Active Tectonics in the Assam Seismic Gap Between the Meizoseismal Zone of AD 1934 and 1950 Earthquakes Along Eastern Himalayan Front, India

J. Earth Syst. Sci. (2018) 127:66 c Indian Academy of Sciences https://doi.org/10.1007/s12040-018-0967-7

Active tectonics in the Assam seismic gap between the meizoseismal zone of AD 1934 and 1950 earthquakes along eastern Himalayan front, India

Arjun Pandey1,3,IshwarSingh1, Rajeeb Lochan Mishra1,4, Priyanka Singh Rao2, Hari B Srivastava 3 and R Jayangondaperumal1,*

1Wadia Institute of Himalayan Geology, Dehradun 248 001, India. 2Geological Survey of India, SU: WB & AN, ER, Kolkata 700 016, India. 3Department of Geology, Banaras Hindu University, Varanasi 221 005, India. 4Present address: Ravenshaw University, Cuttack 753 003, India. *Corresponding author. e-mail: [email protected]

MS received 31 July 2017; revised 10 November 2017; accepted 23 November 2017; published online 25 June 2018

The Assam Seismic Gap has witnessed a long seismic quiescence since the Mw∼8.4 great Assam earthquake of AD 1950. Owing to its improper connectivity over the last decades, this segment of the Himalaya has long remained inadequately explored by geoscientists. Recent geodetic measurements in the eastern Himalaya using GPS document a discrepancy between the geologic and geodetic convergence rates. West to east increase in convergence rate added with shorter time span earthquakes like the 1697 Sadiya, 1714 (Mw∼8) Bhutan and 1950 (Mw∼8.4) Tibet–Assam, makes this discrepancy more composite and crucial in terms of seismic hazard assessment. To understand the scenario of palaeoearthquake surface rupturing and deformation of youngest landforms between the meizoseismal areas of Mw∼8.1 1934 and 1950 earthquakes, the area between the Manas and Dhanshiri Rivers along the Himalayan Frontal Thrust (HFT) was traversed. The general deformation pattern reﬂects north-dipping thrust faults. However, back facing scarps were also observed in conjugation to the discontinuous scarps along the frontal thrust. Preliminary mapping along with the published literature suggests that, in the eastern Himalayan front the deformation is taking place largely by the thrust sheet translation without producing a prominent fault-related folds, unlike that of the central and western Himalayas. Keywords. Active tectonics; eastern Himalaya; earthquake; palaeoseismology; Assam Gap.

