Corresponding author: R. Caputo, Department of Earth Sciences, University of Ferrara, via G. Saragat 1, IT-44122 Ferrara, Italy (email@example.com)
 We investigate the broader epicentral area of the M = 7.8, 1905 Kangra earthquake(s), north India, affecting the sub-Himalayan hills. The tectonics of the area is characterized by two major rentrants (Kangra and Dehradun) interposed by the Nahan Salient. The first-order topography between the Himalayan Frontal Thrust and the Main Boundary Thrust shows a marked lateral variation along strike of the mean gradient, characterized by a very small mean slope angle (∼1°) in correspondence with the reentrants and higher values (∼3°) in the salient. These tectonic and topographic features also show a good correspondence with the peculiar macroseismic field of the 1905 event(s), which is characterized by two distinct intensity maxima, separated by a distance of ∼150 km, clearly overlapping the two major tectonic reentrants. In this paper, based on available geological and geophysical information and a critical analysis of the general mechanical constraints, the seismogenic volume of the external sector of the chain is investigated in terms of critical taper model attempting to clarify the possible correlations between tectonics, topography and seismicity in the sub-Himalayan belt. Based on different assumptions, three possible seismotectonic scenarios are explored in order to constrain their likelihood and therefore to suggest a potential seismic gap in the area corresponding to the Nahan Salient, which may experience an event of significant magnitude in the future.
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 Tectonic processes have long been understood to build the topography of the Earth's surface [e.g., Tricart, 1974; Burbank and Anderson, 2001]. Once a land surface is uplifted, it is subjected to a competition between endogenic tectonic processes and exogenic surface processes. The concurrent role of both phenomena tends to modify the landscape of large regions of the Earth. In the past few decades, the study of the landscape has gained enormous ground in the interpretation of the interaction between tectonic and surface processes [e.g., Lavé and Avouac, 2000; Burbank and Anderson, 2001; Delcaillau et al., 2006; Singh, 2008; Caputo et al., 2010]. Investigations of landforms of different dimensions and distributions have led to the interpretation of tectonic processes from the local to the regional and up to the continental scale [Summerfield, 2000]. The significance of such studies in terms of seismic hazard assessment is still debated because of the lack of one-to-one relationships between morphological and seismotectonic parameters, while only a few papers clearly document such correspondence [Kirby et al., 2008]. There is also great uncertainty about prehistoric and historic seismic data, especially for the older events. In such an investigating scenario, this research is devoted to better understanding of the relationships between landforms and seismotectonic behavior by focusing on the Indian sub-Himalayan fold-thrust belt. The area investigated in the present paper (about 25,000 km2) is located in northern India, 100–200 km NE of New Delhi, one of the major cities of the subcontinent (Figure 1). The inferred results are certainly of interest for the recent seismotectonic evolution of the studied area and will also provide some insights for reevaluating the seismic hazard assessment of the broader region.
 From a tectonic point of view, the sub-Himalayan belt is delimited by two major thrust faults, the Himalayan Frontal Thrust (HFT) and the Main Boundary Thrust (MBT) (Figure 2). Tectonic activity along the Indian thrust faults is documented by either slow secular creep or episodic seismicity along some major faults [e.g., Kayal, 2001; Bilham, 2004; Gitis et al., 2008]. Both evidences of the ongoing crustal deformation and tectonic convergence should be considered complementary. If investigated together, they could provide either specific information or a broader time-space perspective of the seismotectonic behavior of a region, therefore allowing (1) better understanding of the seismic cycle and (2) improvement of seismic hazard assessment estimates, especially in areas where literature/data on the topic are scant or completely lacking.
 The importance of this kind of study lies in the fact that the Himalayan region is characterized by a significant historical seismicity, which includes the greatest earthquakes of the 20th century that heavily affected the sub-Himalayan belt [Seeber and Armbruster, 1981; Khattri, 1987; Ambraseys and Bilham, 2000; Bilham et al., 2001]. The late Quaternary tectonic unrest and the consequent evolving topography within the sub-Himalayan mountain belt provide an opportunity to investigate relief development versus seismicity, especially by using different geospatial data sets as well as geological and seismological information. Present-day topography is a cumulative manifestation of the interaction of geological processes, mainly tectonics, including seismicity through time, and climate. The climate for the investigated region, although varied in time, has remained more or less similar along the strike. Paleoclimatic reconstructions clearly document that though major changes in climatic conditions have occurred since the deposition of the sub-Himalayan sediments, these changes were regionally uniform in pattern and distribution in the Indian part [Singh et al., 2012].
