The Indian landscape has been rocked by numerous moderate to high seismic sequences in the past. Owing to the subduction of the Indian plate into the Eurasian plate at the rate of 50–60 mm/year, several regions of our country come under the list of highly seismic-prone areas. Apart from this, numerous other fault lines in the interiors of the country have been a risk factor for other regions.
Keeping this in mind, we present a case study of three of the most significant seismic sequences which disrupted the Indian landscape and caused a widespread socio-economic loss. We have considered the Uttarkashi Earthquake of 1991, the Bhuj Earthquake of 2001 and finally the Sikkim Earthquake of 2011, spanning approximately two decades, which would lay down the pattern of initiatives and development that took place over that period, signifying the seriousness in the system regarding earthquake management. These studies will also shed light on the building typologies present in those regions and their associated structural failures during seismic occurrences.
6.1 Bhuj earthquake (2001)
One of the most devastating seismic activities witnessed by the Indian Subcontinent was the earthquake in Bhuj on Republic Day, January 26 2001, at 08:46:00 (Indian Standard Time). The epicentre was located at 23.36o North and 70.34o East with a focal depth of 16 km (10 mi), approximately 9 km south-southwest of Chobari Village in the Kutch district in Gujarat, India. As recorded by the USGS, the moment magnitude was 7.9 with an Intensity X (extreme) on the Modified Mercalli Index (MMI).
Official site reconnaissance carried out by the EEFIT organization reports more than 20,000 casualties, over 167,000 people got injured, and more than 1,000,000 structures were affected, including bridges, R.C. buildings, temples, and stone masonry structures, small/large block masonry structures and more. The total economic loss was estimated at around 5 billion USD.
Extensive building damage was observed on ground surveys carried out by the researchers. Extensive damage was observed in both the old non-engineered structures as well as in the newer engineered structures. In the Kutch region, for instance, extensive structural damage was observed owing to various factors like the use of random-rubble masonry practises, use of huge blocks of stones (25cm x 40cm x 60cm) with low grade/strength mortar, separation of thick masonry of roughly 40-60cm in two distinct wythes and more (Murty et al., 2001).
On a broader scale, damage to both the load-bearing and framed structures was seen. In the on-site survey, the reasons encountered for load-bearing structures were a) Unsymmetrical building design, (b) Insufficient gap between 2 or more buildings, (c) Absence of connecting band in structural elements, and (d) No through stone provision.
Framed structures, even at a distance of 200–300 km from the epicentre, suffered significant damage, reasons for which were a) Violation/ noncompliance to codal provisions, (b) Lack of proper detailing, (c) Short column effect, (d) Soft story effect, (e) Poor artistry, (f) Appendage effect and more (Bokey & Pajgade, 2004).
Even in Ahmedabad, about 250 km from the epicentre, several multi-storeyed buildings suffered total collapse. Upon analysis, it was found that these buildings were built unsymmetrically, with heavy mass concentration at the top of the structure and insufficient footing size. Other everyday observations include (a) Improper detailing at beam-column joint, (b) Discontinuous bars, (c) Absence of lateral ties, and (d) Insufficient lap length provided bars in columns and beams (Murty et al., 2001).
Throwing light on the geotechnical aspects of the Bhuj earthquake, widespread liquefaction was observed. In the Kachchh field, liquefaction was observed over a wide area. Sand boiling due to soil liquefaction was observed over large areas, with salt crusting resulting from drying. It is anticipated in the Rann of Kachchh due to saltwater near the ground level.
Many civil engineering facilities in the Kachchh area and the Navalakhi port in the Saurashtra region were damaged by liquefaction. The Bhuj earthquake revealed some fascinating details about the efficiency of bridges built on liquefied soil foundations. The Bhachau-Vondh bridge location, in particular, was fascinating, with four bridges of various ages and types of design. Once the pillars had liquefied, the superstructure stiffness of the bridges tended to establish the possible cause of the collapse. The piers of the arch bridge were vulnerable to unequal settling. In contrast, the piers of a more recent plate girder bridge were vulnerable to torsion, with piers revolving along the bridge's longitudinal axis (Madabhushi & Haigh, 2005).
