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The latest threat of earthquake-triggered tsunami has abated, but disaster-prone Bangladesh faces a high risk of moderate to strong quakes, experts have warned.
Bangladesh also faces the risk of tsunami as four active sources of earthquake in the Bay of Bengal can generate tremors with a magnitude of over 7 on the Richter scale, affecting the country seriously.
The observatory at Bangladesh University of Engineering and Technology (BUET) recorded 86 tremors of over four magnitude during January 2006-May 2009. Another four earthquakes took place with magnitude of over five during the period.
The meteorological department detected at least 90 earthquakes taking place in the country between May 2007 and July 2008, nine of them above five on the Richter scale and epicentres of 95 percent being within a 600 km radius of Dhaka city.
Experts say it is these minor tremors that indicate the possibility of much more powerful earthquakes hitting the country.
According to a seismic zoning map prepared by BUET, 43 percent areas in Bangladesh are rated high risk, 41 percent moderate and 16 percent low, Mehdi Ahmed Ansary told The Daily Star.
The map, which is being drawn up under the supervision of Ansary with funding provided by the science and information and communications technology ministry, divides the country into three earthquake vulnerability zones.
The current zoning map has, however, not been included in the Bangladesh National Building Code (BNBC) that needs to be urgently updated, pointed out Ansary who is also vice-president of the Bangladesh Earthquake Society.
In the zoning map of 1993, 26 percent of the country was high risk, 38 percent moderate and 36 percent low in terms of earthquake vulnerability.
A.S.M. Maksud Kamal, an expert on earthquake and tsunami preparedness, said four sources of earthquake in the Bay of Bengal are active and can generate tsunami.
He said one of the sources generated an earthquake of 7.8 magnitude in 1762 which generated waves in rivers and other closed water bodies, and around 100 people were killed in boat capsizes at that time in the Buriganga.
Kamal said all the four sources in the Bay called F1, F2, F3 and F4 have a capability of generating earthquakes of over seven magnitude creating tsunami which will affect Bangladesh.
In that case the sea level will rise by 4-5 metres at Nijhum Dwip. The water level will rise 2-3 metres at the Sundarbans, Cox's Bazar and the estuary of the Meghna.
Kamal said seven major earthquakes struck Bangladesh during the last 150 years and only two had the epicentre within the country.
The Srimangal earthquake July 8, 1918 was recorded at 7.6 on the Richter scale and its epicentre was in Balisera valley near Srimangal. Although there was damage, the intensity rapidly decreased due to the shallow focal depth and only minor effects were felt in Dhaka, he said.
The Bengal earthquake of July 14, 1885 caused considerable damage in the Sirajganj-Bogra region and perhaps more severe destruction in Jamalpur-Sherpur-Mymensingh region. The magnitude of the earthquake was more than seven on the Richter scale and the epicentre was at Manikganj, he added.
During the 1762 earthquake in Chittagong-Arakan coast the magnitude was 7.6 but the exact epicentre remained unclear.
The great Indian earthquake of June 12, 1897 that had a magnitude of 8.7 with the epicentre in the central part of the Shillong plateau was recalled as one of the world's worst.
He said any minor earthquake might be due to the activity in the local small fault zones, thus increasing the chances of a major jolt happening. Besides, Ansary felt a strong earthquake could occur in the plate boundaries as the 100-year alarm bells have passed.
Bangladesh is close to the meeting point of the Indian, Eurasian and Burma (Myanmar) plates. The movement of Indian and Eurasian plates has been locked at the foot of the Himalayas for many years, storing strain energy, he said.
When the lock is released, it will let out the strain energy causing major earthquakes that will affect Bangladesh, northeastern India and Myanmar, Ansary explained.
Three strong earthquakes were recorded from the Indian-Eurasian plate, which jolted Bangladesh within 150 years, said Ansary.
Due to the Indian and Eurasian and Myanmar plates, the Bihar-Nepal earthquake took place in 1934 and it was felt as far away as Dinajpur and Rangpur, he said.
The Assam earthquake Aug 15, 1950 had a magnitude of 8.6 on the Richter scale. The tremor was felt throughout Bangladesh but miraculously no damage was reported anywhere.
But the Mandalay earthquake that struck in 1858 with a magnitude of 7.9 affected Chittagong division, he added.