2. Geological Setting
 The Himalaya is one of the most prominent and active intracontinental orogens showing a classical example of topographic relief development in a compressional tectonic setting. The origin of the Himalaya is attributed to the collision of the Indian plate with the Eurasian plate starting about 50 Ma and the persistent convergence that caused a shortening of about 2000–3000 km thereafter [Valdiya, 1998]. This process caused the development of a number of diachronous major crustal-scale detachments accommodating the tectonic shortening between the two plates (Figure 1). Slip and consequent throw (e.g., hanging wall uplift) associated with these contractional structures represent the principal geological process governing the regional and local relief development. The present-day tectonic convergence is concentrated along the southernmost of the major Himalayan thrust faults, i.e., the HFT that marks the tectonic and topographic front of the Himalayan orogen [e.g.,Yeats and Lillie, 1991; Lavé and Avouac, 2000]. Within the investigated area, the HFT trends in an almost NW-SE direction, while the surface trace of the MBT in this sector of NW India is markedly sinuous [Valdiya, 1998] (Figure 2). This geometry generates two structural reentrants (i.e., curved fault traces showing the concavity toward the foreland) interposed by a salient (i.e., convex toward the foreland; Figure 2). Where the sub-Himalayan belt is narrow (Nahan Salient), Tertiary rocks are largely exposed in an imbricate thrust systems; while where it is wide (Kangra and Dehradun Reentrants), alluvial deposits fill in broad synclinal valleys (duns). Two of the major contractional structures internally affecting the sub-Himalayan belt are represented inFigure 2 (Jawalamukhi and the Bilaspur Thrusts [Powers et al., 1998; Dubey et al., 2004]), and their possible role is discussed in section 5.2. Between the Kangra Reentrant and the Nahan Salient, the lateral variation of the geological characteristics is not sharp, suggesting the occurrence of a transition zone (TZ in Figure 2).
 The sub-Himalayan stratigraphic sequence (Table 1) could be grossly divided into the parautochthon and the autochthon units. The parautochthon unit consists of lower Tertiary sediments comprising the Subathu, Dagshai and Kasauli formations, while the autochthon unit is represented by the sediments of middle Miocene to Pleistocene age included in the Siwalik Group [Raiverman, 2002]. The parautochthon unit overthrusts the autochthon unit along few basin-scale faults [Raiverman, 2002]. Both these units are unconformably overlain by Quaternary fluvial terraces all along the major river valleys entrenching the hills [Lavé and Avouac, 2000]. Farther to the southwest, the autochthon unit is affected by active deformation also involving the Holocene Indo-Gangetic alluvial deposits as a consequence of subemergent thrusting and/or fault-related folding at the tip of the range-bounding HFT [Kumar et al., 2006].
Table 1. Simplified Stratigraphy of the Sub-Himalaya in NW Indiaa
 Shuttle Radar Topography Mission (DEM) data have been used for the present investigation [Jarvis et al., 2004]. The resolution of the used maps is 90 m, which is largely sufficient for the purpose of this paper. The DEM was processed to produce a shaded relief model for emphasizing the sub-Himalayan topography (Figure 2) and allowing the creation of serial topographic profiles (Figure 3) as well as swath profiles.
 The sub-Himalayan belt is characterized by low-elevation hills that are variably entrenched and dissected by erosion, thus giving rise to a rough topography [Delcaillau et al., 2006; Singh and Virdi, 2007; Singh, 2008]. In NW India, the sub-Himalayan belt has a width (orthogonal to the HFT) ranging from more than 100 km in the Kangra Reentrant to less than 40 km in the Nahan Salient (Figure 2). The topographic elevation of these hills varies from about 300 m above mean sea level at the base of the range front to maximum values of ∼2000 m. Higher values are commonly found northeast of the MBT. The highly variable altitude distribution across the area suggests the occurrence of a relatively rugged morphology (Figure 2).
 The sub-Himalayan belt represents the hanging wall block of the HFT taking into account the whole detachment plane and not only the frontalmost and locally upward propagating fault tip. In NW India, the trace of the HFT, or its superficial evidence where the fault is blind, conforms to a relatively straight line oriented NW-SE (Figure 2) showing very little lateral variation in strike. This “simple” fault trace geometry is likely due to a combination of (1) a planar geometry of the low-angle thrust plane (HFT), (2) the blind setting of the fault tip along most of the structure's strike and (3) its very young age that prevented any deep erosion. In contrast, as emphasized by the shaded relief model and many topographic profiles (Figure 3), the topography of the sub-Himalayan belt does not show a similarly homogeneous pattern and a uniform relief distribution. Accordingly, three major domains can be recognized: the Kangra area in the northwest, the Nahan area in the central sector and a third southeastern sector surrounding Dehradun (Figures 2 and 4).
