Earthquake round the corner!  

Helal Uddin Ahmed       | Published: July 24, 2018 22:10:43

The world experiences millions of earthquakes each year. But not many can be felt because of their low intensity and magnitude. Generally, a few catastrophic earthquakes occur each year, such as the one that hit Port-au-Prince of Haiti in 2010 and killed over 200,000 people. Approximately 10 large quakes and around 100 moderate ones also strike every year. All these serve as wake-up calls for the people living in earthquake zones. The danger is the greatest for those living in old masonry buildings not built according to earthquake standards. For those living on alluvial or river-flood plains like ours, the danger is magnified many times. Alluvium acts like a liquid during quakes and generates large waves similar to swells on a stormy ocean.

It is a fact that the sliding movements of the earth's tectonic plates, which form the outer and inner shells of the crust, cause earthquakes. The country stands on the Bengal basin, which is one of the most seismically active zones in the world. The land is extremely prone to earthquakes due to its location at the confluence of India, Burma and Eurasia plates. The seismic fault line of these plates run from Afghanistan, and extends up to Myanmar via Pakistan, India and Bangladesh. As a result, this region has experienced some of the worst earthquakes in history. In 1993, ten thousand people died in a major quake that shook the Indian state of Maharashtra. Another earthquake struck Afghanistan in 1998 that killed 4,500. Then the 2001 Bhuj earthquake of Gujarat (magnitude 7.7) in India killed up to 20,000. That was followed by the 2005 Kashmir quake, which claimed over 86,000, mostly in the Pakistan administered territories. In recent times, an earthquake having a magnitude of 7.8 on the Richter scale struck Nepal in April 2015, which killed around 9,000 people. Besides, many also died from minor quakes in India, Pakistan and Bangladesh over the past few decades. The earthquake that struck Chattogram in November 1997 killed at least 23, and the one at Moheshkhali in the Bay of Bengal dismantled most of the residential structures on the island in July 1999.

According to experts, there is a good possibility of a major earthquake striking Bangladesh in the not-too-distant a future. But society and the people at large do not seem to be much concerned about or aware of it due to its negligible incidence over a long stretch of time. The government also appears to be lacking in preparedness for any such eventuality in the immediate future. More than a century has elapsed since the last major earthquakes shook Bangladesh. Those were the Bengal earthquake of 14 July 1885, the Great Assam earthquake of 1897 and the Srimangal earthquake of 1918. The first one's magnitude was estimated at 7.0 and caused extensive damages in Sirajganj, Bogura, Jamalpur, Sherpur and Mymensingh regions. The second and third ones had magnitudes of 8.1 and 7.6 respectively.

The Indian subcontinent has been crashing slowly into Asia for over 40 million years in a geological pile-up that created and continues to feed the Himalayas. As mountains erode, sediments wash into the rivers Ganges and Brahmaputra and flow into the Bay of Bengal at the rate of billion tons per year. These sediments pile up in the delta region of the Ganges and Brahmaputra that encompass Bangladesh. According to the findings of a recent research by a team of scientists led by Michael Steckler of Columbia University released in 2016, a locked and loaded mega-thrust fault lies buried under miles of sediments underneath Bangladesh that could unleash an earthquake of magnitude 9.0 in one of the most densely populated areas of the world. Mega-thrust faults occur at zones where the earth's tectonic plates collide with each other and one plate moves (or 'sub-ducts) under another. This kind of faults produces the worst earthquakes, reaching and exceeding even magnitude 9.0. The recent examples are the 2011 Japan earthquake and the 2004 Banda Aceh earthquake that caused the Indian Ocean tsunami. Most of the mega-thrust faults lie beneath the ocean, and it is rare to find these below lands. It is even rarer and potentially catastrophic to have such a fault directly under a major population centre. The research by Steckler et al indicates that over 140 million people live within a perimeter of 100 kilometres in Bangladesh, India and Myanmar atop the active mega-thrust fault. 

The research utilised over 10 years' data from high-precision GPS receivers placed in northeast India, Bangladesh and Myanmar. It allowed the researchers to visualise the motion of plates and deformations around the fault due to locking together of the two sides. The study suggests that the fault is stuck and has been accumulating stress for over 400 years. The motion is building up tension along the plate boundary and that strain will be ultimately released through an earthquake somewhere along the fault. Although its timing is unknown, the quake could have an estimated magnitude in the range of 8.2 to 9.0. Such a quake would have catastrophic consequences for millions of people living near its epicentre.

Such an earthquake would also cause the sedimentary ground to behave like a liquid in a liquefaction process that could amplify the seismic waves. This would make the shaking even more devastating, as observed during the 2010 Haiti earthquake of magnitude 7.0 that killed over 200,000 people. An 8.0-magnitude earthquake is 32-times more powerful than a 7.0-magnitude one. Consequently, the resulting death toll could be huge if such a quake struck Dhaka (population 18 million) in place of Haiti's Port-au-Prince (population 1 million. The building codes in the rapidly expanding Dhaka have been ignored for long and the citizens remain unsure about what to do in the event of an earthquake. Therefore, apart from concerted efforts on building more earthquake-resistant structures as well as improving contingency plans to cope with relief and supplies, joint collaboration between the regional governments can be helpful in taking preparatory and mitigation measures for short, medium and longer terms.

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