifts, suture zones, sedimentary, and active regions are characterized by high attenuation (Q\(_{\circ}\)<200) (Reshma et al., 2022).