1. Introduction and interest. Study of active faults of a particular area reveals its deformation pattern, which in Active faults are defined as faults along which tec- turn helps quantify the seismic hazard associ- tonic movement occurred during the Quaternary ated with these faults. Due to advancement of Period and are expected to reactivate any time in earthquake geology and its allied disciplines such future span of human civilization (McCalpin 2009; as palaeoseismology and active tectonics, active Jayangondaperumal et al. 2018). Active faults pose fault mapping has been a matter of major con- serious threat to life and property, due to which cern over the last few decades. Palaeoseismological their study has since long been of great importance trench excavations across fault scarps, aided with 1 0123456789().,--: vol V 66 Page 2 of 17 J. Earth Syst. Sci. (2018) 127:66 chronological constraints have been utilized to limit along active faults in the Assam Gap area were the time of the scarp forming palaeoearthquakes, focused on trench investigations and findings of their recurrence intervals as well as the level of surface signatures of historical earthquakes, but seismic hazard imposed over a region in the past these studies very loosely constrain the timing of (McCalpin 2009). Palaeoseismic studies investi- the palaeoearthquake event(s) recorded in their gate the timing, size, rupture extent and return trenches (figure 1a) (Nakata et al. 1998; Lavé time of palaeoearthquake events. Subsurface imag- et al. 2005; Kumar et al. 2010; Jayangondaperu- ing of active faults using high-end geophysical mal et al. 2011; Sapkota et al. 2013; Roux-Mallouf instruments like Ground Penetrating Radar (GPR) et al. 2016; Mishra et al. 2016a; Wesnousky et al. efficiently help in estimating the geometry of active 2016; Priyanka et al. 2017). However, in an attempt faults. to precisely review the timing of the palaeo- As active faults are the outcomes of earthquakes, earthquake event(s) in the eastern Himalaya for global research on active faults gained pace after the first time, Mishra et al. (2016a) deployed Oxcal their role and activity during highly destructive modeling of radiocarbon ages from six trenches earthquakes came into picture. For example, great previously excavated in the central and eastern devastating earthquakes such as Mw ∼6.7 1994 Himalaya. But their inference was later neglected North Ridge (Stein et al. 1992), Mw ∼7.6 1999 by Pierce and Wesnousky (2016) without providing Chi–Chi (Yu et al. 2001), and Mw ∼7.6 2005 Kash- any additional dataset. However, such ambiguity mir (Kaneda et al. 2008) have occurred on active could be overcome by conducting more detailed faults. This has given rise to a serious concern on palaeoseismic investigations in these areas. the issue of seismic hazard imposed by active faults In addition, the Assam Gap (Khattri 1987)has on the human society and infrastructure, thereby witnessed no great earthquake after 1950 Tibet– triggering the need to understand the relationship Assam. However, historical accounts of eastern between active faults and earthquakes. Himalaya report major historical earthquakes in With regard to India, active fault studies have 1697 and 1714 (figure 1a). So, a major concern is been carried out in the Himalayas. Seismicity in that the strain accumulated in the segment might the Himalaya has resulted due to the ongoing reactivate the active faults in the area through northward convergence of India with Eurasia and large earthquakes in the future. In this regard, it the associated crustal deformation (Molnar and is essential to identify the potential active faults Tapponnier 1975; Tapponnier et al. 2001). The in the area. Also, identification of active faults in Himalaya is a platform for intense deformation in the eastern Himalaya will provide a base for future terms of folding and faulting due to continental palaeoseismological research in this area. There- underthrusting that has given rise to thrust sys- fore, in view of the above issues a detailed account tems striking parallel to the Himalayan structural of the active fault scarps identified and mapped in grains (Zhao et al. 1993). An eastward increase in the Assam Gap area between the Manas and Dhan- the convergence rate in the Himalaya has posed shiri Rivers is presented in this study. seismic threat to the region. The significant earthquakes that devastated the central and eastern Himalayan foothills in past few decades are the 2. Geotectonic setting of the study area 1697 Sadiya (Iyengar 1999), Mw ∼8 1714 Bhutan (Iyengar 1999; Hetényi et al. 2016), AD 1255 The Himalaya is the youngest and one of the most (Sapkota et al. 2013; Mishra et al. 2016a), 1934 active continental mountain belts in the world. Bihar–Nepal (Dunn et al. 1939; Sapkota et al. 2013; In this belt, the accumulated strain energy is Bollinger et al. 2014) and Mw ∼8.4 1950 Assam linked to the subsurface internal deformation of earthquake (Chen and Molnar 1977; Priyanka et al. rocks underthrusting along the detachment sur- 2017). These successive earthquakes ruptured the face (Seeber and Armbruster 1981; Kayal 2001). same or different segments of the eastern Himalaya, Along its ∼2500 km length from Namcha-Barwa is still a matter of debate. in the east up to the Nanga Parbat in the west, Recent studies reveal that there is inconsistency it is composed of different tectonic units that are between geodetic rate of ∼18 mm/yr (Vernant bound by three major ∼E–W striking south verg- et al. 2014) and geological Holocene slip rates ing thrusts. The oldest among these major tectonic of 23.4 ± 6.2 mm/yr (Burgess et al. 2012). Pre- units, the Main Central Thrust (MCT), the Higher vious patchy palaeoseismic studies carried out Himalayan crystalline rocks lie over the Lesser J. Earth Syst. Sci. (2018) 127:66 Page 3 of 17 66

Figure 1. (a) Shuttle Radar Topographic Mission (SRTM-30 m) map showing the location of previous paleoseismological trenches excavated at sites: (a and b) Tribeni and Bagmati (Wesnousky et al. 2016), (c) MarhaKhola (Lav´e et al. 2005), (d) Sir Khola (Sapkota et al. 2013), (e) HokseKhola (Nakata et al. 1998), (f) Panijhora (Mishra et al. 2016a, b), (g) Chalsa (Kumar et al. 2010), (h) Sarpang Chu (Le Roux-Mallouf et al. 2016), (i and j) Nameri and Harmutty (Kumar et al. 2010), (k) Marbang (Jayangondaperumal et al. 2011), (l) Pasighat (Priyanka et al. 2017) and fault slip rate studies by: (1) Lave and Avouac (2000), (2) Berthet et al. (2014), (3) Burgess et al. (2012)andDe Sarkar et al. (2014); rupture zones of 1505, 1833, 2015, 1100/1255, 1713 and 1950 earthquakes from previous studies (Vernant et al. 2014); green and red stars denote 2015 and 1934 earthquakes epicenters. (b) Generalized geological map of Assam–Bhutan foothill zone and surrounding region (adapted from Gansser 1983; Bhargava 1995; Grujic et al. 2002). Abbreviations. STD: South Tibetan Detachment, MCT: Main Central Thrust, MBT: Main Boundary Thrust, HFT: Himalayan Frontal Thrust, UK: UraKlippe, SK: SaktengKlippe (location from Yin et al. 2010b). Hollow squares a, b and c denote the studied sections. Moment tensor solutions (Red Beach Ball) reported in GCMT (Global Centroid Moment Tensor) catalog from the year 1976 to 2017 for the study area, shown in the map (source USGS; http://www.globalcmt.org/CMTsearch.html). The small red dots indicate the epicenters of the earthquakes. 66 Page 4 of 17 J. Earth Syst. Sci. (2018) 127:66