 It is widely accepted that the uplift of these hills represents the surface expression of upper crustal tectonic processes mainly associated with sliding along the HFT and particularly its low-angle basal detachment and its secondary out-of-sequence branches [e.g.,Delcaillau et al., 2006; Singh, 2008]. Indeed, while activity along the low-angle basal detachment generates a uniform uplift of the hanging wall block, the out-of-sequence thrusts, which are commonly associated with fault propagation folding, cause localized vertical movements characterized by strongly uplifting belts (i.e., along the anticlinal axis) alternating with relatively subsiding sectors (Figure 2). Due to progressive both local and regional uplift, fluvial entrenching is constantly at work producing a very dense hydrography. As a consequence, the present-day stream network has highly dissected the landscape in this area, thereby developing badland-type morphology [Singh and Jain, 2009].
 During the last 2 centuries, the great Himalayan earthquakes have occurred at an average rate of one event about every 30 years [Seeber and Armbruster, 1981]. The investigated area within the sub-Himalayan belt has been the site of one, out of the four greatest earthquakes of the past century (1905 Kangra, M = 8.4 fromSeeber and Armbruster ; revised to Ms = 7.8 ± 0.05 by Ambraseys and Douglas ). The macroseismic field of this event is characterized by two intensity maxima (X and VIII+ Rossi-Forel scale [Middlemiss, 1910]) separated by a gap of about 150 km (Figure 5). Both intensity maxima are located close to the surface trace of the MBT in correspondence with the Kangra area (Imax= X; Rossi-Forel scale), in the northwest, and the Dehradun area (Imax = VIII+), in the southeast, showing a lower intensity zone in the interposed Nahan area (Figure 5).
 Although instrumental data were very poor at that time, the huge amount of collected in situ information and the macroseismic field reconstructed in great detail [Middlemiss, 1910] strongly suggest the occurrence of two distinct rupture areas. Their focal depth has been estimated between 15 and 28 km, both on low-angle dipping planes. A recent reevaluation of the macroseismic field along with the geodetic observations and the scrutiny of original seismograms seem to confirm the occurrence of two distinct earthquakes characterized by Ms = 7.8 and Ms = 7.0, the second probably triggered by the first, a few minutes apart [Hough et al., 2005].
 It has been also suggested that the Dehradun secondary intensity maximum could be due to a site effect [Srivastava et al., 2010]. However, the dimensions and the distance of this highly damaged southern area, but especially the occurrence of similar geological and morphological settings closer to the meizoseismal area (west and south of Kangra; Figure 5), do not favor this hypothesis.
 What still remains uncertain is whether both rupture areas represent two distinct and independent faults that were reactivated almost contemporaneously likely due to a dynamic stress transfer and triggering effect or a unique continuous shear zone characterized by two major patches of coseismic slip interposed by a large and strong barrier [Bilham, 2001; Wallace et al., 2005]. It is noteworthy that both intensity maxima occur where the MBT generates the reentrants on either side of the Nahan Salient, which in contrast was relatively unharmed in terms of damages. Site effects for the 1905 Kangra earthquake have been discussed in detail by many workers. Various documents and firsthand accounts were prepared since [e.g., Middlemiss, 1910]. Mainly, these documents and accounts formed the basis of scientific research and investigations that followed, even today, but they are beyond the goals of this paper. In section 5, we try to understand if there is any relation between the 1905 seismicity, the long-term tectonic activity and the landscape.
5. Critical Taper Model: Assumptions and Constraints
 In order to attempt to unravel the possible correlations between relief distribution, major seismic events and fault geometry, in this section we discuss the geometrical and mechanical aspects characterizing the external sector of the Himalayan thrust belt and analyze them in the frame of the critical taper model [Davis et al., 1983; Dahlen et al., 1984; Dahlen, 1990]. As far as there are several independent geometrical and mechanical parameters playing a role in the evolution of an accretionary wedge, especially concerning the supercritical, critical or subcritical conditions of the taper, we first try to constrain possible realistic ranges for the principal parameters and then discuss and analyze different geological and seismotectonic settings.
5.1. Pore Fluid Pressure
 It is evident from the boreholes and seismic profiles across the sub-Himalaya from India [Powers et al., 1998] and nearby Pakistan [Jaumé and Lillie, 1998] that suprahydrostatic pressures (locally close to geostatic values) have been encountered in several drilled layers, though often showing vertical variability. In the Kangra Reentrant, wells are increasingly overpressured with depth [Powers et al., 1998]. The increased overpressures are likely of tectonic origin, as reported from the Lilla well south of the Salt range thrust [Jaumé and Lillie, 1998], where strain is accumulating along a zone of intense deformation (both thrusting and folding) within the Kangra region Also, according to the Coulomb wedge theory of Davis et al. , where the wedge is close to critical failure, one would expect similar overpressures throughout the thrust wedge. This situation is plausible, as interseismic strain accumulation is documented across the Dehradun Reentrant [Yeats and Lillie, 1991; Gahalaut and Chander, 1997].
