Strong earthquake overdue in India

But scientists at a loss to pinpoint when it is likely

Statistics and historical earthquake list say that an earthquake of magnitude 8 or above is overdue in Uttarakhand or Assam. It is, however, not possible to say that it may happen today or 50 years from now as we neither know where the accumulation of stress has reached the elastic limit nor when, says a scientist

Amid reports that a massive earthquake is overdue in some parts of the Himalayas, a section of Indian scientists say no unusual seismic activity or abnormal increase in changes in the earth's surface have been detected in India's northeast. In addition, they pointed out that no two strong earthquakes have come in the Himalayas immediately after one another.April 25's strong earthquake measuring 7.9 on the Richter Scale has killed thousands and caused widespread destruction in Nepal. The tremors also caused deaths and damages in India besides Tibet.

The 2,500-km-long Himalayan arc extending from Kashmir in the northwest to Arunachal Pradesh in the northeast is seismically "very active", experts from the Gandhinagar-based Institute of Seismological Research (ISR) said. The movement of tectonic plates generates stress over time and rocks at the surface break in response. When the strain accumulates, every 150-200 km stretch of the 2,500-km-long Himalayas can be hit by a high magnitude earthquake in 150-200 years.

Based on historical data, they say a strong quake was overdue in Uttarakhand or Assam but other indicators have not shown any abnormal spikes. "Statistics and historical earthquake list say that an earthquake of magnitude 8 or above is overdue in Uttarakhand or Assam. It is, however, not possible to say that it may happen today or 50 years from now as we neither know where the accumulation of stress has reached the elastic limit nor when," A.P. Singh, scientist, ISR, told IANS.

He said: "It is to be mentioned that there is no unusual increase in seismicity and the GPS (Global Positioning System) network spread all over northeast India has not shown any abnormal increase in crustal deformation in recent years. Hence no sign of large earthquake coming is observed." According to the US Geological Survey, crustal deformation refers to the changing earth's surface caused by tectonic forces that are accumulated in the crust and then cause earthquakes.

So understanding the details of deformation and its effects on faults is important to figure out which faults are most likely to produce the next earthquake. But Singh said current technology can't help predict when the quakes will strike. "It has been observed in the past that aftershocks are lower in intensity compared to the original shock. Secondly, in Himalaya no two strong earthquakes had come immediately after one another. So, the report that a bigger earthquake may come soon is not scientifically founded," clarified Pallabee Choudhury, another scientist at ISR.

There are three thrust belts in the Himalayas and all are seismic in nature. The Nepal quake emerged from the main boundary thrust, said senior scientist Sankar Kumar Nath at the IIT Kharagpur, adding all Himalayan earthquakes are severe. "If you look at the Himalayas, we have had large earthquakes and greatest of the great earthquakes are still possible. Whenever there is an earthquake, it generates a slip of about several meters. This is due to a large magnitude (around 8). The Himalayas has the potential to generate a 9 magnitude earthquake," Nath said.

Guwahati and Srinagar are the two Indian cities at highest risk of being devastated by an earthquake, with 36 other cities in areas prone to earthquakes, according to government data. Guwahati and Srinagar fall in what is called “very severe intensity zone”, or zone V, the highest-risk earthquake zone.Eight cities, including Delhi, fall in the “severe intensity zone” or zone IV, according to a seismic zoning map issued by the Bureau of Indian Standards and quoted in this National Disaster Management (NDM) report. The other 30 cities fall in the “moderate intensity zone”, or zone III.

By Sahana Ghosh