Himalayan metamorphic, and is tectonically inactive (Gansser 1964; Nakata 1989). To the south of MCT lies the Main Boundary Thrust (MBT), which has thrusted the Lesser Himalayan rocks al Thrust. over the Siwaliks (Valdiya et al. 1992; Valdiya 2003, Dubey 2014). The youngest and the most active of these thrusts, the Himalayan Frontal Thrust (HFT) has overridden the Tertiary Siwalik rocks over the Quaternary alluvium (Nakata 1972). The HFT has truncated the Quaternary fluvial terraces and the alluvium fans in the form of discontinuous fault scarps (Nakata 1972; Wesnousky et al. 1999). The active faults, in and around the whole Himalayan arc are the direct indicators of recent crustal movement due to the collision between the Indian and Eurasian plates. Earlier, few active faults in the Sub Himalayan foothills of Dehradun, Darjeeling and southern Bhutan were mapped by Nakata (1972, 1989), subsequently palaeoseismological investigation was embarked across these mapped fault scarps by various researchers in NW and Central Himalaya (e.g., Kumar et al. 2006; 2010; Malik et al. 2010; Rajendran et al. 2015; Jayangondaperumal et al. 2013, 2017; Kumahara and Jayangondaperumal 2013). The eastern Himalaya is a quite interesting and characteristically distinct geological domain that has preserved geological sections from Archean to Recent (Nandy 1975). The oldest sedimentary records in the Assam-Arakan basin are found to be of Upper Cretaceous period. The Gondwana rocks represent the rift-drift tectonism and lie in a narrow belt along the Himalayan foothills. The Siwalik foredeep molasses lie in front of the Himalaya and the Tipam molasses lie in front of the Indo-Myanmar border. Both of these stratigraphic sequences are differentiated by the Brahmaputra Alluvium, which is a vast repository of the Qua- ternary deposits and sediments. The study area is located between the Manas and the Dhanshiri river valleys and it lies between the meizoseismal zones of 1934 Bihar–Nepal and 1950 Tibet–Assam earthquakes in the Assam Seis- mic Gap region (figure 1). The geology of the area comprises Neogene sediments, which are thrusted over the Quaternary alluvium from the south by the low angle MFT or HFT, and its northern limit is delimited by the steep angled MBT by virtue of which the older metamorphic rocks were thrusted over the Sub-Himalaya (figure 2). The Neogene sediments are classified into two litho- logical units, namely, the Mio-Pliocene Siwaliks and the Diklai conglomerates. The Siwaliks which Figure 2. Detailed geological map of the Bhutan foothills. Abbreviations. HFT: Himalayan Frontal Thrust, MBT: Main Boundary Thrust, MCT: Main Centr J. Earth Syst. Sci. (2018) 127:66 Page 5 of 17 66 cover a major portion of the Sub-Himalaya in the 3. Materials and methods region, occurs in patches but are absent near Gale- phu town. This is known as ‘gap’ (figure 2; Biswas The methodology employed in the present work et al. 1979). Here the older metamorphic rocks are involved a 3-dimensional perspective mapping of in direct contact with the Quaternary alluvium. truncated geomorphic markers (terraces, fan, bar, This can be explained as either the Siwaliks were flood plains and other associated geomorphic mark- eroded by trunk streams flowing in the area, or ers) in high spatial resolution (2.5 m) Cartosat- it has not been exposed yet. Heim and Gansser 1A satellite imageries with Socet GXP v4.1.0 (1939) described these gaps as a result of ‘relief software. These mapped geomorphic markers are thrusting’. subsequently cross-checked in the field and are The Neogene Siwalik sediments are basically of precisely mapped by high resolution topographic freshwater origin and they are categorized into survey instruments such as the Real Time Kine- three units from older to younger. The first unit matics Global Positioning System (RTK-GPS) which is named as Formation-I (Lower Siwa- and robotic Total Station (i.e., electronic distance liks), consists of an argillaceous unit having thin meter). alterations of dark grey claystone, shale, mud- Several measures must be adopted while map- stone and very fine grained sandstone. Occasional ping the active faults in an area. Various geomor- patches of carbonaceous shale and coal is also phic markers like fault scarp, back tilting, deflected present in it. The Formation-II (middle Siwaliks), streams, beheaded stream; uplifted and beveled which deposited in a gradational contact with the bedrock terraces, pressure ridges, etc., are the Formation-I, consists of interbedded sequences of direct evidences of a tectonic movement around sand, silt, clay and mudstone. Sandstone is cross- active faults in an area. Offset of geomorphic land- stratified and having grey to dark grey color also forms like fluvial terraces or alluvial fan surfaces contains carbonized wood pieces. This formation is of Quaternary time scale in a thrust regime is an different from the Formation-I by its characteris- essential criterion in identification of active faults. tic sandstone, well developed bedding, and more The orientation of local drainage flows with respect compaction. The Formation-III contains lenticu- strike of the fault scarp is the most important crite- lar bodies of conglomerates made up of quartzite ria for the identification of the active faults. Usually in a sandy matrix. It basically comprises the the streams flow perpendicular to the strike of the upper conglomerate beds of the Siwalik Group fault scarp. Strath terraces show river incision due and is exposed among the major river valleys. to tectonic uplift over a thrust fault. The lower boundary of this formation is gradational. The Diklai conglomerates are lithologically distinct from the underlying Formation-III and essentially contain pale yellow to brown conglom- 4. Active fault mapping and landform erate unit having pebbles, cobbles and boulders evolution in the study area of quartzite. These formations are delimited in the north by the MBT, above which rocks of the 4.1 Active faults between Manas and Aei rivers Gondwana and the metamorphic group are overridden. The former is deposited in a patchy and East of the Gelephu town in southern central discontinuous manner. The Gondwana Group com- Bhutan, abandoned fluvial terraces are preserved prises highly sheared and crushed pale to brown, along the Manas and Aei rivers (figure 3). The micaceous feldspathic sandstone with interbedded Quaternary alluvium is truncated by a ∼NE–SW yellow arkose and black carbonaceous slaty shale. trending frontal fault scarp (south facing or toward This formation is separated by the MCT along modern foreland basin) and three NNW facing which the metamorphic group of rocks comprising back scarps (scarp facing toward mountain side or low to medium grade metasediments like phyllites, hinterland, and hence forth it is referred as back graphite schists and quartzites, are thrusted over scarp) striking parallel to the frontal scarp. Siwa- the Gondwana group of rocks. At some places liks are not seen in this area, due to which the like Galephu, the older metamorphic rocks are Quaternary fluvial gravels are in direct contact in direct contact with the Quaternary alluvium with the older metamorphic rocks. The strike of whereas at others they are with the Siwaliks the mountain front changes from ∼E–W to NE– (figure 1b). SW due to the formation of a large re-entrant. 66 Page 6 of 17 J. Earth Syst. Sci. (2018) 127:66

