Southern States
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Tamil Nadu
Chennai, Salem, Pondy quake prone?
By Dr. SM. Ramasamy
In the recent years, the occurrence of earth tremors and earthquakes/seismicities has become a frequent phenomenon causing greater anxiety to the planners, the administrators and the scientists. A lot of information and concepts are afloat in the media, that too as and when any episode strikes Tamil Nadu. But surprisingly these are mostly bookish concept and the real scientific information on the earthquake susceptibility of south India, reasons for seismicity, the locales prone for earthquakes, future predictability on the earthquakes, etc., have not been made known.
Many are of the view that because of the movement of the plates and their collision, the earthquakes occur. Of course, this is true in the larger perspective and the earthquake occurrences in plate boundaries like Himalayan belts can be explained so. But as far as south India is concerned, all the earthquakes/tremors have not occurred in plate boundaries but only within a plate and hence this opinion is far from true.
One view was that overexploitation of groundwater is the causative factor for the recent earthquakes in Tamil Nadu. Over extraction of groundwater can probably cause land subsidence with mega pentagon shaped cracks, but can never induce earthquakes/tremors/seismicities.
The studies conducted all over the world show clearly that the regions prone for earthquakes can be fairly assessed by duly and carefully studying the pulses of the earth but definitely not the time of its future occurrence.
The Centre for Remote Sensing, Bharathidasan University, is one of the centres in the country which is conducting focussed studies in south India on the tectonic movements leading to the earthquakes on the basis of anomalies and the signals observed in riverine, coastal and hydrological systems of Tamil Nadu and adjacent parts using high technologies like remote sensing. The studies conducted have indicated that India which has made a North North Easterly (NNE) journey of 6,000 km during the last 60 million years (at the rate of approximately 10 cm a year) because of the north easterly directed compressive force, collided with the Eurasian plate. Such a collision has resulted in the phenomenal Himalayan mountain building activity at the junction of Indian and Eurasian plates. Now, the same Himalaya itself has become a rigid mass obstructing the further northerly movement of the Indian plate because of which the whole Indian plate is whirling like a worm as the north easterly force rising from the south of Indian landmass in the Indian Ocean and the Arabian Sea is still active. Such whirling is manifested in the form of a series of east - west trending alternating arches and deeps from Cape Comorin in the south to the Himalayas in the north.
Further, deeper remote sensing studies in parts of Tamil Nadu and Kerala show that because of such still ongoing compressive force, the North-South trending faults/cracks are opening up like lenses, the North East-South West trending fault/cracks are moving in an anticlockwise direction and the North West-South East trending faults are moving in a clockwise direction.
The NE-SW trending faults appear to be more prominent and take a near westerly swing in the Kerala coast, shifted the very young coastal land forms and continue upto the Laccadives and Maldive islands as observed from satellite photographs. These NE-SW trending faults seem to be more active as seen from the geomorphic anomalies which may be due to the additional north easterly pressure exerted by the rising Carlsberg ridge in the Arabian Sea.
During the last nearly 200 years, over 170 earthquakes/tremors have occurred in south India. Integration of such occurrences with such ongoing land/fault movements show that the maximum number (77) of earthquakes have occurred along North East-South West faults/cracks followed by North-South, East-West and North West-South East trending faults/cracks. Even the recently occurred earthquakes/tremors in Pondicherry and Chennai on August 25 this year are also falling along such North East-South West trending faults. All the earthquakes occurred in Kerala during the last two years have also fallen along only such North East-South West trending faults.
All these indicate that the NE-SW trending cracks found in parts of Tamil Nadu are more susceptible for earthquakes/seismicities followed by N-S faults/cracks. However, the analysis on the distribution of such nearly 170 earth tremors recorded in the last 200 years show four major clusters, one along Chennai-Salem, the second along Pondicherry-Tiruchy, the third along the Eastern part of Tanjore delta and the fourth in Tirunelveli- Kalakkadu regions. Amongst these, the first two clusters are falling along NE-SW trending cracks and the last two are coinciding with N-S trending cracks.
Thus, all the NE-SW trending faults in Tamil Nadu are more vulnerable to earthquakes followed by N-S cracks in general and Chennai, Salem, Pondicherry and west of Tirunelveli region, in particular. Hence these are not due to plate movements, but due to mid plate deformation and breakage. However, the area needs warranting studies and the Centre for Remote Sensing is contemplating more deeper studies.
(The author is Director, Centre for Remote Sensing, Bharathidasan University, Tiruchi).
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