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Abstract:
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The Andaman-Nicobar Islands in the Bay of Bengal lies in a zone where the
Indian plate subducts beneath the Burmese microplate, and therefore forms a belt
of frequent earthquakes. Few efforts, not withstanding the available historical and
instrumental data were not effectively used before the Mw 9.3 Sumatra-Andaman
earthquake to draw any inference on the spatial and temporal distribution of large
subduction zone earthquakes in this region. An attempt to constrain the active
crustal deformation of the Andaman-Nicobar arc in the background of the December
26, 2004 Great Sumatra-Andaman megathrust earthquake is made here,
thereby presenting a unique data set representing the pre-seismic convergence and
co-seismic displacement.Understanding the mechanisms of the subduction zone earthquakes is both
challenging sCientifically and important for assessing the related earthquake hazards.
In many subduction zones, thrust earthquakes may have characteristic patterns
in space and time. However, the mechanism of mega events still remains
largely unresolved.Large subduction zone earthquakes are usually associated with high amplitude
co-seismic deformation above the plate boundary megathrust and the elastic relaxation
of the fore-arc. These are expressed as vertical changes in land level with the
up-dip part of the rupture surface uplifted and the areas above the down-dip edge
subsided. One of the most characteristic pattern associated with the inter-seismic
era is that the deformation is in an opposite sense that of co-seismic period.This work was started in 2002 to understand the tectonic deformation along the
Andaman-Nicobar arc using seismological, geological and geodetic data. The occurrence
of the 2004 megathrust earthquake gave a new dimension to this study, by
providing an opportunity to examine the co-seismic deformation associated with
the greatest earthquake to have occurred since the advent of Global Positioning
System (GPS) and broadband seismometry.
The major objectives of this study are to assess the pre-seismic stress regimes, to
determine the pre-seismic convergence rate, to analyze and interpret the pattern of
co-seismic displacement and slip on various segments and to look out for any possible
recurrence interval for megathrust event occurrence for Andaman-Nicobar
subduction zone. This thesis is arranged in six chapters with further subdivisions
dealing all the above aspects. |