Morphotectonic Study of Frontal Siwalik Hills, near Gandhiri, Kangra, Himachal

The densely populated foothill zone of western Himalaya between the Beas river and the Chakki stream in and around Kangra is being critically examined by us considering the seismo-tectonic destructive potential of the 1905 (Mw 7.8) Kangra earthquake which has not been evaluated so far. We use Resourcesat 1: LISS III Ortho satellite imageries and field studies to qua-litatively and quantitatively assess the active tectonic setup of the terrain. Quantitative morphotectonic analysis viz. hypsometric integral, drainage basin asymmetry, stream sinuosity index, ratio of valley floor width to valley height, stream length gradient index, basin shape index and mountain front sinuosity index are being evaluated on high resolution digital elevation model. The five sub-drainage basins of the Gandhiri stream, the Sukar stream and the Duhg stream of Kangra district of Himachal Pradesh, India are being morphotectonically analyzed. The value of hypsometric integral, drainage basin asymmetry, stream sinuosity index, ratio of valley floor width to valley height, stream length gradient index, basin shape index and mountain front sinuosity index reveal that the terrain is tectonically active. An active dextral strike slip fault with significant oblique slip component has been inferred and is being named as the Gandhiri Fault. This fault crosses all the five sub-drainage basins and results in multiple stream offsets. The Index of Relative Tectonic Activity (IRAT) has been established for different sub-drainage basins. The study reveals that the terrain near Gandhiri in Kangra district of Himachal Pradesh is seismo-tectonically active and proper building codes should be followed in construction activity.


Introduction
The Kangra (1905, Mw 7.8) earthquake, along with the Bihar-Nepal earthquake (1934, Mw 8.1), the Assam (1950, Mw 8.4) quake and other recent earthquakes has highlighted the importance of identification of active faults and their characterization so as to comprehend the quantum of seismic vulnerability of the densely populated Himalayan foothill region. As per media reports 20,000 lives were lost in Kangra quake; 12,000 lives were lost in the Bihar-Nepal earthquake and around 5000 people were killed in the Assam earthquake. As per 2011 census of India, the population of Kangra district is more than 1.5 million. There is an urgent need to characterize the tectonic nature of the populated foothill region and to identify the active faults in the terrain which can pose a serious seismic threat. The tectonic activity in the Himalaya is due to the continuous movement of the Indian plate in the northward direction, at an average rate of 50 mm per year [1]. This has made the foothill terrain a high stress and strain zone. Morphotectonic analysis is being increasingly used to assess the relative impact of tectonic forces and the surface stability in different parts of the terrain spread over a broad region [2]- [16]. The use of geomorphic indices such as the stream length gradient index [17], drainage basin asymmetry [18], hypsometric integral [19], ratio of valley floor width to valley height [20], drainage basin shape [20] [21] and mountain front sinuosity index [20] in active tectonic studies has been successfully demonstrated by numerous researchers including [4] [5] [20] [22] [23] [24] and others. The study area ( Figure 1) lies in the frontal Siwalik terrain between the Beas river and the Chakki stream. The NW-SE trending Gandhiri stream drains the frontal Siwalik terrain and runs for the 23 km. The Gandhiri stream crosses the Soan Thrust, where the Quaternary sediments are overlain by the Middle Siwalik subgroup, joins the Beas river in the foothill region. In this study, we attempt to closely examine the terrain near Gandhiri in Kangra district of Himachal Pradesh to identify the active fault present in the region which could facilitate the possible damages in the event of a large magnitude earthquake striking the region. The current study would help built up relevant data required for a comprehensive evaluation of the seismo-tectonics of the region. The main sections in this paper are geological setup, materials and methods, results comprising drainage basin, morphotectonic indices viz. hypsometric integral, drainage basin asymmetry, stream length gradient index, mountain front sinuosity index, stream sinuosity index, basin shape index, valley floor width to valley height ratio and field investigation followed by discussion and conclusion.