 Indeed, within the 6–8 km thick Pliocene-Quaternary sedimentary sequences infilling the Siwalik Foredeep, the Chinji and Nagri formations (Lower-Middle Siwalik Group;Table 1) mainly consist of clays and shales [Valdiya, 1998]. These sediments, characterized by sandstones (siltstones) alternating with argillaceous layers, in all likelihood, are also expected to hamper a complete seepage of water during progressive burial and consequent compaction. This condition therefore contributed to the increase of the pore fluid pressure within the lower clastic units of the Siwalik Foredeep, which are now close to, and probably contain at their base, the recently formed HFT. Accordingly, we can reasonably assume a suprahydrostatic fluid pressure ratio (i.e., λ> ∼0.42). For comparison with other orogenic critical tapers, both accretionary wedges and continental fold-and-thrust belts, theλ ratio commonly varies with depth, but it predominantly ranges between 0.6 and 0.8 [e.g., Fertl, 1976; Hyndman et al., 1993; Lallemand et al., 1994; Moore et al., 1995; Adam and Reuther, 2000; Calderoni et al., 2009; Roure et al., 2010; Saffer, 2010]. These values are also confirmed by the results from analogue [e.g., Cobbold et al., 2001; Mourgues and Cobbold, 2006; Graveleau et al., 2012] and numerical [e.g., Saffer and Bekins, 2006] modeling. Although in the calculations here carried out and discussed in section 6 we tested a broader set of λvalues, from about hydrostatic (0.4) up to (almost) lithostatic (0.99), we consider the above range (0.6–0.8) as preferred ones for the investigated area as suggested by the available information from nearby sectors of the sub-Himalaya belt [e.g.,Powers et al., 1998; Jaumé and Lillie, 1998].
 Also as concerns the fluid pressure ratio within the basal shear zone, λb, a comparison with other accretionary wedges shows it is commonly greater than λ within the wedge [e.g., Saffer and Bekins, 2002, 2006; Hayward et al., 2003]. According to numerical modeling and for the sake of simplicity, in our calculations we tested the λb varying from 0 to 50% greater than λ.
5.2. Topographic Gradient
 The overall topographic gradient and its lateral variability can be emphasized from the digital elevation data by analyzing serial topographic profiles across the sub-Himalayan belt (Figure 3). For this purpose, we traced numerous profiles, roughly parallel to each other, perpendicular to the HFT and with interdistance of 6–9 km (Figure 3a). As mentioned above, the two reentrants are characterized by the occurrence of few major duns. However, for the purpose of this paper we consider the mean slope of the profiles between the fault trace of the HFT (or its surface projection) and the MBT. In practice, these straight lines represent the topographic envelope of the accretionary wedge and are referred to in the following as α angle (Figures 3b, 3c, and 3d).
 From NW to SE, the results of this analysis (Figure 4) show a quite uniform mean slope of 0.9° ± 0.2° across the Kangra Reentrant, while the topographic gradient markedly increases in the Nahan Salient, where the relief is generally higher and characterized by steeper values (mean α= 2.6° ± 0.2°). Farther southeast within the Dehradun Reentrant, the mean slope again decreases with a mean value of 1.0° ± 0.1°, similar to the Kangra area. It is noteworthy the occurrence of very narrow zones marking both salient-reentrant transitions and characterized by intermediateα values (Figure 4). Finally, the two southernmost profiles at the limit of the investigated area show another sharp increase of the α angle to ∼1.8° ± 0.3°, allowing therefore definition of the southeastern “morphological” boundary of the Dehradun Reentrant (Figure 4).
 In order to test if there is any relation between the α angle and the distance between the HFT and the MBT, we plotted the corresponding values on Figure 6a, where it is evident that these two parameters have no statistical relation as clearly confirmed by the very low value of the R2test relative to the best fit regression line. In fact, along the profiles crossing the two reentrants the angular value is almost constant and close to 1° for a wide range of HFT-MBT distances (from 20 to >100 km;Figure 4). Also, for the profiles crossing the area of the Nahan Salient it does not seem to occur with any statistically meaningful relationship with the thrust distance.
 As mentioned in section 2, north of the Kangra Reentrant, the sub-Himalayan belt is affected by two major reverse faults like the Jawalamukhi Thrust and the Bilaspur Thrust (Figure 2) [Powers et al., 1998]. If we take into account the distance between the HFT and these structures instead of the MBT (Figure 6b), a slightly better statistical correlation between thrusts distance and α angle seems to exist. Indeed, the result of the R2 test is much better (R2 = 0.7), thus allowing tentative inference of a possible relationship between the two parameters. If this is the case, we should therefore assume that the accretionary wedge is mechanically constrained on the internal side by the Jawalamukhi and Bilaspur Thrusts and not by the MBT. The former structures disappear and merge with the latter structure in the sector south of the Nahan Salient (Figure 2) possibly confirming the above trend (Figure 6b).
