Earthquake

Earthquake trembling or shaking movement of the earth's surface. Most earthquakes are minor tremors, while larger earthquakes usually begin with slight tremors, rapidly take the form of one or more violent shocks, and end in vibrations of gradually diminishing force called aftershocks. Earthquake is a form of energy of wave motion, which originates in a limited region and then spreads out in all directions from the source of disturbance. It usually lasts for a few seconds to a minute. The point within the earth where earthquake waves originate is called the focus, from where the vibrations spread in all directions. They reach the surface first at the point immediately above the focus and this point is called the epicentre. It is at the epicentre where the shock of the earthquake is first experienced. On the basis of the depth of focus, an earthquake may be termed as shallow focus (0-70 km), intermediate focus (70-300 km), and deep focus (> 300 km). The most common measure of earthquake size is the Richter's magnitude (M). The Richter scale uses the maximum surface wave amplitude in the seismogram and the difference in the arrival times of primary (P) and secondary (S) waves for determining magnitude (M). The magnitude is related to roughly logarithm of energy, E in ergs.

Earthquakes originate due to various reasons, which fall into two major categories viz non-tectonic and tectonic. The origin of tectonic earthquakes is explained with the help of 'elastic rebound theory'. Earthquakes are distributed unevenly on the globe. However, it has been observed that most of the destructive earthquakes originate within two well-defined zones or belts namely, 'the circum-Pacific belt' and 'the Mediterranean-Himalayan seismic belt'.

Although Bangladesh is extremely vulnerable to seismic activity, the nature and the level of this activity is yet to be defined. In Bangladesh complete earthquake monitoring facilities are not available. The Meteorological Department of Bangladesh established a seismic observatory at Chittagong in 1954. This remains the only observatory in the country.

Accurate historical information on earthquakes is very important in evaluating the seismicity of Bangladesh in close coincidences with the geotectonic elements. Information on earthquakes in and around Bangladesh is available for the last 250 years. The earthquake record suggests that since 1900 more than 100 moderate to large earthquakes occurred in Bangladesh, out of which more than 65 events occurred after 1960. This brings to light an increased frequency of earthquakes in the last 30 years. This increase in earthquake activity is an indication of fresh tectonic activity or propagation of fractures from the adjacent seismic zones.

Before the coming of the Europeans, there was no definite record of earthquakes. Following is a chronology of important earthquakes from 1548.

