Available evidence about frequent tremors in the dams-bound Idukki district of Kerala does not appear to indicate a ‘swarm' at this time, an expert has said.

“Tremors are frequent in Idukki. Occasionally, there are a few larger events that are felt by the population,” said Mr Stacey Martin, a leading seismologist and researcher who runs the Amateur Seismic Centre (ww.asc-india.org) at Pune.

A swarm refers to a series of earthquakes being reported at a place during a relatively short period of time ahead of a major event. Earthquakes associated with volcanic activity often occur in swarms, though swarms also have been observed in many non-volcanic regions.

There is also no evidence to suggest this earthquake was related to the tremor in Dharapuram in Tamil Nadu earlier this month or the quake in the Lakshadweep or Laccadive Sea. The northern Indian Ocean, including the Lakshadweep Sea, is often considered to be an unlikely candidate for earthquakes of any size, Mr Martin said.

However, earthquakes have occurred in this region in the past. The largest known earthquake in the Lakshadweep Sea was in 1938. It was centred in the northern part of the sea in the Gulf of Mannar and had a magnitude of 5.8 on the Richter scale. Earthquakes in the M6.0 to 7.0 magnitude range have also occurred in the Indian Ocean in a zone running parallel to the Equator. It is common for quakes in this region to be felt in Sri Lanka and in south India.

Last year, Mr Martin prepared a catalogue of ‘felt intensity data' for 570 earthquakes in India from 1636 to 2009 for a scientific publication on historical earthquakes in India. Of these, the quake on February 9, 1823, in the Northern Indian Ocean caused tremors in many parts of Sri Lanka and, to a slightly lesser extent, in southern India. On February 29, 1944, Maldives experienced a quake, and tremors were felt strongly at Madurai. It was felt for 30 seconds at Coonoor. It was also felt at many places in Sri Lanka including at Colombo, Galle, Nawalapitiya, Nuwara Eilya, Ratnapura and Talawakele.

The Lakshadweep islands are situated in the Arabian Sea and have moderate seismicity. It is classified as seismic zone III, referred as moderate damage risk zone with respect to quakes.

Although there is very little observed seismicity, the faults running parallel to the west coast of the mainland have the potential of generating M6.5 quakes, said a national task force that studied the vulnerability of the region to natural calamities.

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