 In order to obtain regionally averaged values of the mean slope to be used in the frame of the critical taper model, we also traced several swath profiles with different sampling widths, from 10 to 80 km (50 km in the Nahan Salient), across the two reentrants and the interposed salient (Figure 3e). Among the three “cumulative” profiles obtained using this technique (minimum, mean and maximum), the maximum one is certainly the best representative of the mean slope (Figures 3f, 3g, and 3h). The obtained angular values are very similar to the above ones being 0.9°, 2.9° and 1.1° for the Kangra Reentrant, the Nahan Salient and the Dehradun Reentrant, respectively. Taking into account the results of both methodological approaches and the overall uncertainties in the measurements, in the calculations for the taper model we will conservatively consider a range of α values for the three sectors: 0.7°–1.1°, 2.4°–3.1° and 0.8°–1.2°, respectively.
5.3. Basal Detachment Geometry
 Another crucial role is played by the geometry of the basal detachment. In this regard, it is still uncertain whether it is represented in detail by multiple distinct stepping segments (i.e., flat ramp geometry) or by a unique continuous sliding surface from which secondary branch faults depart. Based on comparison with similar convergent systems worldwide [e.g., Guarnieri et al., 2002; Bigi et al., 2003; Suppe, 2007; Park et al., 2010] and seismic reflection profiles crossing the area (Figure 7), the latter hypothesis is more likely. However, a gently dipping basal detachment without strong angular variations is also suggested by focal mechanisms for large earthquakes [Ni and Barazangi, 1984]. In line with this assumption, it is important to analyze the possible occurrence of both downdip and along-strike variations of this major shear zone. In terms of geometrical characteristics, downdip variations of the basal detachment are well documented as a large-scale feature, like the so-called Himalayan midcrustal ramp [Gahalaut and Kalpna, 2001; Avouac et al., 2001], also referred to as Basement Thrust Front [Kayal et al., 2003]. However, this major ramp has no influence in the recent evolution of the external sector of the critical taper due to its structurally internal position relative to the frontalmost thrust.
 Moreover, no information is available for the occurrence of minor ramps within the external sector of the basal fault, their exact location, downdip width, their real dip angle and especially the angular variation with respect to the upper and lower flat segments. In general, the occurrence of a frontal ramp along a thrust fault with undulated geometry generates a stress concentration within the surrounding crustal volume. Even assuming, as a first approximation, a uniform distribution of the friction resistance along the sliding surface (i.e., μb = const), “local” stress variations induced by a downdip undulated geometry could produce a clustering of minor seismic events and possibly a relatively higher frequency of strong earthquakes [Kayal, 2001]. Unfortunately, the incompleteness of the historical seismic record for the region [Ambraseys, 2000; Bilham, 2001] does not allow definite neglect of the hypothesis of a relatively shorter (but neither longer) recurrence interval of strong events for the area.
 Available information as obtained from seismic reflection and geological profiles and borehole data was systematically investigated for constraining the dip angle of the basal detachment (β) in the most external sector of the decollement, here represented by the HFT, underlying the sub-Himalayan belt (Figure 7). The dip angle of the basal detachment in the Kangra Reentrant has been constrained at ∼7° from exploratory wells and balanced cross sections [Mukhopadhyay and Mishra, 1999]. A similar value has been proposed also by Thakur , though Powers et al.  propose a lower one (2.5°). Further information is available immediately north of our investigated area, where DiPietro and Pogue  and Yin [2006, and references therein] propose a βangle of ∼5°. Moreover, based on the depth-to-basement contours [Geological Survey of India, 2000] and reasonably assuming that the shear zone runs parallel and close to this regional monocline, the dip angle of the detachment can be estimated for the most external sector of the thrust at ∼3°, increasing downdip to 10°–15°. The latter certainly represents a minimum value because a larger cutoff angle likely occurs within the Dharamsala formation underlying the Siwalik Group. In summary, for the taper model we will consider β ranging between 4° and 7°.
 As concerns the Dehradun sector of the HFT, Powers et al.  propose a balanced cross section showing a β angle of 6° (Figure 7). The basal detachment can be also seismologically constrained based on the 1991 Uttarkashi earthquake and its aftershocks sequence [Cotton et al., 1996], but only along its deepest and northeastern segment where it ranges between 5° and 7°. A slightly lower value (4°–5°) has been suggested for the southwestern segment based on balanced cross sections of the HFT [Mukhopadhyay and Mishra, 2004]. Accordingly, for the taper model we will consider β ranging between 4° and 7°.
 As stated above, moving toward more internal sectors of the Himalayan chain and a deeper setting of the basal detachment, the dip angle is generally steeper, where northeast of the Main Central Thrust (MCT) it possibly reaches values of 10° and up to 15° (midcrustal ramp; Figure 1b). Regarding the problem of variations in strike of the Himalayan midcrustal ramp, the available geological and geophysical information does not allow resolution of it. In any case it would play a minor role for the general seismotectonic evolution especially in the frame of the critical taper model.
