Recently we have been hearing a lot about minor earthquakes recorded in and around New Delhi, Uttarakhand and Assam. Whenever an earthquake occurs, within a few minutes we get the information about its location and the area it has affected, and its magnitude. But despite that, we know very little about the seismic vulnerability of different regions in the country. The study of earthquakes is really fascinating because it sits at the intersection of multiple disciplines like geography, geology, archaeology, palaeontology, mechanics and many other fields as well. The analysis of earthquakes is like a forensic investigation into nature’s crime scenes. So let’s delve a little bit into this interesting phenomenon.
We often talk about how some regions of India experience earthquakes more than the others. Based on past earthquakes recorded and mentioned in history we have divided India into 4 earthquake zones (or Seismic zones) on the scale of II to V. Here zone II means little to no risk of seismic activity and zone V means high seismic risk, e.g. Himalayan and Kutch region. But is it that easy to understand? Sometimes we have surprises too.
It is important to note that India sits on a major plate boundary of the Indian and Eurasian plate. Indian plate is constantly moving under the Eurasian plate at a current rate of 5 cm/year. It is the same process that led to the formation of the mighty Himalayas. This movement leads to a massive stress accumulation in the rocks at depths as high as hundreds of kilometres, which is released in the form of seismic energy causing earthquakes in the adjacent regions. The amount of energy we are talking about here can go up to 100 to 1000 times the energy released during the Hiroshima nuclear explosion. However, we tend to experience only a fraction of it. The plate movement also leads to the formation of “subduction zones” which are the regions where the plate which sinks below the other plate melts under heat and pressure and forms magma, e.g. Indo-Myanmar border.
Indian peninsular region is considered relatively more stable compared to the Himalayas and northeast India. It has rarely experienced any major earthquake events in history except a few. Earlier, this region was not considered seismically active. Two events, Koyna earthquake (1967) and Latur earthquake (1993), changed the perception after causing heavy damages and led to changes in zonal classification. Interestingly, the Koyna earthquake was triggered by human activities. This resulted due to reservoir induced seismicity which in turn happened because of increased pressure over the subsurface rocks due to seepage of the Koyna dam’s reservoir water. However, it was the great earthquake of Bhuj (2001) that compelled the entire community of earthquake engineers in India to carry out location-specific seismic studies. Now we know that almost 54% of the Indian region is vulnerable to earthquakes.
In an earlier paragraph, I discussed about subduction zones. India’s only active volcano, the Barren Island sits on a subduction zone boundary which extends from Assam to Indonesia. You might be surprised to know that the root cause of the devastating Indian Ocean Tsunami of 2004 was a major earthquake in Sumatra, which lies on this boundary! This highlights the importance of considering hazards which are by-products of an earthquake event, like, landslide, fire, tsunami etc. Most of the damage in hilly areas during an earthquake is often caused by triggered landslides.
Because of the research work carried out till now, we have understood that a seismic zone cannot be a single parameter for seismic study of an area inside it. There will be local factors which will affect the response of a particular site during an earthquake such as regional geology, sediment depth, rock type and distance from any active earthquake sources in the region. Regions of Bihar, although not close to Nepal, suffered heavily during the 1934 Nepal earthquake because of a sediment depth ranging in a few kilometres, which amplified the earthquake vibrations. As evident, it’s a very complex equation of multiple factors and numerous unknowns, I think that’s what makes this study so exciting!
Apart from earthquake studies, the motion of Indian subcontinent has always excited the researchers. It’s intriguing to learn how such a huge landmass broke its ties with Africa and travelled all the way to meet Tibet and formed the highest mountains, the Himalayas. Well, that’s a story for some other time.