Chronology

1548	The first recorded earthquake was a terrible one. Sylhet and Chittagong were violently shaken, the earth opened in many places and threw up water and mud of a sulphurous smell.
1642	More severe damage occurred in Sylhet district. Buildings were cracked but there was no loss of life.
1663	Severe earthquake in assam, which continued for half an hour and Sylhet district was not free from its shock.
1762	The great earthquake of April 2, which raised the coast of Foul island by 2.74m and the northwest coast of Chedua island by 6.71m above sea level and also caused a permanent submergence of 155.40 sq km near Chittagong. The earthquake proved very violent in Dhaka and along the eastern bank of the meghna as far as Chittagong. In Dhaka 500 persons lost their lives, the rivers and jheels were agitated and rose high above their usual levels and when they receded their banks were strewn with dead fish. A large river dried up, a tract of land sank and 200 people with all their cattle were lost. Two volcanoes were said to have opened in the Sitakunda hills.
1775	Severe earthquake in Dhaka around April 10, but no loss of life.
1812	Severe earthquake in many places of Bangladesh around May 11. The earthquake proved violent in Sylhet
1865	Terrible shock was felt, during the second earthquake occurred in the winter of 1865, although no serious damage occurred.
1869	Known as Cachar Earthquake. Severely felt in Sylhet but no loss of life. The steeple of the church was shattered, the walls of the courthouse and the circuit bungalow cracked and in the eastern part of the district the banks of many rivers caved in.
1885	Known as the Bengal Earthquake. Occurred on 14 July with 7.0 magnitude and the epicentre was at Manikganj. This event was generally associated with the deep-seated Jamuna Fault.
1889	Occurred on 10 January with 7.5 magnitude and the epicentre at Jaintia Hills. It affected Sylhet town and surrounding areas.
1897	Known as the Great India Earthquake with a magnitude of 8.7 and epicentre at Shillong Plateau. The great earthquake occurred on 12 June at 5.15 pm, caused serious damage to masonry buildings in Sylhet town where the death toll rose to 545. This was due to the collapse of the masonry buildings. The tremor was felt throughout Bengal, from the south Lushai Hills on the east to Shahbad on the west. In Mymensingh, many public buildings of the district town, including the Justice House, were wrecked and very few of the two-storied brick-built houses belonging to zamindars survived. Heavy damage was done to the bridges on the Dhaka-Mymensingh railway and traffic was suspended for about a fortnight. The river communication of the district was seriously affected (brahmaputra). Loss of life was not great, but loss of property was estimated at five million Rupees. Rajshahi suffered severe shocks, especially on the eastern side, and 15 persons died. In Dhaka damage to property was heavy. In Tippera masonry buildings and old temples suffered a lot and the total damage was estimated at Rs 9,000.
1918	Known as the Srimangal Earthquake. Occurred on 18 July with a magnitude of 7.6 and epicentre at Srimangal, Maulvi Bazar. Intense damage occurred in Srimangal, but in Dhaka only minor effects were observed.
1930	Known as the Dhubri Earthquake. Occurred on 3 July with a magnitude of 7.1 and the epicentre at Dhubri, Assam. The earthquake caused major damage in the eastern parts of Rangpur district.
1934	Known as the Bihar-Nepal Earthquake. Occurred on 15 January with a magnitude of 8.3 and the epicentre at Darbhanga of Bihar, India. The earthquake caused great damage in Bihar, Nepal and Uttar Pradesh but did not affect any part of Bangladesh. Another earhquake occured on 3 July with a magnitude of 7.1 and the epicentre at Dhubri of Assam, India. The earthquake caused considerable damages in greater Rangpur district of Bangladesh.
1950	Known as the Assam Earthquake. Occurred on 15 August with a magnitude of 8.4 with the epicentre in Assam, India. The tremor was felt throughout Bangladesh but no damage was reported.
1997	Occurred on 22 November in Chittagong with a magnitude of 6.0. It caused minor damage around Chittagong town.
1999	Occurred on 22 July at Maheshkhali Island with the epicentre in the same place, a magnitude of 5.2. Severely felt around Maheshkhali island and the adjoining sea. Houses cracked and in some cases collapsed.
2003	Occurred on 27 July at Kolabunia union of Barkal upazila, Rangamati district with magnitude 5.1. The time was at 05:17:26.8 hours.
2006	Known as the Narail earthquake. This tremor occurred on 5 August. The earthquake had a magnitude estimated 4.2 on the Richter scale with epicentre located 110 km southwest of Dhaka near Narail. The tremor was widely felt in Dhaka as well as other places in the country.
2008	Known as the Manikganj earthquake. A minor earthquake jolted Dhaka and surroundings on the evening of 20 March 2008 and created considerable panic among the city dwellers. The earthquake had a magnitude measured 3.8 on the Richter scale. The epicenter was situated in Manikganj 41 km west-northwest of DUEO. It was believed to be originated from the Madhupur fault.
2008	Known as Mymensingh earthquake, this temblor occurred in the middle of the night of 27 July 2008.The epicentre was located 12 km northeast of Mymensingh city and120 km north of Dhaka. It had a magnitude estimated 5.1 on the Richter scale. Apart from Mymensingh where the earthquake caused panic, tremors from this earthquake were felt in many parts of the Dhaka metropolitan area.
2008	Known as Chandpur Earthquake. An earthquake with couple of aftershocks jolted Dhaka on the evening of 20 September 2008 just before Iftar. It caused tremendous panic among the city dwellers. The epicenter was 50 km southeast of Dhaka near Kachua of Chandpur. The magnitude was 4.5 on the Richter scale.
2009	Known as eastern Bhutan earthquake. A strong earthquake occurred on the day of Eid-ul-Fitr, 21 September 2009. The epicenter was situated in eastern Bhutan, 410 km north-northeast of Dhaka. It originated from the Main Central Thrust (MCT). This distant quake had a magnitude of 6.1, but shook most of Bangladesh including Dhaka.
2009	Known as Bay of Bengal Earthquake. Occurred on 11 August at Bangladesh time 01:55:35:.61 hours. The epicentre was located at the North Andaman Islands of the Bay of Bengal and seacoast of Myanmar. The magnitude recorded was 7.5. Though no significant damage was reported throughout Bangladesh the tremor was felt strongly in Dhaka.
2010	Occured on 10 September night at 11:30 pm local time. The tremor was felt in Dhaka and its surrounding areas with magnitude 4.8. The epicentre was 45 km southwest from Dhaka.
2011	Occured on 6:30 pm local time with magnitude 6.8. It lasted for 2 minutes. The tremor felt was strong enough in Capital city Dhaka and the districts of northern part of Bangladesh The epicentre was 500 north from Dhaka in Indian Sikkims’s capital Gangtok.