Figure 3. (a) Map showing the different levels of terraces preserved along the Manas and Aei rivers. Black outlined hollow square denotes the location for figure 3(b). Map prepared in a 3D mode in SOCET GXP 4.1.0. using high resolution Cartosat- 1A satellite imagery. Line X–Y denotes the cross-section profile across the scarps shown in figure 2(b). (b) Satellite imagery showing the 3D view of backscarps and frontal thrust offsetting the T1 terrace preserved along the left bank of Manas River (source: Google Earth). X–Y is the profile line for figure 4. Location of the figure is shown in figure 3(a).

The area has been mapped using Cartosat-1A bank of the Aei River (a tributary of Manas River) imageries in Socet GXP v4.1.0. and ArcGIS soft- a palaeo-alluvial fan surface is preserved. The sur- ware (figure 3). Two levels of uplifted fluvial face may be interpreted to have formed by the Aei terraces varying in height from ∼40–80 m are pre- River, which subsequently shifted its course due to served along the bank of the Manas River. The the tectonic activity along the HFT leading to the higher terrace T2 preserved on both river banks is truncation of the fan surface. This surface is gullied paired, and has an undulating tread. The T2 ter- by small, first and second order streams. race makes a vertical scarp when it meets to the T1 The river is highly braided in nature. It is evi- terrace. The lower T1 terrace widely extends and dent from the imageries as well as from the field has been deformed both by a frontal and three back observations that the frontal scarp has been mod- scarps. From the topographic profile (figure 4), the ified by the Manas River and its tributaries. The backscarps are observed to have much higher eleva- extension of the scarp is not seen on the western tion as compared to the frontal scarp. On the right bank of the river. The frontal as well as the two J. Earth Syst. Sci. (2018) 127:66 Page 7 of 17 66

Figure 4. Topographic profile across the scarps preserved along the left bank of Manas River (location of the profile shown in figure 3).

