Is Mumbai Safe from Tremblers? by V. K. Joshi (Bijji) SignUp
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Is Mumbai Safe from Tremblers?
by V. K. Joshi (Bijji) Bookmark and Share
 


Mumbai is one of the most populous states of the country. The Wikipedia says, 'by some means it is one of the most populous cities in the world'. As per the 2006 census the population of Mumbai is 13 million. With all the suburbs added the population of the metro is already touching the 25 million mark. As per the projections Mumbai will rank the 4th in the world by 2015 in terms of population.

With the population already bursting at the seams the skyline of Mumbai has completely changed. Watch the city from the across the Vashi Creek and you feel as if Manhattan has been transported to our country. It appears that the dreams of the visionaries to make Mumbai a Chicago or for that matter any one of the best developed metro of the world are not mere figment of imagination. The metro is developing at a fast pace especially in the housing sector. The high-rise buildings attempting to tear across the sky are a proof of this. Sometimes one wonders, 'What will happen if there is an earthquake?' The builders claim their constructions are earthquake resistant. Are the complexes being built structurally safe against earthquake? How vulnerable is Mumbai in the event of an earthquake are some of the questions that often dog the mind. 

Mumbai is located in the Peninsular India and compared to the Himalayan Plate boundary region is considered less hazardous as far as earthquakes are concerned. Yet the disastrous earthquakes of peninsular India at Koyna (1967, Mw = 6.3), Killari (1993, Mw = 6.1), Jabalpur (1999, Mw = 5.8) and Bhuj (2001, Mw = 7.7) disprove this myth. The intra-plate earthquakes unlike the plate boundary shakings are less frequent but kill more people.

The seismologists probing the causes of earthquakes in the past tell that faults that are active are one of the perpetrators of earthquakes. These make an area seismically active. Mumbai, for example lies in the Panvel zone, which is seismically active. This zone extends in north-northwest direction along the west coast. Deccan Traps, the volcanic rocks those appear very stable on the surface have a system of north-northwest running faults underneath, with some of the faults being quite large.

It is well known that people do not die of earthquakes. They perish under their own houses, because of the ground motion the buildings collapse. Precise information about the ground motion that has taken place in the past in the 300 km radius of the building being constructed is vital in the engineering practice. However, the existing Indian code IS-1893 does not provide quantified seismic hazard in this fashion say S.T.G Raghu Kant and R.N. Iyengar of IIT, Guwahati and Indian Institute of Science, Bangalore in one of the recent publication. Such information is mandatory in case of nuclear installation etc. But in developed countries even for hospitals and schools such data is considered essential. These are the places where patients or children are lodged en masse. Remember the mass destruction of 400 children at Anjar on the fateful 26 January, 2001, when the earthquake at Bhuj cut short their lives in school!

The seismic location of Mumbai is quite vulnerable. The Peninsular India is made of Dharvad, Aravali and Singhbhum proto-continents. Seismic activity is more at the intersection of these proto-continents. The period from 1870-1920 had been a period of quiescence, whereas, the period before and after that has shown a higher level of seismicity. During the 35 years between 1963 and 1998, the Koyna region has faced 1, 02, 715 tremors, of which 79 were above magnitude of 4 Richter and seven were above magnitude of 5 Richter. Seismologists conclude that Killari type of disasters can occur on discrete faults in regions generally presumed to be a-seismic (see box). The dreadful night of 30 September, 1993 will always haunt the living people of 66 villages of Latur and Osmanabad districts of Maharashtra, when an earthquake of 6.4 on Richter scale cut short the lives of nearly 20, 000 people.

Mumbai lies on Dharwad proto-continent say Raghu Kant and Iyengar. Koyna which was shaken out of its slumber on 11 December, 1967 also lies on the same proto-continent. Ground is not as stable under the Deccan basalts. D.N. Wadia a doyen of Indian Geology had found evidences that indicated that the old land surface along the coast at Mumbai had subsided by ten metres. This is one of the major signatures of the recent tectonic activity in Mumbai that has been tormenting the subsurface discreetly. Seismologists say that Panvel flexure another geological feature of significance is still active. The might of this flexure can be understood from the fact that it has changed the geomorphology of the basalt hills that dominate the Mumbai skyline. The same basalt up to Thane creek constitutes plateau like ranges, but beyond that the inverted cone shaped mountains take its place. Beyond Panvel once again they become plateaus. Parallel to Panvel fault, V Subramaniyam an IIT Bombay Professor and seismologist of great repute has published that faults along Thane Creek, Panvel Creek and Dharmator Creek in and around Mumbai are active.

Raghu Kant and Iyengar say that 23 faults that can cause ground motion in and around Mumbai have been identified. It is time that microsesimic zonation maps of Mumbai are prepared so that the probabilities of ground motion in the eventuality of an earthquake can be worked out for each locality with precision. The buildings thereafter have to be designed accordingly. There is no harm in constructing high rise buildings provided all safety factors have been duly considered and accounted for in the engineering design. 

In a recent publication in Bulletin of Seismological Society of America, (February, 2007) Kishore Jaiswal, a research scholar in seismology, in IIT Bombay and his Professor Ravi Sinha have stated, 'seismic hazard in Mumbai is higher than the standard design specification now used to build earthquake resistant structures.' Meaning thereby that the level of seismic safety of structures designs based on current standards is lower than its desired level. It is imperative that the future designs of high rise buildings have to incorporate the arrangement to withstand the shock at a given locality. A design therefore suitable and safe for Powai area may not be safe for Bandra.

Year

Month

Intensity (MMI)/Magnitude (R)

1618

May

IX

1832

Oct

VI

1906

March

VI

1929

February

V

1933

July

V

1951

April

VIII

1966

May

V

1967

April

4.5 (R)

1967

June

4.2 (R)

1993

September

6.4 (R)

1998

May

3.8

2005

March

5.1

2005

June

3.7

2005

August

4.1

Tremblers that Shook Mumbai in the Past

Active faults are like the signatures of the past earthquakes. Their study opens many vistas of the earthquakes that have visited an area in the past and the possibilities of recurrence as well.

For Mumbai these faults are like terrorists, whose presence has been established in and around the city. Precautions are needed to safeguard the masses from their dreadful activities.

8-Apr-2007
More by :  V. K. Joshi (Bijji)
 
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