Hyderabad: The 3.4 magnitude earthquake, 29 km north-west of Nagarjunasagar, which was detected by the National Geophysical Research Institute (NGRI) on July 26, has once again brought seismic monitoring at large reservoirs into focus. The epicentre of the quake was between Yacharam and Nagarjunasagar dam.
NGRI chief scientist D. Srinagesh said that it was “one small quake, nothing alarming” and no aftershocks were recorded. NGRI gets real-time data within a span of 10 seconds from Nagarjunasagar, Srisailam and Sripada Yellamapally project.
“We have set up seismological recording apparatus at Polavaram in Andhra Pradesh too,” said Mr Srinagesh. NGRI doesn’t have recording equipment at Mallanna-sagar and Kaleswaram.
Study to check if dams trigger quakes
It was the 6.3 magnitude earthquake near the Koyna dam in Maharashtra in 1967 that highlighted the importance of seismic monitoring near water reservoirs.
Most areas in Andhra Pradesh and Telangana state fall under Zone II. Some regions, including Kottagudem, Badrachalam and the Singareni belt in TS, and the coastal belt in AP fall in Zone III, according to the Seismic Zoning Map of India. These areas can experience earthquakes of up to 6.0 magnitude.
NGRI chief scientist D. Srinagesh said that in constructing a dam precautions and mandatory provisions of the Central Water Commission are followed.
“In India we do not have big dams like the Hoover Dam in the US, with huge storage capacity. So dams triggering an earthquake is not a possibility. Nagarjunasagar and Srisailam have been there for a few decades but nothing has happened.”
Three analog seismographs are located at Nagarjunasagar, Srisailam and Sriramsagar dams to monitor seismic activity. The NGRI uses data accrued from such monitoring to assess the level of seismicity in the region and to know whether this activity is influenced by the water levels in the reservoir.
No seismic activity has been observed in the vicinity of the three reservoirs except this latest quake. In 2003-04 earthquakes of smaller magnitude were recorded, but at distances ranging from 50 to 100 km from the reservoir sites.
These earthquakes were located also by supplementary data from Kothagudem, Cuddapah and Hyderabad.
