Studying interference of modes and multi-pathing in regional surface wave propagation using synthetic tests and polarization analysis

Kharjana, Inashua (2023) Studying interference of modes and multi-pathing in regional surface wave propagation using synthetic tests and polarization analysis. Masters thesis, Indian Institute of Science Education and Research Kolkata.

Text (MS dissertation of Inashua Kharjana (18MS108)) Thesis_18MS108.pdf - Submitted Version Restricted to Repository staff only Download (50MB)

Abstract

Modeling of fundamental model surface wave dispersion data is one of the widely used methods for constraining the shear-wave velocity structure of the lithosphere. Accurate measurement of 1D path-average dispersion curves from surface waveforms is the first and most important step in this exercise. These 1D dispersion measurements are performed under two broad assumptions: (a) great-circle arc propagation of the surface wave energy and (b) fundamental mode propagation without any coupling/interference with higher modes. In this study, both these assumptions are tested for 14000 regional surface wave paths propagating across India, Himalaya and Tibet. Motivation of this work stems from previous studies, which claimed measurable off-greatcircle propagation of regional short-to-intermediate period surface waves (T = 10–100 s). Such ray path deviation can cause artefacts in anomaly patterns observed in tomographic maps. In addition, there are significant frequency ranges over which the fundamental and the higher modes actually overlap, and thus it is important to test both assumptions using a regional dataset before interpreting surface waves tomography maps. The analysis involves (a) multimode interference analysis and (b) multi-taper polarization analysis. Synthetic multimode dispersion curves are computed for a path-average 3D CRUST1.0 crustal model over IASP91 mantle model. These are compared with the observed dispersion curves for the same ray paths and the segment of the fundamental mode dispersion which is without any higher mode overlap is used for the tomography. Through this analysis it is observed that ray paths propagating through thick sedimentary basins (e.g. the Bay of Bengal and Himalayan foreland basin) and along edges of laterally varying crustal structure (e.g. ocean-continent boundaries and the Himalaya) had higher mode overlap at lower periods (10-40 s). Rayleigh wave polarization analysis isolated multi-pathed energy as a function of period and these periods were removed from the tomography. About 6-7% of the total dataset was removed after the multimode interference analysis and about 5% was removed after the polarization analysis. The shortlisted dataset was used to generate group velocity tomography maps. These are compared with the ones obtained previously and significant improvements are highlighted.

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