Some engineering aspects of Chamoli earthquake

Some engineering aspects of Chamoli earthquake

Kusala Rajendran, C. P. Rajendran
Centre for Earth Science Studies, Trivandrum 695 031

A damaging earthquake, even though tragic, can also be an opportunity to improve preparedness so as to cut losses in the future earthquakes. This earthquake also offered some engineering lessons that have far reaching implications.

The Chamoli earthquake (magnitude 6.5) of 29 March 1999 in Garhwal Himalaya caused significant losses: about 100 persons dead, about 400 persons injured, about 22,000 houses damaged. A total of about 400,000 persons in about 800 villages were affected by the earthquake. A damaging earthquake, even though tragic, can also be an opportunity to improve preparedness so as to cut losses in the future earthquakes. This earthquake also offered some engineering lessons that have far reaching implications.

The earthquake took place in a region classified as the highest seismic zone (zone V) by the Indian code. Zone V implies that the area has potential for shaking intensity of IX (and above) on the modified Mercalli intensity scale. As against this, the area sustained shaking intensity of VIII; this too was limited to a few locations only. In that sense, the region experienced less than design intensity of earthquake shaking.

The Uttarkashi earthquake of 1991, which also occurred in the Garhwal Himalaya, was also of magnitude 6.5 and occurred about 100 km north-west of the present event. Because of that earthquake, awareness about the earthquake resistant features seems to have improved considerably in the region and today the local population appears quite receptive to such provisions. During our post-earthquake reconnaissance of the area, we came across numerous houses in rural areas built with traditional random rubble stone masonry walls but with reinforced concrete lintel band. Even though most deaths and injuries were caused by collapses in houses in random rubble masonry, those with lintel band showed excellent performance. A meaningful programme to seismically strengthen the entire building stock in the region is needed.

It is well known that the earthquake damage at a site depends not only on the earthquake size and distance of the earthquake source, but also on the local site conditions. This earthquake caused much more damage to settlements located on river terraces as compared to those located on hard rock. For instance, while Chamoli experienced shaking intensity of VIII, Gopeshwar, located on a higher elevation on the hill slope, but just 2 km aerial distance away, showed intensity V. Part of Delhi is on the sedimentary deposits of Yamuna river and constructions in such areas could be more vulnerable to damage. Extensive studies are needed on this aspect.

Another serious issue is damage sustained by a few buildings in Delhi during this earthquake. This damage, even though minor, is a matter of concern because it was caused by a moderate earthquake occurring about 280 km away. It shows that Delhi is vulnerable to damage not only due to earthquakes in its vicinity but also due to large earthquakes occurring in the far away from Himalaya.

Last 11 years are unique in the country’s earthquake history. Five earthquakes of magnitude exceeding 6.0 (Bihar 1988, Uttarkashi 1991, Killari, Latur 1993, Jabalpur 1997, and now Chamoli 1999) have occurred during this period. This has created some awareness at all levels: political, bureaucratic, scientific, and common man. It is the right time that a strong programme be initiated in the country to minimize the earthquake risk. The problem of India can no longer be kept on a low priority

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