| Re: Felt earthquake tremours in Mumbai
Today morning, at our Broad Band Seismic Observatory, we recorded three earthquakes in Western India with epicentres at three locations:
(1) At 8.54 am IST the Rann of Kutch area got jolted with a quake of 4.1 magnitude ( we recorded it as 3.5).
(2) At 10.57 am IST the area around Koyna was jolted with a quake of 4.9 magnitude ( we recorded 4.9). The epicentre was 110 kms NE of Ratnagiri, falling in Satara dist (Maharashtra)
(3) At 11.47 am IST the area in the vicinity of Koyna near Satara town (the epicentre) was jolted with a quake of magnitude 3.8.
The depth of the seismic activity was around 10 kms from surface for the quakes in Sl. Nos. (2) and (3).These quakes jolted most parts of Western Maharashtra.
The quake (4.9 M) epicentre is incidentally just 100 kms from Jaitapur the proposed site for the Nuclear Power Corporation (NPC) nuclear power plant.
Both the areas are in the Risky Zone IV of the Bureau of Indian Standards Seismic Zonation Map of India.
Both the areas have a past history of being earthquake prone.The Bhuj earthquake (M 7.6) of 26/01/2001 is quite fresh on our minds. In 1819, another devastating quake had destroyed areas in this region. The region has experienced above normal levels of microseismicity throughout the past 200 years.Damaging earthquakes occurred in 1845, 1846, 1856, 1857, 1869 and 1956 in the same general region as the 1819 and 2001 earthquakes.
The Koyna earthquake ( M 6.3) of 11/12/1967 devastated many homes and killed 200 people. Eversince the Koyna dam was built in the mid 1960's, the region has had a past history of Reservoir Induced Seismicity (RIS) and is one of the very prominent examples in the world of this phenomenon in the field of geoseismology.In March 2005, a quake of 5.1 magnitude had rattled this area.There were no loss of lives or major loss of property, other than the fact that many houses developed cracks. But like today's quake, there was panic all around.
Our metros like Mumbai which is mostly built on tidal flats and reclaimed land is per se quite unsafe area seismically. Pune, though nearer to Koyna is better off as there is a hard rock terrain all around.The foundations built on softer material have always been unsafe and stand to get toppled if the ground motion frequency of seismic waves match that of the structure. With the presence of water these are prone to liquefaction. In Greater Mumbai the ground water table depth is a average 2-3 metres below surface, which is again quite a deadly combination.The multi- storeyed structures are more unsafe than the structures with lesser heights, taking the build quality as the same in both cases. | 
14th April 2012, 11:00
