The stable shelf zone occupies the northwestern part of Bangladesh and is composed of the Pre-cambrian Indian platform. The area is primarily characterized by shallow thickness of sedimentary rocks above the Precambrian basement complex. This unit is geologically stable that has not been affected by folding. This zone is characterized by intra plate vertical tectonics with signatures of block uplift and subsidence (horst and grabens) within the Precambrian basement rocks.
The basement of the Indian platform slopes to the northwest and southeast from a central ridge. Taken into account the basement rock depth and its nature of slope the stable shelf zone has been further segmented into three parts: the Rangpur saddle, the Dinajpur slope and the Bogra slope [Bakhtine, 1966; Guha, 1978].
In Rangpur Saddle the basement is the most uplifted and is covered with thin sedimentary deposits. This segment connects the Indian Shield with the shilling massif and the Mikir Hills. Dinajpur slope is the northern slope part of Rangpur saddle and it inhabits the north-west portion of Rangpur-Dinajpur districts and it slopes down gently towards the Himalayan foredeep region. Bogra Slope represents the southern slope of Rangpur Saddle which is gently dipping southeast towards the hinge zone.
2. The hinge zone:
The junction segment between the stable platform and the foredeep zone is a zone of weakness running from Mymensingh to Calcutta in a south-west direction. This 25 km long narrow strip of flexure zone is known as the hinge zone. This NE-SW oriented zone (Fig. 3) runs from the Naga thrust in north-east India up to the west coast of Bay of Bengal in the south-west. [Uddin and Lundberg, 2004].This zone is thought of as zone of a series of deep-seated normal faults in the basement complex which divides the continental crust of Indian platform from the Bengal foredeep zone.
3. The Bengal foredeep zone:
The central foredeep zone is a vast sunken area between the stable shelf part in the west and the Indo-Burman hills in the east. Most of Bangladesh is occupied by this tectonic element. This zone is thought to be occurred by the Precambrian basement complex subsided due to the uplift of the Arakan-Yoma anticlinorium in the east during Oligocene period and subsequent accumulation of larger volume of clastic sediments. This foredeep zone comprises the active margin zone of the Bengal basin.
Based on the geotectonic behaviour, the central foredeep zone has been further divided into two major fragments: (a) Deep basin area and (b) The folded belt area [Hossain, 1989].
a. Deep Basin Area:
The deep basin area of the foredeep zone is occupied by a massive thickness of sediments with about 20 km thick pile of sediments in the deeper parts and it retains three prominent depression features in this region, the Sylhet trough, Hatiya trough and the Faridpur trough. Sylhet trough and Faridpur trough are separated by a relatively uplifted zone, known as the Madhupur high. This high area is believed to be uplifted in recent times as evidenced by the upliftment of two isolated uplifted Pleistocene blocks, Madhupur and Lalmai hills.
Faridpur Trough is situated adjacentent to the hinge zone and is characterized by a general gravity low with development of Neogene sediments [Hossain, 1989]. Barisal gravity high is located between the Faridpur and Hatiya trough of the foredeep region and this zone is described by few authors that it is attributed to a gravity and magnetic anomaly due to the presence of a magmatic body at depth.
The Hatiya trough is the deepest trough in the foredeep zone and it is located in the active margin. The Sylhet trough occupies the north-eastern part of Bangladesh and it is also known as the Surma basin. This area is a fast subsiding basin and is very close to the Dauki fault system in the north. Shillong massif forms the northern boundary of Surma basin while the Dauki Fault system separates the basin from the massif part.
b. Folded belt Area:
Folded Belt embodies the most noticeable tectonic element of active margin setting in the Bengal foredeep area with a large number of narrow, elongated and N-S trending folds in the eastern part of Bangladesh, Tripura, part of Assam, Mizoram and Myanmar territory (Fig. 3). This area is also characterized by numerous faulting controlled by subduction and transpression force regime. These folds are thought to be the western extension of the Arakan-Yoma anticlinorium in the east [Hossain, 1989]. Structural complexity of the folded flank increases onward from west to east into the Indo- Burman ranges. And it is demonstrating the influence of the compressional tectonic forces created by the overriding Burmese plate on the tertiary sediments of the folded belt.
2.1 Seismicity and historical earthquakes:
Seismicity of an area is directly related to the propagation of seismic wave energy at the surface of the earth that is produced by the release of enormous energy at shallow depth or deep inside the earth. This energy release can be attributed to the tectonic instability along zones of crustal weakness and corresponding propagation of energy causes various nature and intensity of ruptures and faulting. Earthquake epicenter location and the concurrence of an epicenter at a fault can provide valuable information about the position of an active fault zone. In north-eastern and south-eastern parts of Bangladesh, indication of paleo-seismic activities and subsequent propagation of fault ruptures inside the country are strongly assumed by researchers. These fault locations have been determined mostly by surface expression with aeromagnetic and gravity survey, aerial photos and satellite image interpretation. Some trenching investigation and seismic data acquisition at potential sites also indicate some vulnerable areas with new ruptures or recurrence of older ruptures. In fact earthquake epicenter location matches reasonably well with the zones of ruptures that were identified by surface expression and other geophysical tools. A number of small to moderate earthquakes are occurring in the country almost every year and significant damaging earthquakes have occurred in and around the country in last few centuries.
The earthquake history in and around Bangladesh suggests that although the country have been experiencing small to moderate earthquake intensity quite frequently over the years but the frequencies of events and their corresponding epicenters are higher in number in the close neighborhood. Crustal weak zones along the plate boundaries and their corresponding fault zones are the main driving force for distribution of epicenters in this region. Assessment of the seismicity map of the region (Fig.4) shows that the Arakan-Yoma anticlinorium in the east, the Naga thrust zone in the northeast, the Himalayan Arc, the Shillong plateau and the Dauki fault system in the north are seismically the most active zones around Bangladesh.