Geological Setup
In the Kangra reentrant, rocks of the Mid-Miocene-Mid-Pleistocene Siwalik foreland group are exposed in the study area. The Middle and Upper Siwalik subgroups are better exposed. The lithostratigraphy of the Siwalik group exposed in the study area is depicted in Table 1. The active tectonic setup of the terrain  Alternating sequence of well indurated sandstones and purple maroon clays between the Beas river and the Chakki stream is poorly understood. In this study, we attempt to quantitatively analyzed the morphotectonic setup of the terrain on the basis of the active tectonic indices viz. stream length gradient index [17], drainage basin asymmetry [18], hypsometric integral [19], ratio of valley floor width to valley height [20], drainage basin shape [20] [21] and mountain front sinuosity [20]. The Bharwain anticline and the Dhinosar syncline comprise the major structural features of the study area. The northward dipping forelimb of the Dhinosar syncline forms a thrust junction with the Quaternary deposits. This thrust has been named as Soan Thrust. The Gandhiri stream (locally termed as Khad) drains the Bharwain anticline. This anticline has numerous minor faults which have developed parallel to sub-parallel to the major thrusting zone of the Himalaya by high oblique convergence rate along the Soan Thrust. The tectonic activity in the Kangra region has been documented by [25]- [36] and others.
The Bharwain anticline and the Dhinosar syncline form the major regional structural features in the area under investigation. The forelimb of the Dhinosar syncline dips towards North direction and is thrusted over the Quaternary deposits in the South. Crustal shortening of the foothill thrust at a rate of 14 ± 2 mm/yr [39] contributes to the tectonics in the region. Numerous minor faults have developed in parallel to sub-parallel orientation with the major thrusting zones of the Himalaya. In the study area, Middle Siwalik subgroup is represented by 5 -7 m thick massive horizontal beds of sandstone, siltstone, clays and inter-bedded pebbly beds. The outer Himalayan range consists of the Siwalik sequence which is folded and faulted due to fault propagation folding [40]. A shortening about 23 km since 1.9 to 1.5 Ma, across the Siwalik sequence in the Kangra reentrant has been reported [41]. Along the Soan Thrust, crustal shortening rate of 3 ± 0.3 mm/yr and slip rate of 3.3 ± 0.3 mm/yr is observed [33].
This in turn has resulted in accumulation of stress and strain in the terrain surrounding Gandhiri lineament zone. Geological map ( Figure 2) has been prepared by field traverse which supports the previous literature and different geological investigations.

Materials and Methods
The frontal terrain between the Beas river and the Chakki stream is thickly covered by vegetation.

Hypsometric Curve and Integral (HI)
Hypsometric analysis expresses the relative area distribution from one drainage basin to the surrounding terrain [19]. Hypsometric integral was calculated for each sub-drainage basin. The curves related to 5 sub-drainage basins have been plotted between relative area and relative height (Figure 7). The degree of convexity and concavity of the curve is used to differentiate between younger and the mature landscapes. The calculation procedure used is   The hypsometric curve has been prepared by data extracted from digital ele-

Drainage Basin Asymmetry (AF)
Asymmetry factor is a successful parameter for evaluating the active tectonic tilting in basins in the possible seismic potential zone by envisioning neotectonic structures [24] [46]. The asymmetry factor (AF) can distinguish tilting on basin scales and is sensitive to tilting in directions opposite to the course of the main channel [18]. It can also distinguish tectonic tilting of small scale basin or on mega scale. The asymmetry factor (AF) is related to the trends of the main channel and is sensitive to both the sides of the basin. A scenario in which the left or right side of the basin has larger tributaries than the opposite side indicates the tectonically rising of that segment. In such a case, the tributaries do headward erosion, and lengthening of channel takes place. It is calculated by the formula: where, A r is the Area of the right side of sub-drainage basin (towards downstream) and A t represents the basin area. AF values vary from 42.74 to 55.70.
The results indicate that the five sub-drainage basins of the Gandhiri stream are tilted towards East. The first sub-drainage basin exhibits the highest order of tilting.