Higher approach embankments in the current highway bridge seem to have caused large lateral forces on the bridge decks. The abutments could not withstand these large lateral forces until the base soil had liquefied. During this earthquake, the new bridge at Surajbari offered an excellent example of soil-structure interaction, with the base soil involved in the movements experienced by the bridge piers and decks.
6.2 Sikkim earthquake (2011)
On September 18 2011, Sikkim was struck by a strong earthquake of magnitude 6.9 Mw which caused several damages, including structural damage and loss of lives. The peak ground acceleration, PGA, was recorded at about 0.15g. The tremble was followed by three main aftershocks of more than 4.2 Mw magnitudes, each of which could damage the URM with weak tensile and shear strength. The maximum intensity of ground shaking in the region was estimated to be VI + on the MSK scale. An approximate economic loss of $14 billion was calculated in the disaster.
Most structures observed in Sikkim primarily fall under masonry (brick, block and stone), RCC and wooden building categories. The buildings are observed to have flat or sloping roofs of different materials, RCC, wood, etcetera. In general, government buildings suffered more damage as compared to their private counterparts. Also, more damage was observed in the newer structures compared to the older ones. Various types of damage patterns were observed after the earthquake, (a) collapse due to ground shaking amplification and lateral spreading, (b) pounding of buildings, (c) collapse due to out-of-plane rotation, (d) generation of structural cracks, (e) plastic hinge formation (Dutta et al. 2015).
Sikkim lies in the main boundary thrust(MBT) and main central thrust(MCT), collectively known as main thrust faults, due to which several earthquakes have struck the same region. One such quake of magnitude 5.7 Mw occurred in 2006, which incurred some damage to the structures such as the URM building and infill walls, upon which the 2011 quake caused the other impact (Kaushik et al. 2006).
There were some instances where the buildings were not damaged, including both engineered and non-engineered ones—the sound bearing capacity of the ground upon which the building was built. Also, the structures made from locally available materials such as timber and bamboo, which have a better shock-absorbing capacity, performed well although non-engineered (DMMC Uttarakhand, 2012). The structures/portions retrofitted before the earthquake did not damage or suffered negligible damage.
We can conclude that bye-laws were not adhered to, and there is an urgent need to improve and implement techno-legal guidelines. Apart from that, soft or open ground stories should be avoided. Retrofitting measures through bracing and intense beams may be adopted to arrest the problem of plastic hinges. Failures such as pounding can be avoided by providing the minimum necessary distance depending on the height of the adjacent buildings. Ground improvement before the construction of buildings should be a significant concern, especially in the hilly areas, as they cause a slope failure. Also, joints could be strengthened to avoid out-of-plane rotation. Structures could be made out of wood and bamboo as they provide lightweight roofing. In general, it can be seen that the traditional structures performed well; however, they are still not recognized under the building codes provided by the BIS. The experts should research the viability of traditional structures as they cost very little compared to the RCC buildings.
