Geodynamics and potentiality of large earthquakes in densely populated
Bangladesh: Constrained from GPS
Abstract
Bangladesh, a small and over populated country in Southeast Asia
occupies most of the Bengal Basin that results from sediments derived
from the collision of India with Asia. The basin is filled with a 19 km
thick sequence of Cenozoic sediments deposited by the mighty rivers
Ganges and Brahmaputra. Unconsolidated Holocene sediments susceptible to
seismic amplification characterize the upper part of the Cenozoic
sequence. Bangladesh sits a top on three tectonic plates; India, Tibet
and Burma. The India plate is colliding with the Tibet subplate to the
north, which gives rise to great Himalayas, while to the east it is
subducting beneath Burma and Sunda slivers, which gave rise to
Indo-Burma arc. The Surma basin of NE Bangladesh is being underthrust
under the Shillong massif producing the 2-km high plateau. The
Indo-Burma fold and thrust belt results from the oblique subduction of
the thick sediments of the Bengal Basin on the India plate that has
deformed into a series of north-south trending en-echelon folds and
thrust faults. The faults rooting these folds and the underlying
megathrust are capable of generating devastating earthquakes in and
around Bangladesh. Past earthquakes have brought changes to the
landscape, avulsion of rivers Brahmaputra and Meghna, migration of human
settlements, and widespread sand liquefactions and sand and/or mud
eruptions. Our GPS study demonstrated that the landward extension of
Andaman-Sumatra subduction zone into Indo-Burma subduction in deltaic
Bangladesh is active. The present day India-Burma oblique convergence
rate is 17 mm/y and that the décollement beneath the fold-thrust belt is
locked (Steckler et. al., 2016). The western part of the subduction zone
over a shallow décollement shows little seismicity whereas the eastern
part shows moderate seismicity of magnitude 4 to 6. Based on the GPS
velocity across the fold belt and seismicity the Indo-Burma subduction
zone can be potentially be divided into locked western segment and
slipping eastern segment, analogous to Cascadia subduction zone. Fold
belt parallel shortening across Dauki Fault in Shillong is 7 mm/yr,
which is another potential source of a large earthquake. The huge
population might be severely ravaged by devastating earthquakes from
both these sources.