Interfan area >>Geology >>Neotectonics

		see structural map of East-Gangetic-Plains see detailed structural map of Baghmati Basin

Basement and Surface Faults

The upstream part of the Baghmati River basin in the Kathmandu region is influenced by the Himalayan tectonics. Besides the major thrusts i.e. Main Central Thrust (MCT) and Main Boundary Thrust (MBT) (Fig. 1.11), The Kathmandu Himalaya is traversed by several other faults. Some of them mark the lithological boundaries in this area (Table 1.3). They include Bhimpedi-Kathmandu fault (NE-SW) upthrown in SE part (Valdiya, 1976), Tasar fault, Kulikhani fault, Manari fault, Chau fault, Motihari-Everest fault and Motihari-Gourishankar fault (Arita et al.,1973; Valdiya, 1976; Dasgupta et al., 1987). It is also reported that that Bhimpedi-Kathmandu fault, Motihari-Gourishankar and Motihari-Everest are the seismically active faults (Valdiya, 1976; Dasgupta et al., 1987).

On the basis of the gravity data, seismic surveys and deep drilling data from the Gangetic foredeep, some basement faults have been demarcated by various workers (Sastri, et al. 1971; Rao, 1973; Karunakaran and Rao, 1976; Agrawal, 1977; Mukhopadhyay, et al., 1986; Verma et al., 1988; Dasgupta, 1993). These investigations clearly bring out two types of basement faults that are present below the foredeep alluvium cover. One set of faults orient sub-parallel to the Himalayan trend and the other is transverse to it. In the north Bihar region, the important faults are West and East Patna faults in the East Ganga Basin and the Monghyr–Saharasa Ridge with its bounding faults i.e. one passing through Rajgir and Barauni towards the NE and the other from east of Bhagalpur towards N/NNW. These faults are known as transverse faults, as the trend of these faults is transverse to the trend of Himalayan faults (Valdiya, 1976; Dasgupta et al., 1987). These transverse faults were formed during a rifting phase immediately before or contemporaneous with Permo-Carboniferous Gondwana sedimentation (Dasgupta, 1993). These transverse faults in the Himalayan region vary in direction form NW to NE direction and form a set of conjugate faults. Dasgupta et al.,(1987) interpreted that the northward movement of Indian plate is causing the activation of these subsurface transverse fault and added that a substantial part of convergence of Indian plate is accommodated by strike-slip motion across the Himalaya, in addition to normal faulting in certain areas.  

Another set of E-W trending faults has been recorded only in the western part of the Gangetic plain (Hari Narain and Kaila, 1982; Srivastava, et al., 1983). There is a possibility of occurrence of roughly E-W trending subsurface faults in the eastern Gangetic plain below the Gangetic foredeep, close to the mountain front (Dasgupta, 1993). Reactivation of these faults is possible in response to the Himalayan tectonics. These faults are perhaps the oldest and originated during the Precambrian (mid Proterozoic) period as possible passive margin normal faults (Hari Narain and Kaila, 1982; Srivastava, et al. 1983, Dasgupta, 1993).

Besides these subsurface faults, some surface faults have also been reported from the alluvial fan area of north Bihar Plains. The surficial faults in and around Gandak megafan include Rohini fault, Gandak fault, Rapti fault and Ghaghra-Ganga Fault (Raiverman, et al., 1983; Mohindra et al., 1992). The Gandak River is following the Gandak fault, which is trending NW-SE. Mohindra et al. (1992) have also inferred that the flood basin area in between the Burhi Gandak River and Kosi River is tectonically active and rapidly subsiding. Gohain and Parkash (1990) have worked on the Kosi alluvial fan and presented the geological map of the Kosi fan and the adjoining area. The map shows two large-scale thrusts trending in the NE-SW direction and NW-SE direction. Also, several other active faults are shown in the adjoining area of the Kosi megafan. Agarwal and Bhoj (1992) also interpreted several NNW-SSE and NNE-SSW trending faults in the Kosi River basin on the basis of remote sensing and geophysical studies. In general, the north Bihar Plains provide an opportunity to study the fluvial geomorphology in a subsiding basin.

Earthquake Occurrence

Occurrence of earthquake indicates the tectonic instability in the area and suggests that the faults are presently active. The earthquake activity in the Himalaya is attributed to the Alpine–Himalayan seismicity belt. Most well located epicenters reported after 1961, are concentrated in a 50 km wide zone, between MBT and MCT in the Lesser Himalaya, mainly south of MCT. On a regional scale, the seismic data reflects that the Nepal and Sikkim Himalaya and its foredeep region (which includes north Bihar Plains) are quite active tectonically. This area has recorded 131 seismic events (source: ISC Bulletin, in Dasgupta et al., 1987) during the period of 1963-1980. In the last 100 years few great earthquakes have occurred in the Himalaya, affecting the north Bihar region.

Dasgupta et al. (1987) have plotted these focal points and observed the concentration of seismic events in association with transverse features in the Nepal-Sikkim Himalaya and its foredeep region (north Bihar Plains) (Fig. 1.11). The Focal mechanism solution of the 16 large magnitude earthquakes has shown either strike-slip or normal mechanism or strike-slip with normal component. The nodal plane of these solutions ranges from NW-NE and also includes some N-S trending plane. Dasgupta et al. (1987) have also reported some folds with N-S axial trend. These data suggest that the present-day N-S lineament and all these transverse features are seismically active. The presence of N-S lineaments is also supported by occurrence of N-S fractures in Muzaffarpur during the major earthquake of 1988 (Sinha and Chatterjee, 1993). The major earthquakes in the region are further discussed in detail.

