Although it is a known fact that ground motion during an earthquake gets amplified in sedimentary terrain, geophysical studies in the Himalayan arc have revealed that such amplification would be five times greater in such sites as compared to hard rock regions.
Under an on-going five-year project on ‘Hazard assessment for earthquakes and tsunamis’, scientists from the Hyderabad-based National Geophysical Research Institute (NGRI) are characterising earthquake sources and monitoring seismic activity in the Himalayan region as well as in the sub-continent.
Apart from North-east India, particularly Assam and Arunachal Pradesh, the seismological studies are underway in Gujarat, western Maharashtra and parts of Andhra Pradesh and Telangana.
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With scientists undertaking site-specific ground motion studies as part of the larger project, it was found that though the ground-shaking during the recent Nepal earthquake on April 25 was of high intensity, the acceleration was not of the order normally associated with such large temblors. This was because higher frequencies were not generated at the source of the earthquake. “In case of such large earthquakes, we should have had greater ground motion, which didn't happen. Otherwise, it would have been catastrophic as the damage will be more when the ground-shaking is more vigorous”, says Dr. D. Srinagesh, Chief Scientist and Head of the Seismology Observatory at NGRI.
Over a period of time, scientists have been receiving data from 26 strong motion velocity metres that have been installed at different places.
Pointing out that ground motion gets amplified if the site has thick pile of sediments, Dr. Srinagesh said the studies showed that the sedimentary thickness in the Indo-Gangetic plain varied between a few hundred metres to more than five kilometres. “The greater the thickness, the greater will be the amplification”, he added.
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Observing that no major earthquake has happened in the last 200 hundred years in Garhwal-Kumaon Himalayan region, a close watch was necessary. Since prediction was not possible, studies were directed towards hazard assessment and mitigation in the Garhwal-Kumaon region, which could be probably affected by a large earthquake, he said.
Uttarakhand, Uttar Pradesh and Haryana, including Chandigarh were being monitored through ground motion studies.
Seismic hazard assessment at micro-level was also being done for major cities in Uttar Pradesh including Lucknow, Kanpur and Allahabad. Seismic and tsunami hazard studies have also been taken up along the East and West Coasts of India for proposed power plants and other projects, including major irrigation schemes.
Dr. Srinagesh said that in a bid to create awareness on earthquakes, NGRI was conducting outreach programmes in schools in Seismic Zones V, IV and III and targeting 50,000-100,000 students each year.