A total of 27 micro-earthquakes, with the largest being of the magnitude of 0.3 on Richter scale, were recorded in the last five days at Sachivayalanagar of Vanasthalipuram.

The National Geophysical Research Institute (NGRI), which installed two seismological stations here following reports of subterranean sounds by locals, has been monitoring the seismological activity since October 23 and recording these micro-earthquakes.

According to a statement issued by the NGRI here on Wednesday, further analysis of the seismograms had shown that these micro-earthquakes were of high frequency and were in the audio range and hence the sounds were being heard. Most of these shocks had zero or less magnitude which could not be felt by people, but only the sound could be heard.

The NGRI found these small shocks to be occurring within 300 metres to 400 metres below the ground in a localised area around Vanasthalipuram. Such shocks were a natural phenomena commonly observed in shield regions and sometimes caused due to adjustment in the very shallow layers of the earth's crust following heavy rain.

Usually, the phenomenon dies down within short duration of time without causing any damage and thus there was no cause of any concern, the NGRI assured.

Most of the shocks had zero magnitude which could not be felt, but the sound could be heard

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