5.4. Rock Strength
 Downdip and/or along-strike variations of the mechanical behavior could also influence the evolution of the taper and hence represent a crucial issue for better understanding the seismotectonics of the external Himalayan thrust belt. In particular, we should consider both the mean rock strength within the accretionary wedge (μ) and the mean friction along the basal detachment (μb). For example, in the frame of the investigated area, it would be important to know if the shear zone underlying the region interposed between the two 1905 macroseismic intensity maxima (i.e., Nahan Salient; Figure 5) is characterized by extremely weak rocks, prone to deformation mainly by very low magnitude seismicity (“creep”), or, in contrast, by rocks stronger than the two lateral segments of the detachment underlying the Kangra and Dehradun Reentrants. In fact, it has been observed that in the Nahan Salient, the internal strength of the rocks (Bilaspur limestone) involved in thrusting increases [Raiverman, 2002]. The latter hypothesis is also in agreement with the information available for the 1905 seismic event, and it possibly explains the local halting of the rupture propagation process in correspondence with the Nahan Salient, which somehow constrained the overall seismogenic area and hence limited the released seismic moment. The importance of lateral rheological changes within fold-and-thrust belts has been recently highlighted byBuiter  based on a review of compressional wedge models.
 On the other hand, this would also imply that (1) the accretionary wedge is here well coupled with the footwall block, (2) shear stress is at present accumulating within the Nahan Salient, (3) the corresponding fault segment represents a barrier and (4) when critical conditions will be reached, a likely strong earthquake will thus affect the region.
 Also for the southeastern fault segment possibly reactivated during the Ms = 7.0, 1905 earthquake and producing the secondary local intensity maximum (Imax = VIII) near Dehradun (Figure 5) [Hough et al., 2005], a considerable stress accumulation has been suggested [Bilham, 2001]. The above hypothesis should be carefully considered for reassessing the seismic hazard of the region.
6. Critical Taper Model: Possible Scenarios
 Based on geological information, it is possible to suggest different seismotectonic scenarios, which under some assumptions could provide inferences and constraints for improving the seismic hazard assessment of the investigated region. Also, based on the constraints summarized in Table 2, we have applied the critical taper model [e.g., Davis et al., 1983; Dahlen et al., 1984; Dahlen, 1990] to the three major sectors of the sub-Himalayan belt (Kangra and Dehradun Reentrants and Nahan Salient). In order to facilitate the discussion, numerous values ofα versus β have been calculated by systematically varying the principal parameters (μ: internal friction of the wedge; μb: friction along the basal detachment; λ: ratio between pore fluid pressure and vertical stress component within the wedge; λb: ratio between pore fluid pressure and vertical stress component within the basal detachment shear zone) following the well-known equations proposed byDavis et al.  for subaerial wedges:
where K is approximated as follows, considering φ = atan(μ):
Accordingly, we generated several plots using different sets of critical taper conditions and tested for a range of μb/μ values (0.2 to 1.2) as shown in Figures 8 and 9. Using these graphs and the constraints for the different parameters discussed in section 5, in this section we discuss various mechanical scenarios trying to define the more realistic ones in terms of seismotectonic behavior of the broader region.
Table 2. The Angular (α and β) and Mechanical (μ, μb, λ, and λb) Parameters Used for Applying the Critical Taper Model to the Kangra Reentrant (KR), Nahan Salient (NS) and Dehradun Reentrant (DR), the Tested Ranges and Their Preferred Valuesa
Preferred values are given in parentheses.
 Further constraints are based on the laterally homogeneous paleogeographic, and hence a more uniform lithological, distribution within the Siwalik Foredeep [e.g., Valdiya, 1998]. This permits the reasonable assumption that both internal friction, μ, and pore fluid pressure, λ, within the wedge are laterally uniform, at least as a first approximation. Although with a larger uncertainty, a similar assumption may also hold for λb at least in a geological time perspective.
 As a first end-member scenario we assume the occurrence of critical taper conditions in all three sectors and take into account the possibility of a NW to SE variable basal friction,μb. For example, assuming λ = λb = 0.7 and μ = 0.65, the estimated range of values for μb is 0.26–0.36, 0.45–0.51 and 0.27–0.38, in the Kangra Reentrant, Nahan Salient and Dehradun Reentrant, respectively (Figure 8). This mechanical variation along strike would imply that much stronger materials are involved within the shear zone of the salient thrust segment with respect to the two nearby reentrants (or weaker materials in the two reentrants).