Figure 5. Digital Elevation Model (DEM) of Cartosat-1A satellite imagery (draped with contours of 15 m interval) showing three back scarps and NE–SW striking frontal fault scarp which offsets the lowermost T1 terrace. Yellow arrowheads pointing towards each other denote the strike of the fault scarp. The red solid line indicates the trace of frontal scarp. X–Y is the profile line for figure 4. backscarps extends up to a length of ∼4 km, but subjected to active tectonic deformation in the they do not cut or displace the fan surface pre- present day. served at the bank of the Aei River. This leads to an interpretation that the earthquake occurred prior to formation of a fan surface. 4.2 Active faults along the Lokhaitora River The profile obtained from 90 m SRTM (figure 4) clearly depicts the presence of back scarp. The second area of our study is the exit of the On the left bank of the Manas River, truncated Lokhaitora River in Upper Assam (figure 1, square surfaces are preserved which is dissected by three ‘b’ and figure 6). Long and wide fluvial terraces back facing scarps that are formed due to move- have been uplifted and truncated along the bank ment along back thrust faults (figures 3–5). The of the Lokhaitora River along the HFT. A ∼15 m smaller terrace T1 preserved on the left bank high fault scarp preserved on the right bank of the of the river has an elevation of ∼80 m and is river has been identified (figures 7–9). 66 Page 8 of 17 J. Earth Syst. Sci. (2018) 127:66

Figure 6. (a) Map showing the different levels of terraces preserved along the Lokhaitora River. Map prepared in 3D mode in Socet GXP v4.1.0. using high resolution Cartosat-1A satellite imagery. Line A–B denotes the cross-section profile across the scarps shown in figure 6(b). (b) Topographic profile across the T2 terrace preserved along the left bank of Lokhaitora River (location of the profile shown in figure 6a) obtained using high resolution kinematic GPS.

Three levels of fluvial terraces, T3 through T1, of height ∼48 m is exposed beneath the terrace in the decreasing order of their elevation towards T2. The bedrock is overlain by a matrix-supported the current river grade were mapped and later sub- fining-upward unit-2 composed of gravel, pebble stantiated in the field (figure 6a). The terrace T3, and cobble with exceptionally large boulders that which stands at a height of ∼85 m above the cur- grade upwards into a medium to fine bioturbated rent river grade, is unpaired and extends linearly, silty sand unit-3 (figure 10a). The Siwalik is show- and is developed only on the right bank of the river. ing 30◦–40◦ monotonously dipping towards north. The uplifted and disjointed middle terrace T2 at Since in the field we did not encounter any frontal the frontal part of the mountain range was mapped fold geometry. The visible strath contact is hori- with RTK-GPS (figure 6b). The terrace T2 stands zontal and parallel to the river profile (figures 10a at a height of ∼ 60 m and has been preserved all and 11a). The lowest terrace T1 lies at a height of along the length of the river but in patches. At ∼20 m above the present river grade and is contin- the exit of the Lakhoitara River, Siwalik bedrock uous along the length of the river. At the exit of J. Earth Syst. Sci. (2018) 127:66 Page 9 of 17 66

Figure 7. Digital Elevation Model (DEM) of Cartosat-1A satellite imagery (draped with contours of 7 m interval) showing an E–W striking fault scarp which oﬀsets the lowermost T1 terrace. Yellow arrowheads pointing towards each other denote the strike of the fault scarp.

Figure 8. 3D anaglyph showing a ∼15 m high E–W striking scarp at the right bank of the Lokhaitora River. Arrows pointing towards each other denotes the trace of the active fault scarp.

Figure 9. Field photograph of the ∼15 m high E–W striking scarp at the right bank of the Lokhaitora River. Arrows pointing towards each other denotes the strike of the fault scarp. Location of the photograph shown in ﬁgure 6(a). 66 Page 10 of 17 J. Earth Syst. Sci. (2018) 127:66

Figure 10. Field photographs in (a) and (b) showing the cross-sectional view of the T2 and T1 terraces at the bank of the Lokhaitora River, respectively. Location of the photographs is shown in ﬁgure 6(a) (log is provided in ﬁgure 11).

