Tuesday, April 19, 2011

March 11, 2011 Japan earthquake gives insight into "greater earthquakes"

Syed Akbar
Hyderabad:  The recent earthquake and subsequent tsunami in Japan may have left a trail of death and destruction, but scientists here feel that it has given them enough information on the mechanism of "greater
earthquakes" and the stability of buildings including nuclear power plants falling under high seismic zones.
"Normally buildings collapse when the acceleration caused by an earthquake is substantial. The acceleration of an earthquake is measured against Gravity i.e 1.0 g. The acceleration is said to be of 1 g if a boulder is lifted off the earth under the impact of an earthquake.  In the Bhuj earthquake the gravity was just 0.35 g and hundreds of buildings had collapsed. But in the case of the earthquake in Japan the gravity was as high as 2.7 g. Such a high gravity was never recorded and yet the buildings were in tact. The structure of nuclear power plants too were in tact and they did not cave in," observes senior seismologist from NGRI-CSIR Dr RK Chadda.
Stating that the Japan earthquake has left lessons, both positive and negative, for seismologists around the world, Dr Chadda said the positive factor was that if buildings were constructed in accordance with the seismic norms, they will not collapse however high the gravity may be. "The negative side is that since most of the nuclear power plants are located near sea, the backup system should be erected on a high platform. The earthquake as such did not damage the nuclear power plants in Japan, but the tsunami had submerged the
backup systems as they were not erected on a high pedestal," he pointed out.
He said nuclear power plants in India will be not be affected even if a tsunami strikes the Indian coasts, Dr Chadda pointed out that the Kalpakkam power plant had its backup system erected 30 mts high. India faces tsunami threat only from two points - Karachi and Sumatra - and only when an earthquake of high magnitude strikes at these places, there may be a tsunami to the East or West Indian coasts. "But the golden lining is that we will have one to two hours lead time to switch off the nuclear plants and take up other
evacuation works," he added.
Dr K Rajendran of Indian Institute of Science sees an "element of surprise" at the magnitude and the underestimated fallouts of this calamity, as all this was happening in one of the most seismically active and best-studied regions of the world, with a high level of preparedness.
"Part of the problem was that the earthquake expected to occur in the concerned source area was one of smaller magnitude (7.4 to 8.2), following a 30-40 year seismic cycle," Dr Rajendran, who has published a research paper on the Japan quake, pointed out.
Earthquakes of greater magnitude occur once in a cycle of 1000 years, and the Japan earthquake is listed one among them. Most subduction zones feature two types of cycles - one of shorter duration characterised by quakes of magnitude 7.5, and another of longer duration leading to great tsunamigenic events. The quake measuring 9.0 seems to be part of the super cycle, with its predecessor occurring way back in 869 CE.
According to Dr Rajendran's study, "the 9.2 quake that occurred in 2004 at  Sumatra had ruptured 1300 km length of the plate boundary between Indian and Eurasian plates. It is considered to have marked the end of a super cycle, with an estimated duration of 500 -1000 years".

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