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).