Figure 11. Schematic log for the terraces preserved at the exit of Lokhaitora River for the river sections. Log for (a) the T2 terrace and (b) the T1 terrace (see ﬁgure 10 for photo). J. Earth Syst. Sci. (2018) 127:66 Page 11 of 17 66

Figure 12. (a) Map showing different levels of terraces preserved along the Dhanshiri River. Map prepared in a 3D mode in SOCET GXP 4.1.0 using high resolution Cartosat-1A satellite imagery. Line P–Q denotes the cross-section profile across the scarps shown in figure 12(b). (b) Topographic profile across the T2 and T1 terraces preserved along the left bank of Dhanshiri River (location of the profile shown in figure 12a) obtained using high resolution kinematic GPS.

the river, the terrace T1 is displaced by the HFT required in this regard and it is beyond the scope to form a ∼15 m high frontal scarp. Terrace T1 of this paper. having Siwalik bedrock around ∼6.1 m, followed by a unit-2 composed of clast supported fabricated 4.3 Active faults along the Dhanshiri River gravels, pebbles, cobbles with few boulders. This unit contains two fining upwards sequences fol- At the exit of the Dhanshiri River between longi- lowed by a medium to fine grained silty sand unit-3 tudes 90–93◦E (figure 1, square ‘c’), four disjointed (figures 10band11b). The formation of three lev- fluvial terraces have been preserved (figure 12a). els of terraces leads to the interpretation that the Here, traces of HFT have been identified by the river has been migrated from its course repeatedly, presence of extensive shearing at the river exit. which can be due to repeated tectonic uplift in Because of border restrictions, the entire stretch the region or due climatic impact as is expected of terraces could not be mapped, but terraces T1 in a high monsoon area. However, detailed study is and T2 were mapped using RTK-GPS (figure 12b). 66 Page 12 of 17 J. Earth Syst. Sci. (2018) 127:66

Four levels of fluvial terraces have been preserved along the Dhanshiri River. In order of decreasing elevation with respect to the current river grade, the fluvial terraces are categorized as T4 through T1. The highest terrace T4 stands at a height of ∼110 m above the current river grade. This terrace is the most extensive among the four levels of terraces, and is deposited in a tongue-like fashion only on the west bank of the river as unpaired terrace. This might be due to the fact that the river is eroding its eastern bank more frequently than its western bank. The unpaired terrace T3, preserved in small patches, is at a height of ∼80 m from the present river grade. At several places it forms a steep contact with the T1 terrace. The wider extent of the T3 terrace suggests longer res- idence time of the river before it gets abundant, either tectonically or climatically. The terrace T2 standing at a height of ∼35 m contains clast supported disorganized gravel, cobble and pebble in a sandy matrix as evident from the cross-section (figures 13 and 14). The lowest terrace T1 consisting of matrix supported cobble and pebble in a sandy matrix is at a height of ∼20 m from the current river bed. Terraces T2 and T1 have been sampled for the estimation of the active deformational rates in the area.

5. Fault plane characteristics of the study Figure 13. Schematic log for the T2 terrace at the right left area bank of the Dhanshiri River.

The available centroid moment tensor solutions can be attributed to the overthrusting of the Indian reported by USGS (United States Geological Sur- continental basement and its Palaeogene–Neogene vey) in the region corresponds to a combination of cover along a stack of imbricate thrusts (Chen and thrust as well as strike-slip having an oblique slip Molnar 1990; Hallet and Molnar 2001). component (figure 1). The earthquakes reported in the GCMT (Global Centroid Moment Ten- sor) catalogue have their epicenters placed in a 6. Discussion and conclusion latitude ranges from 26.91 to 27.20◦N and lon- gitude 91.62 to 91.94◦E having their magnitudes The present study provides the first report on Mw > 5.0. Out of the three reported earthquakes active faults between the Manas and Dhanshiri in the region, two were having a pure thrust focal River valleys. The sense of motion along the mechanism and are seem to be associated with observed fault scarps in this area is interpreted to MCT. The strike/dip/rake for these two earth- be thrusting. At the Gelephu re-entrant, besides quakes are reported to be 281/6/94 and 293/7/107, one frontal south facing scarp there are three respectively. The third one observed north of MBT NNW facing backthrust scarps which are also vis- is characterized by strike-slip faulting. The fault ible in the SRTM profile (figure 4) as well as plane associated with this earthquake is having in the Cartosat-1A Digital Elevation Model (fig- strike/dip/rake of 321/73/-173. These are mainly ure 5). In NW Himalaya along northern limb of shallow focus earthquakes with their hypocenters Janauri anticline, on the basis of palaeoseismic lies at depth ranges from 12 to 15 km. The dom- investigations on a backthrust it was suggested inance of shallow focus earthquakes in the region that the bulk strain is being accommodated in the J. Earth Syst. Sci. (2018) 127:66 Page 13 of 17 66