The Bihar-Nepal earthquake of August 26, 1833 was a violent earthquake of Magnitude 7.0-7.5 (Dunn, et al., 1939; Mittal and Srivastava, 1962). However, not much information is available of this earthquake. Another, earthquake of Magnitude 8.4 (approx.), occurred on 15^th January 1934, with its epicenter at latitude 26⁰21¢ N, longitude 86⁰12¢ E. It was located south of MBT (Dunn et al., 1939). Initially, it was interpreted to have occurred due to movement along a fracture zone between Motihari and Purnea (Dunn et al. 1939). However, Krishnaswamy (1962) interpreted that origin of this earthquake from the West Patna fault while Singh and Gupta (1980) deduced thrust and strike-slip faulting mechanism solution for this event and interpreted a plane striking nearly E-W and dipping south as the fault plane for this event.  

Recently, a major earthquake measuring 6.6 on Richter scale with its epicenter close to Bihar-Nepal border, shocked the northern Bihar and Nepal region on August 21, 1988. Banghar (1991) determined the fault plane solution of this earthquake and shown that this earthquake occurred due to faulting with component of strike-slip as well as normal fault. One of the nodal planes of this mechanism solution had the strike similar to the trend of the Himalaya, whereas the strike of other nodal plane was transverse to its trend in the epicentral region. The nodal plane striking 30⁰N and dipping 60⁰ NW was inferred as a fault plane. This fault plane was marked by left lateral sense of motion along the fault with some normal component. The inferred orientation of the slip vector was nearly northeast. The occurrence of this event at a focal depth of 71 km suggested that the transverse faults extended to deeper depth in this region. This earthquake occurred due to rupture along East Patna Fault (Banghar, 1991). Besides this, all other faults between Motihari and Kishanganj have equal potential for seismic hazard as they form part of the same fault system (Dasgupta, 1993).

Effect of Neotectonics on the River Systems of the Indo-Gangetic Plains

The neotectonic activity in the Indo-Gangetic Plains and its influence on the drainage network have been studied by several workers. Even though the thickness of alluvium in Indo-Gangetic Plains varies from 2 km to 6 km (Parkash and Kumar, 1991), several parts of the Indo-Gangetic Basin show the influence of neotectonic activity and subsurface structure on the drainage pattern. Kumar (1981) interpreted the migration of Yamuna River due to neotectonism. Bajpai (1983) related the N to NE flowing drainage in the southern marginal part of Gangetic Plain with the basement lineaments. Tangri (1986) proposed the neotectonism as the cause for migration and change in length of Ghaghra River system. The neotectonic features of western Gangetic plains were interpreted by Singhai et al. (1991) and Kumar et al. (1996) and it was concluded that in this area, the flow of Yamuna, Ganga and Ramganga rivers is confined along the faults. Srivastava et al. (1994) noted the confinement of river flow along the faults in the area between Ramganga  and Rapti rivers, part of the Gangetic Plain. Singh and Ghosh (1994) observed that the SW-NE trending ‘reactivated old basement lineaments’ are controlling the rivers in the southern part of Gangetic Plain. Ghosh (1994) studied the structural features in the Indo-Gangetic Plains and observed that the alignment of river flow, tilting of alluvial fans in the area is controlled by surface faults. It was added that these faults are also controlling the channel flow over these fan areas. The neotectonic control on the drainage system of Indo-Gangetic Plains was illustrated by Singh et al. (1996) through the use of morphotectonic elements. These morphotectonic elements include the skewed megafan, preferential alignment of river channels, sudden change in the direction of river alignment, knick points, distorted meanders, straightening of river course and asymmetrical terrace. Gupta (1997) observed the structural control on the origin and development of the megafans in the Ganga Basin.

The eastern region of the Indo-Gangetic Plains also shows the effect of neotectonic activities on the drainage pattern. The migratory trend of Brahmaputra River is controlled by tectonic movements within the Bengal Basin (Morgan and McIntyre, 1959). Abrupt cut in the stream meander in Brahmaputra River basin before achieving their maturity stage was interpreted as the consequence of reactivation of dormant subsurface faults during neotectonic activity (Bharali et al., 1993). Sharma (1993) also noted that the courses of many rivers of Brahmaputra valley are controlled by subsurface fracture and few faults. Singh et al. (1998) showed that Ganga-Padma fault is controlling the flow direction of Ganga River new Farakka, West Bengal.

Similar controls were observed in the north Bihar Plains also. Arogyaswamy (1971) showed the significance of subsurface structure on the lateral migration of Kosi River. Agarwal and Bhoj (1992) also related the migration of Kosi River with the activation of subsurface faults in the Kosi basin. Mohindra and Parkash (1994) observed that the faults in the Gandak mega fan like Rapti, Gandak and Ghagra-Ganga faults are controlling the river flow in the area. Sinha (1996) suggested that most of the channel movement in this area viz. avulsions, cutoffs were caused by neotectonics and/or sedimentological readjustments.

Neotectonic activities in the Himalayan region have resulted in unstable slopes, loose and fragmented rocks and higher river gradient. All these effects contribute to high sediment load of these rivers and, as a result, the beds to these rivers are rising rapidly. Specially, in South-Eastern Nepal, sedimentation in the river reaches in the 'Bhabhar' belt  are quite  high. The  bed  of  the  Kosi  River in  Bihar is now  at a higher level than the floodplain (Valdiya, 1987). It seems therefore that rapid subsidence combined with very high sedimentation rates has resulted in reduction in carrying capacity of channels in downstream reaches, which experience frequent and severe flooding.