IN 1996, Kerala, considered a seismicallysafe zone, witnessed a tremor of magnitude 2.8 (on the Richter scale). This isnot 4D isolated case, for the state haslately been seeing an increase in the frequency of tremors in its central andsouthern parts. Studies by C P Rajendran and Kusala Rajendran, earthscientists at the Centre for Earth ScienceStudies (CESS), Thiruvananthapuram,show that this increase in tremors inrelatively stable regions is associatedwith the reactivation of ancient faults.
The reseaTdhers@ say an increasein the occurrence of micro-tremorswas reported fr@m the districts ofIdukki, Thrissur and parts of Thiruvananthapuranm during the pasteight years. Historically, the largestearthquake (magnitude 5.5) thatoccurred in southern India was in theyear 1900 at Coimbatore. As Kerala isnot known to be a high-seismicity zone,an earthquake of magnitude 4.3 thatoccurred near Wadakkancherri inThrissur district in 1994 has confoundedscientists and caused much alarmamong residents. According to a CESSreport, the earthquake was felt in anarea of 1,000 sq km in Thrissur,Palakkad and Malappuram in thestate. The earthquake was followed bymore than 100 small tremors lastingupto February 1995.
Seismic data collected by CESS scientists indicate occurrence of minor earthquakes within the confines of a regionalstructure called 'Palghat gap'. This is alarge tectonic structure that passesthrough the district of Palakkad inKerala, and is marked by high-risinghills to its north and south. The reportsuggests that "in the backdrop ofobserved seismicity and likelihoodof causative structures, the regioncomprising the 'Palghat gap' may beconsidered seismogenic".
According to the researchers, aninteresting relationship betweenmicro-tremors and rainfall hasbeen observed in Kerala. A majoritythese tremors have occurred duringperiods of increased rainfall. Thestate receives its major share of rainfallfrom June to September and Octoberto November every year. Also, almostall the tremors reported have occurredin the vicinity of river channels,which suggests that earthquakes areassociated with weak zones andincreased water pressure. This, theypoint out, is mainly due to changesin land-use patterns in Kerala.Deforestation, quarrying, filling oflow-lying areas, erosion and siltation ofriver beds after the hydrodynamicnature of the regions.
Past experiences of earthquakes thatoccurred in Killari and Koyna show thata better understanding of their mechanisms and causative structures is needed.An increase in the number of seismicstations to monitor background activityis required. In the case of Kerala, betterunderstanding of background seismicityand the hydrodynamics of selecteddrainage basins is required before it istoo late, say CESS scientists.