A Multi-Scale Seismic Attenuation Study: Lg Coda Q Mapping of India-Himalaya-Tibet and 3D Body-wave Attenuation of the Kashmir `Seismic Gap'

Chaudhuri, Dibyajyoti (2026) A Multi-Scale Seismic Attenuation Study: Lg Coda Q Mapping of India-Himalaya-Tibet and 3D Body-wave Attenuation of the Kashmir `Seismic Gap'. PhD thesis, Indian Institute of Science Education and Research Kolkata.

Text (PhD thesis of Dibyajyoti Chaudhuri (18RS086)) 18RS086.pdf - Submitted Version Restricted to Repository staff only Download (14MB)

Abstract

Seismic attenuation provides valuable insights into Earth's structure and deformation that complement seismic velocity models. Previous studies in the India-Himalaya-Tibet region, though abundant, have mostly been limited to local tectonic domains, and few have quantified and mapped the intrinsic and scattering components of attenuation. In the first part of this work, Lg coda quality factor (Q₀) and its frequency dependence, η are estimated from 5009 event station pairs across India and Tibet by stacking spectral amplitude ratios along the coda. These single-trace measurements are spatially averaged and mapped to reveal lateral variations in attenuation. Comparisons with surface-wave tomography highlight a prominent east west attenuation contrast across Tibet, linked to variations in the style of Indian Plate underthrusting. Low-Q sedimentary basins are clearly expressed, while attenuation variations across the Indian cratons likely reflect differing degrees of crustal heterogeneity. The second part of this work develops a 3D attenuation model of the Kashmir 'Seismic Gap' in the NW Himalaya using local earthquake waveforms from the Jammu and Kashmir seismological network. 3D S-wave attenuation is computed using coda-normalization, and frequency-dependent seismic absorption (from coda quality factors) and scattering (from peak delay times) are jointly separated and mapped. The results show high attenuation and absorption within the sedimentary and meta-sedimentary units of the Sub-Himalaya and the Lesser Himalayan Duplex in the Kishtwar Window, and low attenuation with weak scattering in the Higher Himalayan crystalline. Elevated attenuation and scattering within the Kishtwar Window and its surroundings are attributed to concentrated seismicity associated with the frontal ramp of the Main Himalayan Thrust. The Kashmir Valley exhibits lateral contrasts, indicating varying sedimentary thickness, while parts of the Indian upper crust beneath the Higher Himalaya and Pir-Panjal Ranges show high attenuation.

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