 Alternatively, we could also neglect the hypothesis of a uniform λb all along the basal detachment. Calculations and graphs in Figure 9 show that pore fluid pressure within the shear zone of the Kangra and Dehradun Reentrants should be respectively 20–35% and 15–30% greater than within the wedge (λ), while no additional overpressure would be required in the Nahan Salient. Whatever the case (relatively larger μb or smaller λb), in the central sector of the investigated area a better stress coupling should be envisaged between the accretionary wedge and the underthrusting footwall. A possible cause for these lateral variations could be the recent to subrecent involvement of inherited positive and negative basement structures hidden by the foredeep deposits and representing the northerly extension of the orogens of the Indian shield [Raiverman et al., 1983]. Based on geophysical investigations in the Indo-Gangetic plains south of the HFT,Rao  suggested the occurrence of a basement high south of the Nahan Salient and lows or depressions in the two adjoining reentrants.
 Recently, Rajendra Prasad et al.  discuss the curved geometry of the MBT suggesting it is partly controlled by the sedimentary thickness of the Siwalik foreland basin and arguing this exerts a principal control on the thrust wedge thickness in the Lesser Himalaya, i.e., north of the MBT. In reality, the major contention is represented by the geometry of the MHT and particularly whether the lateral variation is due to a lateral ramp [Dubey et al., 2004] or a smooth surface. Also there is no expression of recesses and salient in the HFT and so it seems plausible that whatever be the case the MBT does not have a strong control on the accretionary wedge width. Only future and more detailed investigations will possibly confirm this subsoil setting. In summary, this scenario seems in agreement with the hypothesis of the Nahan Salient representing a seismic barrier, which possibly halted the coseismic propagation during the 1905 event.
 As a second seismotectonic scenario, we assume a priori that the mechanical parameters have laterally uniform values within their uncertainty range. Whatever the exact value of μ and λ, within the assigned ranges of reliability (see Table 2), and taking into account the α and β angles of the three sectors (as previously discussed), critical taper conditions along strike of the investigated area are necessarily uneven. The causative faults of the two major 1905 shocks (Ms= 7.8 and 7.0) were likely two segments of the basal detachment, as far as secondary higher-angle reverse faults branching upward from it (1) are probably too small for releasing such amount of seismic moment (6–6.5 × 1020 and ∼4 × 1019 N m [Kanamori and Anderson, 1975; Wells and Coppersmith, 1994]) and (2) would have produced more clear evidence of differential uplift. In a recent review of the macroseismic data paired with geodetic investigations, Hough et al. support the hypothesis of a detachment reactivation during the 1905 events neglecting the out-of-sequence thrusting hypothesis. If this is the case, the taper theory would suggest that the Himalayan wedge in correspondence with the Kangra and Dehradun Reentrants are in critical conditions [Davis et al., 1983] (Figure 10b), while based on the above assumption (second seismotectonic scenario: lateral uniformity of the mechanical properties within the taper), calculations and graphs (Figures 8 and 9) clearly show that the basal friction, μb, should be strongly reduced (60–80% of μ) and/or pore fluid pressure along the shear zone, λb, should be 20–30% in excess of λ. In both conclusions, the wedge in the Nahan Salient should be consequently in supercritical conditions. According to theory, this state of stress of the taper would imply an internal deformation associated with the activity of normal faults on the top of the wedge and/or the propagation of the frontalmost thrust (Figure 10c). The two processes would decrease the α and βangles, respectively. Although the hypothesis of strong-to-great earthquakes associated with the wedge propagation into the foreland along its basal detachment could be not completely neglected, it is unlikely due to the occurrence of a basement high in correspondence with the Nahan Salient [Rao, 1973; Raiverman et al., 1983]. Moreover, the presence of imbricate thrust systems of Tertiary rocks within the Nahan Salient are clear evidence of contractional internal deformation within the wedge, an evidence in striking contrast with the above conclusion.
 As a third seismotectonic scenario, we could assume again that mechanical parameters within the accretionary wedge have laterally uniform values, but considering the Nahan Salient in critical conditions. In this case, the two reentrants would be in subcritical conditions (Figure 10a). Consequently, in order to approach dynamically stable critical conditions, the two reentrants should increase the α + βangle and this commonly occurs by out-of-sequence thrusting. The duns and the growing folds that characterize the two reentrants [Singh and Tandon, 2008; Singh, 2008] possibly suggest the occurrence of this mechanisms and hence this last hypothesis in the long term.
 In the present study, the relationship between tectonics, topography and the 1905 event within the external sector of NW sub-Himalayan fold-thrust belt is analyzed. A clear-cut lateral variation is observed in the elevation distribution of the sub-Himalayan belt moving along the strike of the two major thrusts, HFT and MBT. Based on serial topographic and swath profiles, the measured mean slope,α, is generally lower (0.7°–1.2°) in the Kangra and Dehradun Reentrants with respect to the Nahan Salient (2.4°–3.1°). It has also been found that the deforming sub-Himalayan wedge is mechanically constrained internally by the Jwalamukhi and Bilaspur Thrusts.
