Increasing seismic vulnerabilities for Bengal basin & the northeast India: Environmental impact of ground water depletion
Dr. Tridib Bandopadhyay
Earthquake is caused from tectonic activity deep inside the earth. The effect is the shaking of ground and everything on the ground resulting in large-scale devastation. The main geological feature is the fault-rupture in any direction away from the epicenter and soil stratification. Seismic hazards, the outcome of earthquake, result in slope instability, ground collapse or subsidence, liquefaction of soil base, structural destruction, tsunami etc. in the near-field, as well as, far-fields along line of faults. Seismic vulnerability analysis defines the damage prognosis in the light of potentiality of an earthquake in seismically active locations. While an earthquake at any region cannot be avoided being a natural phenomenon, the anthropogenic environmental impacts can advance the clock and increase the possibility of far major devastations from the quake. This present study, based on secondary data, tries to find the potential vulnerabilities of unrestricted groundwater extraction and the resultant increase in vulnerabilities of soil liquefaction and ground collapse from any major earthquakes passing through the traversing fault-lines, from the Northeast to the Bengal Basin or vice versa. The convergence of Indian and Eurasian plates has developed a mountainous topography and led to occurrences of earthquakes in the region. According to widely accepted model of earthquake occurrences at this collision-plate boundary, this convergence is accommodated on the Main Himalayan Thrust Front. The detachment is the surface between the underthrusting Indian shield rocks and the overlying Himalayan rocks. Bureau of Indian Standards marked the Indian landmass into three tectonic provinces, viz. Himalayan region, Indo-Gangetic basin and Peninsular India. Four seismic zones are designated as II, III, IV and V. The peak horizontal acceleration (PHA) and spectral accelerations for periods 0.1s and 1s have shown high seismic hazard in most parts of the Northeast and the Bengal Basin. While the region close to the Bay of Bengal is placed under Zone III, the most parts of the northeast of Indian region has been placed in zone VI and a few pockets in V, the highest level of seismic hazard potential in Seismic Zonation Map of India. There have been two great earthquakes in 1897 (Shillong) and 1950 (Assam) in the region. Global Seismic Hazard Assessment Programme (GSHAP) also classified this zone under high seismic risk with peak ground acceleration values 0.35 – 0.4g. According to subsequent studies, the last major quake of Mw 7.8 in April, 2015 near Kathmandu (Nepal) did not generate high-frequency waves and unzipped only a small part of the locked energy from the lower edge of the Main Himalayan Thrust Front. The Western Nepal, as well as the areas under north and northeast of India thus remained under potentially increased risk of any major quakes in future. While the sub-Himalayan region extending from the Northeast of the country to the Bay of Bengal is seismically active, the potential vulnerabilities from earthquake for this geographical region can be anticipated from any of these two directions -
Through fault-lines emanating tectonic impulse from Himalayan Thrust Front and its translation through Eocene Hinge Zone, and
The quakes extending from Sumatran Subduction Trench, stretching along Lesser Sunda Islands off southern coast of Sumatra to Andaman Islands.
Geological and geophysical studies indicate evidence of seismic activity for these divergent margins where the seismic zones orient in NE-SW direction. Study of seismicity and fault planes of the past earthquakes in the Bay of Bengal region also focuses attention on deformations of the northern Indian plate. It provides evidence that the intra-plate region of the entire Bay of Bengal and peninsular India over the Indian plate is seismically active. In particularly, the Gangetic delta show higher seismicity in comparison to eastern plate boundary close to the seismically active arc of the eastern Himalayas. While the entire region is formed by sedimentary and saturated alluvial deposits of Barak–Brahmaputra–Ganga river systems, the soil deposits are soft, erratic and geomorphologically divided into fluvial plain, tidal flat, natural levee and fed by numerous channels. The soil stratum covers alternate layers of clay, silt and sand horizons. The recent studies on the last major earthquake of Kathmandu (Nepal, 2015) showed a major contribution of groundwater extraction to trigger enhancement of the devastations. Enormous extraction of ground water, far exceeding the recharge potential, is creating formation of void space below overburden soil-stratum has resulted in the entire area of the Northeast and the Bengal Basin. Confined groundwater is increasingly under pressure due to weight of the overburden and is further pressed by piling of deep columns for high-rise concrete structures for developmental activities. The overburden pressure has been increasing below that region. Due to over-extraction, water-level of underlying aquifers vis-à-vis pore–water pressure is depressing year after year, while due to changing rainfall pattern and reducing tree-cover, the possibility of normal recharge is being restricted. These anthropogenic environmental impacts are likely to enhance the scale of devastations in the event of an earthquake episode, which would act as catalysts for large-scale soil liquefaction and structural devastations.