Earthquake Vulnerability of Guwahati

Ranjan Kumar Padmapati

(The writer can be reached at rkpadmapati@yahoo.co.in)

Of late, Guwahati and other places in Assam are experiencing earthquake jerks of milder intensity, measuring around 3 on the Richter Scale (R). Geologists are of the opinion that such mild jerks release accumulated stress and will avert future major earthquakes. In the recent past, earthquakes in Syria and Turkey killed roughly 47,000 people. Eleven percent of the landmass of India falls under the highest sensitive zone 5. Guwahati is a densely populated city within seismic zone five with high-rise buildings, and hence the probable casualties would be maximum. In general, 59% of the landmass of India is vulnerable to earthquakes of varying intensity. Assam has witnessed two major earthquakes in the past, one in 1897 and the other in 1950, with magnitudes up to 8.7 on the Richter scale. In total, 17 such earthquakes rocked Assam between 1869 and 1988, ranging in intensity from 7 to 8.7 on the Richter scale. The one in 1950 killed more than 4,800 people. Now old Assam-type earthquake-resistant buildings are gradually being replaced by high-rise buildings, and the risk factor has increased many fold. It is aptly said, “Earthquakes do not kill, buildings do”. It has been published in a section of print media that the predominant factor behind the huge toll of deaths in the recent earthquake in Turkey is the “construction amnesty granted on many occasions, granted on political grounds, for violating earthquake building code standards in the past. The last one was granted in May 2018, before the presidential election. And later on, these relaxations became laws. Seven million buildings were benefited by the “construction amnesty,” out of which 5.8 million were residential. Due to undue compromise in construction work, which violated building code standards, the earthquake damaged a huge number of buildings, as reported. It is interesting to note that some buildings constructed following building codes remained intact without any damage.

The contributing factors that are responsible for loss of life in an earthquake are (1) vulnerable housing stock, (2) distance of the epicentre from the earth’s surface, and (3) the time of occurrence. Ninety percent of deaths are due to the collapse of buildings. The 2019 Earthquake Disaster Index Report was published in June 2019. It listed 50 cities, including Guwahati, on the venerable list. No one can predict earthquakes precisely. Hence, the government should be prepared to meet the emergency. From the previous records of earthquakes in India, it is found that “non-engineered” buildings are more prone to collapse, and the damage is extensive. According to the National Disaster Management Authority, at present there is no mechanism that can enforce builders to comply with the provisions of the BIS codes for earthquakes or penalise violators. Hence, it is necessary to bring the builders under regulatory control so that only expert engineers, certified by a statutory authority, are allowed to be involved in the design process of buildings.

Different agencies have carried out surveys of buildings to assess the earthquake vulnerability in Guwahati. In Guwahati, the total building stock is divided into three groups: (A) Traditional Assam Type; (B) Confined and unconfined clay brick masonry houses; and (C) Ductile and non-ductile RC frame structures. A survey conducted by professors Nripendranath Patowari, Jayanta Pathak, and Viswajit Sarma of the AEC noted that out of traditional conventional building stock, 25% would experience extensive to complete damage, and 16% of buildings were classified as vulnerable unconfined masonry houses. About 40% of the building stocks are found to be susceptible to moderate damage; they consist mostly of confined and unconfined masonry houses. Many multi-story buildings have been constructed on hillocks in and around Guwahati. These are prone to landslides and rock falls. These tall buildings, along with hutments on hilltops, are to be surveyed for imminent danger. In the event of an earthquake, buildings swing just like betel nut trees and vibrate as inverted pendulums. Of the 60 municipal wards in Guwahati, 21 are partially or fully susceptible to landslides. Some of the common defects mentioned in the surveys are unsymmetrical walls, building blocks touching each other, building gaps that are not properly maintained, long overhanging projections, badly engineered flats, inferior supervision, defective material, etc. Symmetrical buildings, like squares and rectangles, are less prone to damage. Old buildings that do not conform to BIS codes should be surveyed and instructed for necessary retrofitting. In the recent past, an earthquake with intensity 6.4 (R) rocked entire Assam on April 28, 2021, in which a portion of Bhairabkunda hill in Udalguri had broken away and fell in the river, soil liquefaction took place, and water was oozing out from the ground with force. Many buildings in Guwahati and other places also developed cracks.

The killer Bhuj earthquake on January 26, 2001, with intensity 7.7R, killed about 14 thousand people and flattened 1.2 million buildings. After the quake, the government of Gujarat has undertaken an initiative known as “Earthquake Capacity Building and Risk Reduction Measures”. The damage was analysed, and two factors were assigned as root causes of the mass destructions: (1) general disregard for the existing earthquake building codes and absence of robust regulation enforcement authority; and (2) non-availability of skilled seismic engineering manpower in the state. The government felt that a mere degree in civil engineering is not sufficient without expertise for building design work. To enhance skill, the State of Gujarat introduced an “earthquake engineering syllabus” in diploma, undergraduate, and also postgraduate courses in civil engineering curricula. In Assam too, “Earthquake Engineering” as an elective subject may be introduced along similar lines. It would be still better if a full-fledged Earthquake Engineering Department was set up with a research laboratory to study different aspects of earthquakes related to the North Eastern States.

Masons engaged in building construction play an important role. They are not qualified enough to understand the complex implications of construction engineering work. Non-engineered buildings are more prone to destruction and are constructed by masons, not only in villages but also in cities. The Gujarat government has designed a “certificate course” for masons, ranging from a few days to a few months, so that trained manpower is readily available. As high-rise building failures are attributable to civil structure failures, the design of such structures needs to be under the ambit of a statutory authority, as suggested by the Government of Gujarat. The Assam Council of Professional Engineers (as proposed) will allow engineers to undertake building design work only after passing a written competency examination. The proposed Gujarat model may be studied to enhance earthquake capacity-building measures and risk reduction.