Sikkim quake a warning for the unplanned city
Even the vulnerable and unplanned buildings in Dhaka sustained Sunday’s Sikkim earthquake, as it wakened by the time it struck Bangladesh.
The capital felt a “weak” (3.0 to 4.0 on Richter scale) jolt while the northern districts shook “lightly” (4.0 on Richter scale).
The United States Geological Survey (USGS) made a map following the earthquake Sunday evening to measure the intensity of the quake in different areas. The USGS initially measured it at 6.8 on Richter scale but later revised it at 6.9.
The quake that had its epicentre in Sikkim of India killed at least 71 people in India and Nepal. In Bangladesh, the nearly two-minute quake damaged a number of structures but claimed no life.
Dhaka is one of the top earthquake-prone cities in the world, earthquake expert Prof Humayun Akhter said.
“Sunday’s quake was a wake up call for Bangladesh,” said Asaduzzaman, Dhaka zone director of Geological Survey of Bangladesh.
Citing the USGS map, he said the intensity of the quake was very strong in Sikkim. But the further it travelled from the epicentre, the weaker it got.
So, by the time the quake hit northern Bangladesh, it had already come down to iv on Mercalli scale, which measures the intensity of the jolt felt at different places.
“In Dhaka, the strength of the earthquake was not more than ii or iii [on Mercalli scale],” he said.
At the epicentre, the energy transmitted was 16 centimetre (cm) per second but when the quake struck northern Bangladesh it came down to 4 cm per second. The higher the energy transmitted per second is, the stronger the quake strikes.
“It was less than 1 centimetre per second in Dhaka.”
Had it struck the city at 6.0 to 7.0 magnitude, almost all the buildings constructed on organic soil filling up wetlands would have collapsed, he added.
Only those buildings built on the Madhupur clay and Old Ravy sand in the capital are the most protected while those in eastern Dhaka are risky, says Asaduzzaman, who is preparing a geological map on Dhaka’s soil quality.
Some of the most vulnerable structures in the capital are at Shakharibazar in Old Dhaka, but they sustained as they are on the best quality soil.
According to the Geological Survey of Bangladesh, most areas in Motijheel, Sayedabad, Kamalapur, Segunbagicha, Kakrail, Banani, Gulshan, Dhanmondi, Lalmatia, Khilkhet, Baridhara, Agargaon, Mirpur 1-3, 6, 10-12 sections and parts of Uttara and Uttarkhan are made up of Madhupur clay, and relatively safe for high-rises.
But, buildings in Satarkul, Jatrabari, Meradia, parts of Badda, Mirpur Kalapani and Mirpur 14 and all other newly filled up areas are vulnerable, as most of these places were wetlands even in the recent past.
Urban geology experts stressed the importance of soil tests before constructing multi-storey buildings, saying most owners skip it to save money.
A recent survey done by Comprehensive Disaster Management Programme (CDMP), a government project, says some 78,000 of 3,26,000 buildings in Dhaka, 1,42,000 of 1,80,000 in Chittagong and 24,000 of 52,000 buildings in Sylhet are risky.
Although Bangladesh is by and large seismically active, Dhaka has yet to experience any moderate to large earthquake, experts say.
A study on earthquake in Dhaka city jointly conducted by Lamont-Doherty Earth Observatory, Columbia University, USA and the Department of Geology, Dhaka University in 2004 demonstrated that the western part of the capital from Mirpur-Kalyanpur to Pagla along the Buriganga river and the eastern part from Uttar Khan-Badda to Demra along the Balu river falls in the high risk zone.
-With The Daily Star input
