Editorial

Earthquake disaster risk assessment in Northeast

An earthquake of magnitude 6.6 on the Richter Scale in Nepal that claimed six lives and left a trail of destruction is a grim reminder for the northeastern states in India to update risk assessments of the cities and towns in the region.

Sentinel Digital Desk

An earthquake of magnitude 6.6 on the Richter Scale in Nepal that claimed six lives and left a trail of destruction is a grim reminder for the northeastern states in India to update risk assessments of the cities and towns in the region. An earthquake cannot be predicted but relative risks from it within a city or town can be forecast, provided the data is periodically updated and used for timely revision of design codes for earthquake-resistant building construction. The Northeast region falls in Zone V. The highest among all five seismic zones in India in terms of intensity and frequency of earthquake incidents and risk assessments in the entire region needs to be updated frequently for better preparedness. Vertical growth of Guwahati and other cities and towns in the region due to the accelerated pace of development and economic growth symbolised by a rapid increase in the number of multi-storeyed and high-rise buildings calls for regular review of the preparedness for impending earthquakes. The Earthquake Disaster Risk Index Report, a publication of the National Disaster Management Authority (NDMA) released in September 2019 has quantified the risks in 50 cities and one district located in seismic zones III, IV and V on a pilot basis. The report highlights that in the last 25 years till 2019, India has witnessed several moderate earthquakes that caused around 40,000 deaths, largely due to the collapse of buildings and one of the main reasons for such large casualties is a lack of awareness of earthquake risks and poor construction practices. "The losses could have been reduced if there was preparedness for the expected disaster. Such preparedness can be initiated by an assessment of earthquake disaster risks. It also helps in enabling the mitigation efforts for future earthquake events," the report adds which summarises the rationale behind developing the Earthquake Disaster Risk Index (EDRI). Certain observations in the report about seismic risks within capital cities of the region are quite alarming and call for maximum possible preparedness and review of existing building permission and construction norms. The report states that Aizawl city is observed to have maximum risk with more than 90% of buildings either built on hill slopes or located on sites vulnerable to falling debris from the hill slopes. Besides, 35% of these buildings are located very close to an adjacent and seemingly unsafe building/construction, whose collapse can damage the buildings easily. Shillong and Kohima also have more or less similar conditions as that of Aizawl as these cities are situated in hilly areas with steep to extremely steep sloped terrains, according to the report. The EDRI of a city developed by NDMA, a Hyderabad-based International Institute of Information Technology is based on three factors- topographical condition (seismic hazard), the total number of people and buildings spread in the topography (exposure) and present condition of buildings (vulnerability). EDRI of Aizawl and Gangtok was found to be high while an index of other capital cities, including Guwahati, was classified as 'medium.' The report elaborates that the level of risk in a city depends not only on the vulnerability of individual building typologies in that city but also on factors such as the topography of the city, soil conditions, seismic hazard, population, possibilities of collateral hazards such as liquefaction of soil, landslides and fire, use of buildings -residential, office and commercial uses and floor area ratio or floor space index. It cautions that quality of construction alone does not ensure the safety of a building and a building constructed by following all the design guidelines but built on very loose soil strata or a vulnerable hill slope, is safe under normal loading conditions but "clearly unsafe under severe earthquake shaking". A critical gap in the risk assessment of a city is the lack of a comprehensive oversight mechanism to ensure authorities concerned have enforced all prescribed norms during the construction stage, including soil strata assessment. The report harps on the need for periodic evaluation of the EDRI and planning of Technical structural safety audit of the buildings so that present conditions can be compared from the inventory of surveyed buildings to have a reality check and to gauge the pattern of improvement based on which further appropriate measure shall have to be adopted to reduce the risk factor over the period i.e. from high to moderate, moderate to low. Disaster Management authorities in each state in the region creating and updating the inventory of all buildings and putting in place robust mechanisms of frequent safety audits of these buildings and other structures is crucial for reducing the risk level so that damage can be minimised. Building awareness among residents of the cities on EDRI can motivate them to take voluntary steps for getting their buildings surveyed and assessed for risks and undertaking retrofitting measures to make them safer. It is high time that North-eastern States update their database on earthquake risk and preparedness.