Stream Length Gradient Index (SL)
Stream Length Gradient Index (SL) [17] reveals the gradient anomaly in channel   where ∆H/∆L is the gradient for the respective nick points, and L is the length from the most extended reach to the midpoint of the nick points. Each sub-drainage basin has significant channels that are divided into equal segments of 500 m, and the stream length gradient index (SL) values are calculated for each segment using a digital elevation model, and its average value is taken for each sub-drainage basin for assigning the class relative to the tectonic activity. The average value of this index in the five sub-drainage basins ranges from 47.05 to 101.58. Different classes [3] have been assigned to sub-drainage basins: 1 (SL ≥ 100), 2 (51 ≤ SL > 100) and 3 (SL ≤ 50). This lineament crosses the Gandhiri stream at three locations, so the values of stream length gradient index (SL) along the nick point of Gandhiri Stream are 74, 0.28 and 132.5, which suggests that the segments (L1 & L2) of Gandhiri Fault govern the strike slip movement along the Gandhiri stream. During field observation, well developed shutter ridges indicating active strike slip movement are observed in the sub-drainage basins. The distribution of stream length gradient index (SL) over the Gandhiri stream and their basins is depicted along the lineament (Figure 8). The variation in stream length gradient index (SL) reflects the tectonic activity. Stream profile along with the stream length gradient index (SL) has been plotted for evaluating the channel slope anomalies (Figures 9(a)-(e)). High peaks of stream length gradient index (SL) depict the steep gradient of the channel segment. The low peak may indicate the strike slip movement along the affected segments of the lineament.

Mountain Front Sinuosity Index (Smf)
Mountain front sinuosity index depicts the resultant between two opposing forces, one which develops embayment in mountain front and the other which tends to produce a straight mountain fronts. A strong tectonic influence results    (Table 4), which reveals the high upliftment rate of straight active mountain fronts with less erosional forces.

Basin Shape Index (Bs)
A circular basin shape represents tectonic stability while an elongation of the basin reflects the influence of tectonic forces which tend to alter the shape. The elongated shape of the drainage basin in the faulted region reflects the active tectonic settings [3] [20] [21].
The basin shape (B s ) is calculated by the formula: where B l is the sub-drainage basins length taken from its outlet to the mouth, B w is the width at the widest points of the basin. In this study, B s is calculated from digital elevation model and it is divided into three classes: elongated shape (B s ≥ 4); semi-circular (3 ≤ B s < 4), and circular (B s < 3), as shown in Table 5.

Stream Sinuosity Index (S)
Stream sinuosity is a delicate parameter that records tectonic changes in the Sinuosity indicates a quantitative index of stream meandering and is a distinctive property of channel pattern. It relates to the morphological, sedimentological and hydraulic characteristics of stream channels. The results of the indices in different sub-drainage basins of the Gandhiri stream represents in Table 6.

Valley Floor Width to Valley Height (Vf) Ratio
Valley floor width to valley height ratio (V f ) [47] is a geomorphic index used to separate V and U shaped valleys. The V shaped valley is formed by the high upliftment rate and maintenance of the equilibrium channel slope by a corresponding increase in incision rate of the channel. U shaped valley develops due to side cutting erosion and processes of streams in relatively stable tectonic settings. However, the presence of strike slip fault changes the landscape of the valley and terraces rise as sediments accumulate.
V f value is calculated as:  where "V fw " is the width of the valley floor; "E ld " and "E rd " are the rises of the left and right valley partitions individually, and "E sc " is the rise of the valley floor.
Location of V f of the sub-drainage basins of the Gandhiri stream is shown in Figure 10.  Table 7.
V f is generally low for V shaped valleys, and high for U shaped valleys. Low values of V f are related to higher tectonic activity. V f values of less than 1 are associated with direct dynamic down cutting streams, while V f values more than 1 suggest a flat or broad valleys (U-shaped) with prominent side cutting erosion and relative tectonic stability. Middle Siwalik sandstone at a height of more than four meter in the different sub-drainage basins of the Gandhiri stream ( Figure 12).

Discussion
Geomorphic indices have been used for classifying faults into different tectonic

Conclusion
The present study has been carried out in the Kangra district of Himachal Pradesh where maximum damage was caused due to 1905 Kangra earthquake. Numerous scenario earthquake damage studies have been initiated to assess the present day possible damages in the event of large magnitude earthquake striking the region. The present study which attempts to fill in this gap regarding the seismo-tectonic character of the terrain in and around Gandhiri is significant.
The results of the quantitative morphotectonic analysis reveal that the terrain is tectonically active and an active dextral strike slip fault which has been identified and named as the Gandhiri fault is a significant tectonic element. The index of relative tectonic activity reveals a very high activity for the sub-drainage basins 1 and 3 and relatively low tectonic activity for sub-drainage basin 2, 4 and 5. The data generated and the results thereof suggest that proper seismic codes be incorporated in all the planned constructions and retrofitting of the existing masonry structures.