Status of earthquakes Bangladesh is surrounded by the regions of high seismicity which include the Himalayan Arc and shillong plateau in the north, the Burmese Arc, Arakan Yoma anticlinorium in the east and complex Naga-Disang-Jaflong thrust zones in the northeast. It is also the site of the Dauki Fault system along with numerous subsurface active faults and a flexure zone called Hinge Zone. These weak regions are believed to provide the necessary zones for movements within the basin area.

In the generalised tectonic map of Bangladesh the distribution of epicentres is found to be linear along the Dauki Fault system and random in other regions of Bangladesh. The investigation of the map demonstrates that the epicentres are lying in the weak zones comprising surface or subsurface faults. Most of the events are of moderate rank (magnitude 4-6) and lie at a shallow depth, which suggests that the recent movements occurred in the sediments overlying the basement rocks. In the northeastern region (surma basin), major events are controlled by the Dauki Fault system. The events located in and around the madhupur tract also indicate shallow displacement in the faults separating the block from the alluvium.

The first seismic zoning map of the subcontinent was compiled by the Geological Survey of India in 1935. The Bangladesh Meteorological Department adopted a seismic zoning map in 1972. In 1977, the Government of Bangladesh constituted a Committee of Experts to examine the seismic problem and make appropriate recommendations. The Committee proposed a zoning map of Bangladesh in the same year.

In the zoning map, Bangladesh has been divided into three generalised seismic zones: zone-I, zone-II and zone-III. Zone-I comprising the northern and eastern regions of Bangladesh with the presence of the Dauki Fault system of eastern Sylhet and the deep seated Sylhet Fault, and proximity to the highly disturbed southeastern Assam region with the Jaflong thrust, Naga thrust and Disang thrust, is a zone of high seismic risk with a basic seismic co-efficient of 0.08. Northern Bangladesh comprising greater Rangpur and Dinajpur districts is also a region of high seismicity because of the presence of the Jamuna Fault and the proximity to the active east-west running fault and the Main Boundary Fault to the north in India. The Chittagong-Tripura Folded Belt experiences frequent earthquakes, as just to its east is the Burmese Arc where a large number of shallow depth earthquakes originate. Zone-II comprising the central part of Bangladesh represents the regions of recent uplifted Pleistocene blocks of the Barind and Madhupur Tracts, and the western extension of the folded belt. The Zone-III comprising the southwestern part of Bangladesh is seismically quiet, with an estimated basic seismic co-efficient of 0.04.

Mitigation approach The occurrence of earthquakes in an earthquake prone region cannot be prevented. Rather, all that could be done is to make a prediction and issue a warning for minimising loss of life and property. Although precise prediction is not always possible, an acceptable valid prediction of an earthquake will certainly minimise the loss of life and property. However, as far as Bangladesh is concerned a detailed geological map including the delineation of all crustal faults and lineaments is of prime importance. The Aeromagnetic survey of Bangladesh has already provided the pattern and distribution of such faults and lineaments. By now the delineation of faults within the Tertiary sections are well established, but the situation within the Quaternary section is quite uncertain. It is evident that Quaternary sediments are affected by various earthquake events in Bangladesh pertaining to uplift, subsidence, ground deformation and massive liquefaction. Since water plays an important role in fault creep and fault slip, a small amount of water can produce an effect on a lubricated surface for fault displacement with a stress drop of only 10 to 100 bars.

The earthquake disaster mitigation approach should be followed by (i) pre-disaster physical planning of human settlements, (ii) building measures for minimising the impact of disaster and (iii) management of settlements. [Sifatul Quader Chowdhury and Aftab Alam Khan]

Bibliography MH Ali and JR Choudhury, Assessment of seismic hazard in Bangladesh, Disaster Research Training and Management Centre, Dhaka University, Dhaka, 2001; JR Choudhury and MH Ali, Seismic Zoning of Bangladesh, paper presented in the Seminar on Recent Development Earthquake Disaster Mitigation, Organised by IEB and TAEE, Dhaka, 1994; KM Hossain, Tectonic significance and earthquake occurrences in Bangladesh, 7th Geological Conference, Bangladesh Geological Society, 1989.