The Financial Express

Living under the threat of earthquake

| Updated: October 25, 2017 04:09:39

Living under the threat of earthquake

An earthquake of 6.9 magnitude occurred near Mawlaik in Myanmar on April 13, 2016 that caused mass panic and damage of a few buildings in the country, specifically in the Chittagong city.

This is not an isolated seismic event rather a part of events that have been occurring along the north and eastern margins of the Indian plate boundary located at the juncture of three tectonic plates - the Indian Plate, the Eurasian Plate and the Burmese microplate.

The convergent collision of these three plates forms the two long active tectonic structures - the India-Eurasia plate boundary to the north building the mighty Himalaya Mountain belt, and the India-Burma plate boundary to the east building the Indo Burma Ranges.

The Indian plate, moving approximately 6 cm each year towards the northeast, is being subducted under the Eurasian plate at approximately 4.5 cm/year and the Burmese plate at approximately 3.6 cm/year in the north and east, respectively.

This continuous motion is taken up by active faults, which are broadly distributed over a series of reverse and strike-slip structures. Probability of an earthquake from a given fault depends on the rate of motion and the time since the last rupture.

The active faults of regional scale capable of generating moderate to strong earthquakes are present in and around Bangladesh.

These include about 320 km long east-west trending the Dauki fault, located along the southern edge of the Shillong Plateau (Meghalaya-Bangladesh border), the 150 km long Madhupur fault trending north-south situated between Madhupur Tract and Jamuna flood plain, the Assam-Sylhet fault, about 300 km long trending northeast to southwest located in the southern Surma basin, and the Chittagong-Myanmar plate boundary fault, about 800 km long running parallel to the Chittagong-Myanmar coast.

Other active faults -- the Hafflong-Disang fault to the northeast and Kaladan, Kabaw and Sagaing Faults progressively towards the east of Bangladesh -- can potentially generate large earthquakes.

Among all these faults, a major plate boundary-influenced fault -- the Dauki Fault -- is marking the northern boundary of the Bengal Basin.

The area has experienced high-magnitude earthquakes in the past, and the consequences were devastating, but the devastation may get intensified manifold at present because of huge population density in the poorly-planned urban areas.

The Dauki fault has not been slipped in the recent past, but was it to slip in a single earthquake its potential maximum magnitude would constitute a significant seismic threat to nearby densely populated areas of Bangladesh, India, Bhutan and Nepal.

Moreover, the mighty river systems, the Ganges and the Brahmaputra are potentially being influenced by the recent tectonic deformation.

Owing to the subsequent effects of 1782 earthquake, there was a major change in the Brahmaputra River course by over 100 km when it shifted from Old Brahmaputra course into its current course in Jamuna valley. If there is an earthquake today that could bring changes again in the river courses, which would be a catastrophic event and the whole low-lying and densely populated flood plain areas would be affected by severe floods.

Considering the above risks we, therefore, need to device our preparation and disaster response meticulously. We should immediately record and preserve detailed drawings, images and technical knowledge of all key point installations (KPI), cultural & architectural heritage buildings and monuments.

Construction of earthquake-vulnerability or hazard map for the cities as well as for whole of the country is necessary to be done into three categories based on meticulous study of the historical earthquake events, their magnitudes, and the frequencies -- the time of recurrence intervals within the known active faults and along the plate boundary:

a. Category 1: Maximum vulnerability map considering the possible extreme magnitude of earthquake event.

b. Category 2: By considering all historical high intensity earthquake events within, adjacent and surrounding of Bangladesh that is most likely to be subjected to similar intensity earthquake in near future.

c. Category 3: A moderate vulnerability or hazard map based on most likely possible earthquake event that the area might be subjected.

Our building code should consider and need to implement the above vulnerability or hazard level maps for all types of engineering construction as follows:

a. All KPI installations (e.g., nuclear power plant, metro rail, elevated express way, conventional power plants, oil refinery, airport, and sea port etc.) should be constructed as well as existing KPI need to be strengthened based on the possible extreme magnitude of earthquake events (Category 1: Maximum standard).

b. All public buildings, cyclone centers, schools, colleges, universities, high-rise buildings, commercial facilities and other deep foundation structures should be constructed based on proven earthquake events of the past (Category 2: High standard).

c. Conventional shallow foundation buildings in the cities and rural areas need to be constructed by considering the most likely possible earthquake event (Category 3: Minimum or threshold standard).  It may be suggested that the government and all other concerned authorities will take proper initiatives for the mass awareness about these vulnerability maps and their associated building code standards through knowledge dissemination programmes.

It is also imperative to make a disaster response master plan immediately by considering the possible worst case scenarios.

The plan also needs to be updated and evaluated based on the current state of scientific and technical advancement in every five years interval. This disaster response master plan needs to consider the following major issues:

a) Structured organisational volunteers of adequate numbers ranging from professional expertise to hard labourer should be trained; b) immediately after a major earthquake, geotechnical and engineering professionals must need to conduct quick temporary safety assessment checks and related measures for partly damaged buildings and engineering infrastructures; inform the associated risk to the concerned people or community accordingly; c) making short notice availability of the heavy logistics, food, medicine and other emergency supplies; d) reconstruction should be based on economic feasibility, urgent demand, and considering the importance of cultural & architectural heritage.

Before reconstruction, structural behavioral response of heritage buildings due to earthquake should be studied in details.

Earthquake resistant local and traditional proven technologies need to be considered for reconstruction or rebuilding rather than importing untested technology. Even, locally available construction materials should be used on priority basis.

It is essential to device policy by involving local community for reconstruction, maintenance and management of the heritage sites; e) bottom up approach of the volunteering organisation is needed to be considered for proper disbursement of emergency medical and food supplies. We should bear in mind that coping with severe floods and cyclones are much easier rather facing a strong earthquake.

It is to be remembered that the number of fatalities and cost of property damage primarily depend on the intensity of the earthquake, depth of the hypocentre, distance of the populated area from the epicentre, geology of the area, and quality of the engineering structures.

Fatalities also depend on the timing of occurrence of an earthquake whether that occurs at night or during the day, in the winter or in the summer, in a mountainous region or in a plain land or in coastal region etc. We need to keep in mind that earthquake is a natural calamity; it does not kill people directly.

Buildings and engineering infrastructure damages during earthquake kill people. A well-known study shows that approximately 83 per cent of all fatalities from building collapse in earthquakes over the past three decades occurred in the developing countries that are highly corrupt.

Hence, the total failure or partial damage of man-made engineering structures can be possible to control largely by efficient management, eliminating corruption and holding proper accountability during construction phase.

It is essentially important to safeguard all technical details related to the existing KPI and major engineering infrastructures, construction of three categories of earthquake hazard or vulnerability maps, and a disaster- response master plan for our country.

Probably, even more important is to make the people understand that the risk is real, and to convince them to work accordingly.

The writers are Associate Professor and Professor, respectively, Department of Geological Sciences, Jahangirnagar University

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