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| Movers and shakers June 2005 has turned out to be an unusual month for the northern city of Kalyan, situated on the bank of Ulhas river, with two minor earthquake tremors, both of magnitude 3.0, shaking it and neighbouring Ambernath and Dombivli, on the evening of June 14 and again on the morning of June 18. Though they did not cause any damage, they show that the eastern corridor beyond the Thane Creek is seismically quite active and this is significant. Such low earthquake activity persisted in the northern part of Thane district earlier too, with Khardi, Bhatsa, and Jawhar recording minor tremors between 1983 and 1996. But, after 1998, the earthquake activity shifted southwards to the tract falling between Kalyan and Panvel except for one stray occurrence near Palghar in December, 2001. Two of the most recent tremors May 31, 1998 of magnitude 3.6 and the other that occurred on November 16, 2001 of magnitude 3.0 were epicentred at Ambhe village, 10 km east of Taloja and Khairna, 10 km west of Taloja, close to the Thane Creek. Both shocks that occurred in June, 2005 were epicentred at Kalyan. Why Kalyan rocks The main reason for persistent seismic activity, even if it is of low magnitude, at the corners of a triangle connecting Kalyan, Khairna and Ambhe, is the major geological transition that has taken place in this terrain to the west of Kalyan in the north and Panvel in the south. The rock in this region is a black basalt of volcanic origin, having been poured out of long and deep fissures in a hot, molten state in the geological past. This basalt rock occupies a staggering 5,00,000 sq km of Deccan Volcanic Province that covers major parts of Madhya Pradesh, Maharashtra and Gujarat. The most important aspect is that this basalt lies in a perfectly horizontal position at all places located to the east of the Kalyan-Panvel line as seen clearly in the hill forms flat-topped plateaus and ridges like the ones at Khandala and Mahabaleshwar. But immediately to the west of this line the geological scenario changes drastically and the same black rock occurs here with a distinct slant, called dip, in a westerly direction as observed in the hills around the Powai Lake. Tension in Panvel too Such a geological feature that imparts a tilt to horizontally laid rocks is called a monoclinal flexure and Dr J B Auden, a British geologist who was the first to recognise this feature in this region, christened it the Panvel Flexure since it starts from Panvel at its southern end and runs northward through Kalyan and turns northwest. This important Panvel Flexure owes its origin to a tensional stretching of the rocks in a westerly direction. It is well known that tension always leads to rupture and such a tensional pull of the rocks had consequently led to the formation of many geological faults (which are in fact rupture planes over which the rocks on either side have moved past each other) in the region; these faults had weakened the rocks of the terrain considerably and made them vulnerable to earthquakes. Cost of staying safe Important townships like Navi Mumbai, Belapur, Mumbra, Ambernath, Dombivli and Badlapur besides Kalyan are located in this active seismic belt. Given that earthquakes can neither be predicted nor prevented, precautions are necessary. Following standard building codes for different civil engineering structures during construction is the only precaution. Earthquake-resistant designs which include an additional safety factor in view of the seismic risk, are available for all types of buildings and these increase overall cost of construction only by about 10% which is well worth providing for. How high can you go? Buildings with 40-50 floors are problematic and may be avoided. Seismologists are not able to ensure safety beyond a certain optimum height; a state-level committee had recommended restricting heights to 70 metres (about 22 floors) and this seems reasonable. It is argued that high-rises can be built on raft foundations which will make them float like ships during earthquakes but recent research at Pennsylvania State University has shown that high-rises may not always come back to their original vertical positions after the earthquake vibrations pass off but, instead, remain tilted. Likewise, existing buildings should also be reinforced to withstand earthquake vibrations. California is preparing itself Complacency should give way to caution considering the seismicity of the Kalyan-Panvel corridor. It will be good to increase the level of awareness about earthquake possibilities and get into a mood of preparedness as the Americans are doing in California which is a highly seismic state; their calculations lead them to believe that a major earthquake is due on the San Andreas Fault in the San Francisco area sometime between 2010 and 2030 and they are already gearing themselves towards that possibility after nearly a century their last major earthquake occurred in 1906 at San Francisco. As for Mumbai Mumbai seems to be somewhat favourably located to the west of the Thane Creek in a terrain with fewer faults and so earthquakes have so far avoided the commercial capital of India. However, it still figures in Seismic Zone 3 and therefore can draw its lessons from the Kalyan tremors. An earthquake of magnitude 6.5 occurring anywhere in the Kalyan-Panvel corridor will affect all the places in the Mumbai Island. One can only hope that the Thane Creek will continue to act like the Lakshman Rekha and hold the earthquakes at bay! The writer is an ex-Professor of Geology, IIT Bombay at Powai and an earthquake geologist Finding faults Some of the important faults occurring in this region are: the Thane Creek Fault, the Panvel Creek Fault and the Ulhas River Faults. These faults are responsible for the minor earthquakes in this eastern corridor by getting reactivated periodically. It will be seen that the Ulhas River west of Kalyan flows along many small unusual meandering loops all of which would appear to be faulted segments. These tiny faults and the major fault along the Panvel Flexure itself surround Kalyan closely and make it a potential venue for earthquakes. Likewise, the Thane Creek Fault (which is an established feature by now) had given rise to the Khairna tremor of 16-11-2001 while the Panvel Creek Fault had obviously triggered the Ambhe earthquake of 31-5-1998. It needs to be noted that this major geological transition zone lying to the east of Thane Creek is a highly faulted tract and this makes it vulnerable to earthquakes. |
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