Figure 14. Field photograph showing the cross-sectional view of T2 terrace preserved along the left bank of the Dhanshiri River. Location of the photograph shown in figure 12(a). forethrust and minor is being accommodated in the Dhanshiri River, four levels of terraces are the backthrust (Jayangondaperumal et al. 2017). preserved (figure 12a). The broadest terrace T4 They also suggested that the backthrust is associ- shows the higher dwelling time of the stream before ated with the forethrust in the occurrence of an it deserted the terrace either by tectonic or climatic earthquake. The topographic analysis across the processes. There are several flights of terraces (T1, multiple fault scarps at Gelephu re-entrant reveals T3 and T4) and their aerial extent mapped along both north and south facing scarps are complimen- the frontal thrust, we infer that the area has been tary to each other and backthrust is developed from subjected to repeated tectonic activity. It is evident the master forethrust as demonstrated recently by from the narrow T2 terrace that it has got very Jayangondaperumal et al. (2017) (figure 4). The less time for its deposition since the river incised formation of a frontal scarp along HFT can be its channel very frequently. attributed to a great earthquake event. However, As compared to the western Himalaya, the presence of back thrust and its timing of activity eastern Himalaya is manifested by entirely differ- with respect to the fore thrust can be worked out ent structural and geological framework. As far to understand the convergent tectonics and to deci- as the frontal Himalayan belt is concerned, there pher the pattern of strain release in the area by are less fault scarps present in comparison to that future palaeoseismic investigations. of the western Himalaya. Perhaps the heavy rain- In the Lokhaitora River exit that the lowest T1 fall related surface process in the eastern part of terrace has been deposited all along the length of the Himalaya is the reason for less preservation of river as unpaired terrace. This terrace is truncated small scarps. Moreover, in the western Himalaya, by a ∼E–W trending scarp at the west bank of wide and long valleys within the Siwalik Hills like the Lokhaitora River (figures 7–9). The terrace T2, Dehra ‘dun’ and Pinjore ‘dun’ are characteristically like T1, is formed all along the banks as paired present, whilst these structures are not seen along terrace, and is truncated at the front where it is the eastern frontal sub Himalaya. deposited as an unpaired terrace. The highest ter- The northwestern and central Himalaya con- race T3 in the area is extended up to ∼2kmalong sists of fault related asymmetrical folds verging the river length and has been preserved in the toward south, and their axial trace is parallel to form of an unpaired terrace. Presence of discontin- the strike of the Himalayan structural grains. Some uous, unpaired and truncated Quaternary terraces of the anticlines are the Mohand, Chandigarh, clearly upholds that the area has been subjected to Janauri Anticlines, etc. Contrastingly, along the continuous cycles of tectonic uplift. At the exit of entire stretch of the eastern Himalayan front, there 66 Page 14 of 17 J. Earth Syst. Sci. (2018) 127:66 are only two anticlines, namely, the Banderdewa front along the Arunachal–Assam border, Kumar anticline cross-cutting the Dikrong River (Mishra et al. (2010) excavated a trench at Harmutty west et al. 2016b) and the Balipara anticline at the exit of the meizoseismal zone of the Mw ∼8.41950 of the Kameng River (Burgess et al. 2012). Fur- Assam earthquake. Dating of faulted units from ther, it is evident from the field based structural the trench exposure suggested an earthquake dur- data (figure 2) that the frontal thrust sheets show ing AD 1271–1393 with a co-seismic displacement monotonously north dipping structures. This can of 2.4 m. This event was correlated with the 1950 be said that either the forelimb of the sub Himalaya Assam earthquake. has been completely buried or eroded by the axial Mishra et al. (2016a) re-excavated a trench at Brahmaputra River because it lies near the out- Panijhora village across a fault scarp that is a board of the range front (figure 1a). Interestingly, it continuation of the Chalsa scarp previously exca- is to note that the HFT shows discontinuous south vated by Kumar et al. (2010). Further, they re- facing scarp along its strike. Therefore, the erosion interpreted trench logs of Chalsa, Nameri and or burial of the forelimb can be easily ruled out. Harmutty and re-calibrated their radiocarbon ages Based on our field observations together with high through Bayesian statistics using Oxcal program- resolution satellite imageries it is construed that me. On this basis they interpreted that the AD majority of deformation in the eastern Himalaya is 1255 earthquake might have ruptured the Himala- largely being accommodated by thrust sheet trans- yan front over a length of ∼800 km, validating that lation rather than fault related folds. This implies the Himalaya could generate giant earthquakes of that the geometry of the Main Himalayan Thrust Mw ∼9.0 and rupture lengths as large as 800 km. (MHT) is not same along its strike in the eastern But their inference was later negated by Pierce and