6.3 Uttarkashi earthquake (1991)
An earthquake struck the Garhwal Himalayas in northern India on October 20, 1991, around 2:53 a.m. local time. The earthquake produced severe ground shakings in the Uttarakhand districts of Uttarkashi, Tehri, and Chamoli. According to official reports, 307,000 people were affected in 1,294 communities, with a death count of 768 people and 5,066 wounded. The USGS recorded a surface wave of magnitude 7.1 Mw. The peak ground acceleration was measured to be 0.30 g. A total of 42,400 homes were damaged during the quake. The loss caused by the disaster was estimated to be around $60 million. (Cotton et al., 1996)
Uttarkashi, one of the most earthquake-prone regions in the country, is located in the significant Alpine Himalayan belt, one of the world's most seismic-prone stretches. Seismic activity in the belt is attributed to the movement of the Indian plate in the north direction, at a rate of 0.05–0.06 m per year against the Tibetan Eurasian platform block (Dewey and Bird, 1970) and (Molnar and Tapponnier, 1975), which deforms rocks and piles them in order to build the Upper Himalayas. In addition to many minor faults from the visible tectonic structures, two major thrusts tend from the Northwest to the South East. The shaking intensity was mild, and a variation in intensity was observed over the whole region. In Budhakedar, Krishanpur, Maneri, Uttarkashi, Mahinanda and Bhatwari, the maximum intensity was VIII. The MMI VII quake was in Tehri, Ghansyali and Gangotri. Other reports suggest that the MMI VII also shook Pauri, Karnaprayag and Gopeshwar. India's seismic code categorizes the country into five seismic zones (I to V). Uttarkashi is in zone IV, whereas Tehri and Chamoli are in zone V. According to the seismic zone map of our country, the anticipated MMI for zones I to V is V (or less), VI, VII, VIII, and IX (above), respectively. Hence, it can be inferred that an earthquake of design level struck Uttarkashi and its environs (Jain and Singh et al., 1992).
There was severe damage to rural dwellings, which comprise random rubble masonry supported by a heavy roof. Most private structures and former state properties were built without following the seismic provisions. Uttarkashi has three and four floors of framed, damaged reinforced concrete (R.C.) structures. In a two-storey post office building in Uttarkashi, the shear cracks were produced in the first columns, erected by engineers who worked in the post and telegraph departments from 1985–1986. The powerful floor beams in the frame obliged them to enter the columns of the ground level. Random rubble stone masonry was implemented to construct the retaining walls in the area. A decent number of collapses of such walls were observed in the site inspection. This collapse led to the failure of embankments as the walls were designed similarly (Jain and Singh et al., 1992).
Slopes, retaining walls, and bridges failed, causing significant damage to the area's roads. Due to many landslides and the collapse of a central bridge, the Uttarkashi-Harsil-Nelong Road link was shut down for many days. The Uttarkashi-Lumgaon connection was lost due to the collapse of a recess on the route to the Kishanpur Bridge. On the Uttarkashi-Harsil route, many large landslides occurred, particularly on a 42-kilometre section between Uttarkashi and Bhatwari. The stretch is said to be the shakiest part of the body. While landslides are prevalent along this road during wet seasons, several of the landslides produced by the earthquake were completely new (Jain and Singh et al., 1992).
The Gawana Bridge is a bridge built in 1974, covering 56.0 m. It is located in the direction of Maneri, 6 km. The whole bridge descended from the abutments and fell into the river, cutting the entire region beyond Uttarkashi off. The damage was caused by inadequate rooms and anchor bolts and the lack of acceptable methods to prevent the distance from coming off the supports. According to the Indian seismic codes in those times, the bridges in zone IV should be designed to take the seismic design force from 0.05g to 0.075g, significantly less than the recorded PGA. This seismic occurrence somewhat depicted that the parameter was inadequate (Jain and Singh et al., 1992). This implies that similar parameters are mentioned in the codes but are irrelevant from the on-ground perspective.
Apart from the damage incurred to the houses and residential buildings, the earthquake affected lifeline facilities and other systems. The triggered landslides damaged several electric and telephone poles, leading to total electricity loss and a ten-day communication cut-off. This led to disruption in the communication between the dam and the powerhouse, leading to no electricity generation. (Jain and Singh et al., 1992).
Indeed, we cannot mitigate the damages caused by the seismic occurrences as these phenomena are under the order of nature. However, with proper planning and implementation of effective strategies by the government of any country, we can scale down the extent of damage caused to our society. Even with the advancements in the scientific community, the field of earthquake engineering needs inputs from post-earthquake occurrences to learn about the response of structures to seismic waves, the local geology of a region and more. When these post-event learnings are clubbed with technological advances, robust literature encompassing relevant knowledge for better structural response could be produced.