 Furthermore, using these geometrical characteristics and based on geological constraints the seismotectonic behavior of the investigated area is interpreted in the frame of the critical taper theory [e.g., Davis et al., 1983; Dahlen et al., 1984; Dahlen, 1990]. The results suggest three possible seismotectonic scenarios:
 1. In the first one (Figure 10b), a critical state of stress is assumed for all sectors and therefore the Nahan Salient must be characterized by a basal friction higher than in the two nearby reentrants. As a further inference, this mechanical difference probably acted as a seismological barrier and possibly caused the halting of the coseismic rupture propagation during the 1905 event. In terms of seismic hazard assessment, this behavior was certainly positive at that time for local population because it resulted in a reduction of the released total seismic moment (namely, magnitude) but also implies an ongoing process of stress build up. In fact, as far as convergence continues across the sub-Himalayan belt, and considering that the external sector of the orogenic wedge is entirely within seismogenic depths, elastic stress progressively accumulates within the basal shear zone of the critical taper. In particular, considering 10–15 mm/yr of convergence rate within the investigated area of NW India [Powers et al., 1998; Kumar et al., 2006], and assuming as a worst-case scenario that deformation is entirely concentrating within the basal shear zone, during the last 100+ years the Nahan Salient has undergone a stress accumulation potentially generating ∼1.5 m of coseismic slip associated with a magnitude ∼7.0 earthquake [e.g.,Wells and Coppersmith, 1994]. Taking also into account that (1) in 1905 the previously accumulated elastic stress was not released within the Nahan segment of the detachment and (2) no other strong historical events are known for the area, at least for the last 2–3 centuries, the above mentioned amount of slip (∼1.5 m) should be considered as a very conservative value, and therefore the maximum expected magnitude for a future earthquake potentially hitting the area could be much larger. In line with this, a greater magnitude could be also inferred by estimating the maximum seismic moment (M0 = length × width × mean slip × rigidity) of the supposed Nahan asperity. Considering as a rough approximation of 100 × 50 km2 rupture area, 3 m of slip and a commonly accepted value for rigidity (3.5 × 1010) an Mw = 7.7–7.8 results. Similar values could be also obtained using empirical relationships among seismotectonic parameters. For example, using the Mw versus RLD (subsurface rupture length), Mw versus RW (rupture width) and Mw versus RA (rupture area), equations proposed by Wells and Coppersmith , the estimated magnitudes range between 7.5 and 7.7.
 2. The second and third seismotectonic scenarios (Figures 10a and 10c) are based on the assumption of laterally uniform distribution of the mechanical parameters within the taper. In particular, in the second scenario we consider that the 1905 events likely reactivated the basal detachment in the Kangra and Dehradun Reentrants, and therefore these two sectors are possibly in critical conditions. Even more so, according to theory [e.g., Davis et al., 1983; Dahlen et al., 1984; Dahlen, 1990] the Nahan Salient must be in supercritical stress conditions (Figure 10c). As a consequence, the taper should adjust its geometry by lowering the α angle (mean topographic slope) and/or the dip angle of the basal thrust (β). In the former case, normal faulting should occur, but this phenomenon is not particularly observed in the area [Delcaillau et al., 2006], while the latter mechanism generally occurs due to frontal propagation of the basal detachment into the foreland. However, the possible occurrence of a basement high in correspondence with the Nahan Salient does not favor this hypothesis, which in case of future reactivation would in principle generate a strong earthquake. In summary, this second seismotectonic scenario seems unlikely and the associated seismic hazard assessment should be very low. Accordingly, we do not discuss it further.
 3. Finally, the third seismotectonic scenario assumes that the Nahan Salient is in a critical condition and the two reentrants are in subcritical conditions (Figure 10a). The consequences in this case would be the activity of out-of-sequence thrusting in the latter two sectors and this seems confirmed by the ongoing fault propagation folding and the several associated duns (Figure 2). In terms of seismic hazard assessment, these out-of-sequence faults should be considered minor seismogenic structures compared to a possible rupture area affecting the basal detachment. Although more specific investigations should be carried out on this issue in order to better quantify the Mmax, the seismogenic potential associated with possible out-of-sequence faults is likely reduced. In this third scenario, seismic activity along the basal detachment of the Nahan Salient would however remain expected and possibly associated with strong earthquakes (with Mmax, ≈ 7.5–7.8 as discussed above). Moreover, the seismic hazard assessment would remain high also in the two reentrants notwithstanding the stress release caused by the 1905 events. This is because (seismic) deformation must persist internal to the wedge in order to reach equilibrium (critical conditions).
 Necessary facilities extended by the Scientist-in-Charge, CSIR Centre for Mathematical Modeling and Computer Simulation, Bangalore (India), are gratefully acknowledged. Useful discussions with A. K. Jain, Kamal, and Peter Powers benefited the manuscript. Critical comments of the reviewers helped to improve the overall presentation of the results.