THAT SINKING FEELING, AGAIN
Tuesday’s quake followed by at least 7 tremors in less than two hours, says IMD Nepal records 9 tremors with one measuring 6.8 in Dhading district Over 160 aftershocks have been recorded since April 25 earthquake
Despite being far from the epicenter of the Nepal earthquakes, cities in Andhra Pradesh have felt the tremors because of their alluvial soils and deltaic environments.
Alluvial and loose soil trap amplify seismic energy while regions like Hyderabad, with hard rock subsurface, transmit seismic energy seamlessly.
Scientists say that tremors due to the earthquake in Nepal on Tuesday were felt all along the eastern seaboard till Chennai, apart from the IndoGangetic plains.
Earthquakes trigger seismic waves that sometimes travel throughout the globe, but their intensity weakens as they move farther.
As a consequence, tremors of the two major earthquakes in Nepal shook several parts of Andhra Pradesh despite being hundreds of kilometres away from the epicenter. Andhra Pradesh has experienced stronger than usual tremors because of the soil system, say scientists.
Dr Srinagesh Davuluri, chief scientist, National Geophysical Research Institute (NGRI), said, “Seismic waves carry huge amounts of energy but it wanes as they move farther away. But today we felt strong tremors on the eastern flank, up till Chennai. This is because of a thick file of loose alluvial soils. Wherever there is loose soil, there is amplification of the seismic waves so the tremors are stronger than they would otherwise be.”
This usually happens in regions with loose soil, alluvial soil, places close to riverbeds and delta regions. Dr Srinagesh is also taking up a mega project on the seismic hazards of the Indo-Gangetic plains in northern India, which is a vulnerable zone because of earthquakes in the Himalayas.
But while Andhra Pradesh experienced significant tremors, Hyderabad was immune to the seismic waves.
“Hard rock is an excellent transmitter of seismic waves. But what happens with alluvial soil is that it traps seismic waves and they get reflected in them, which is called site amplification. Tremors are also felt in hard rock areas like Hyderabad, but with exactly the same energy they receive without any amplification,” explained Dr N. Purnachandra Rao, senior principal Scientist, NGRI.