Attenuation and Anisotropic Shear-wave velocity Structure in the Eastern Himalayan Plate Boundary Systems, Northeast India

Sharma, Shubham (2018) Attenuation and Anisotropic Shear-wave velocity Structure in the Eastern Himalayan Plate Boundary Systems, Northeast India. Masters thesis, Indian Institute of Science Education and Research Kolkata.

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Abstract

The Eastern Himalayan Plate Boundary System comprises the Eastern Himalaya, the Himalayan foreland basin, the Shillong Plateau, the Bengal Basin and the Indo-Burman convergence zone. The Himalayan arc meets the Indo-Burman subduction zone orthogonally to form the Eastern Himalayan Syntaxis. This region has experienced several devastating earthquakes in past which caused tremendous loss of life and property. Estimation of ground motions from future earthquakes and understanding the evolution of this complex plate boundary system and are the two broad objectives addressed through this research. This work is divided into (a) study of the lateral variation of seismic attenuation and (b) modeling of anisotropic shear wave velocity structure across the Eastern Himalayan Plate Boundary System, Northeast India. In the first part we use local earthquake coda waveform to estimate the seismic attenuation (Qc) of the Eastern Himalayan and Indo-Burman plate boundary systems. We measure temporal decay of coda amplitude to estimate Qc at frequencies 1 to 14 Hz, and abstract Q₀ and its frequency dependence (η). Single-trace measurements reveal similar Q₀ with strong lateral variation in η. Single trace Qc measurements are combined in a back-projection algorithm to compute 2-D Qc tomographic maps. The Qc maps reveal strong correlation with tectonic settings. At low frequencies (1{5 Hz) we observe relatively lower Qc in the Eastern Himalaya, southern Tibetan Plateau and the Bengal Basin, while the intra-plate region and the Indo-Burman subduction zone has higher Qc. At higher frequencies (>5 Hz) we observe pronounced low Qc beneath Sikkim Himalaya, relatively lower Qc in the intra-plate region and relatively higher Qc in the Bengal Basin and Indo-Burman subduction zone. 2-D variation in Q₀ and η values also reveal strong lateral heterogeneity, and is comparable to other tectonically active regions across the globe. We interpret our results by comparing the Qc maps with S-wave velocity tomography to show that at shallow depth the low Qc and intermediate Vs in the Himalaya is due to structural heterogeneity and deformation in the Himalayan wedge. The low Qc and low Vs in the southern Tibetan Plateau is due to elevated temperatures and partial melts in the crust. The low Qc and low Vs in the Bengal Basin is due to the thick pile of sediments overlying a rift-faulted transitional crust. In the second part we model the Rayleigh and Love wave group velocity dispersion to estimate transversely isotropic shear wave velocity structure. We develop an inversion framework to estimate upper-mantle anisotropy along a 2-D profile across Indian- Eurasian collision zone. The framework uses a starting model which is constituted from two previously available Earth models. We use Genetic Algorithms to perform a global search of the model space and optimizing the misfit between observed and calculated dispersion data. Synthetic tests have been done along 2-D profiles to validate the inversion algorithm and estimate the uncertainties. This procedure will be tested on observed data from the Indian continent-continent collision across northeast India.

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