Himalaya. This provides implications that, in the Wesnousky (2016) without any substantial dataset. eastern Himalayan front the release of earthquake Jayangondaperumal et al. (2011)excavateda purely by faulting of accretionary wedges, but in trench across a fault scarp preserved along the the northwestern as well as in the central Himalaya Marbong-Korong Creek along the HFT in Aruna- the interseismic strain is being accommodated chal Himalaya and suggested it to be the result in the form of fault-related folds. Therefore, a gen- of two subsequent earthquakes post-1012 and 2009 eralized seismotectonic model cannot be applicable cal yr BP. The tilting of sediments that occurred for the eastern Himalayan front for seismic hazard post-2009 cal yr BP was inferred to be the result assessment. of the 1950 Assam earthquake. There have now been a number of palaeoseis- The Sarpang Chu site is a ∼6 m high topographic mic investigations that placed limits on the timing scarp where a river-cut cliff exposes a shallow north and size of palaeoearthquakes along the eastern dipping thrust fault zone between an abandoned Himalayan Frontal Thrust (Kumar et al. 2010; fluvial terrace and modern alluvium. Roux-Mallouf Jayangondaperumal et al. 2011; Mishra et al. et al. (2016) excavated a trench at this site and 2016a; Roux-Mallouf et al. 2016; Priyanka et al. collected charcoal samples for radiocarbon dating 2017). These studies have provided insight into the that yielded ages between AD 782 and 1937. On pattern of strain accommodation and release by the basis of that they have interpreted that two palaeoearthquakes along the Himalayan front. events occurred between AD 1167 and 1487 and In NE Indian Himalaya, Kumar et al. (2010) that youngest event occurred between AD 1524 and identified fault scarps of ∼15 and 9 m height at 1815. Chalsa and Nameri. These studies suggest that the Priyanka et al. (2017) excavated a palaeoseismo- scarps were formed by earthquakes that occurred logical trench across a ∼3.5 m high fault scarp during AD 1059–1266 and 1025–1224 at Chalsa and preserved at the right bank of the Siang River Nameri, with observed co-seismic displacements of at Pasighat town of Arunachal Pradesh. On the 14 and 12 m, respectively. It is inferred that the basis of multi-proxy radiometric dating employed event recorded at these two sites is the same about to the stratigraphic units and detrital charcoals 1100 AD event reported from the eastern Nepal obtained from the trench exposures they suggested (Lave et al. 2005). However, the 1100 AD earth- that the 15th August, 1950 earthquake produced quake was interpreted as 1255 earthquake and its a co-seismic slip of 5.5 ± 0.7 m along the eastern rupture has been extended up to Hokse Khola by Himalayan front. Bollinger et al. (2016) and subsequently by Mishra As compared to the eastern Himalaya, in the et al. (2016a). Further east along the Himalayan central Himalaya, the scenario is more absolved J. Earth Syst. Sci. (2018) 127:66 Page 15 of 17 66 because of closely-spaced trenches. Sapkota et al. the 1714 earthquake is capable of generating an (2013) undertook palaeoseismological investigati- earthquake of Mw 8.1 with a potential slip of ons near the Sir Khola River in Central Nepal. ∼6m. They studied a ∼15 m high natural river cliff The presence of fault scarps and uplifted bedrock section along with a ∼43 m long and 5 m deep terraces in this segment of Himalaya suggests that trench at the eastern bank of the Sir Khola River. the whole area is prone to seismic risk. It is Here the Main Frontal Thrust (MFT) branches also essential to investigate whether all these pre- into right-stepping strands manifested by steep served frontal scarps formed due to one single cumulative scarps. On the basis of six detrital char- event earthquake or multiple events. Consider- coals obtained from the cliff exposure that yielded ing the hypothesis of fault segmentation in the age ranges from AD 1490 to 1960, they reported Himalaya which states that most active faults rup- that the Mw ∼8.21934 Bihar–Nepal earthquake tured only a portion of their total length during ruptured this segment of Himalaya producing a co- large earthquakes (Kumahara and Jayangondape- seismic displacement of ∼3 m. In addition to this, rumal 2013; Mugnier et al. 2013; Grandin et al. they reported evidence of two successive surface 2015; Hetényi et al. 2016; Hubbard et al. 2016), it faulting events in the excavated trench. Thirteen still remains debated whether the whole area is gov- 14 C dates of detrital charcoal samples placed lim- erned by a single active fault or various segments its on the ages of the stratigraphic units in the and splays of the main fault. The theory of segmen- trench. On the basis of four ages in the deformed tation in the Himalaya (Gupta and Gahalaut 2015; unit they suggested the palaeoearthquake occurred Grandin et al. 2015; Hubbard et al. 2016)being a between AD 700 and 1300. This 13th century barrier to the generation of a single giant earth- event was correlated with the great medieval earthquake of magnitude approaching Mw 9.0 (Mishra quake of AD 1255 that reportedly devastated the et al. 2016a) has now been negated through recent town of Kathmandu. They further synthesised GPS studies (Stevens and Avouac 2016). There- the results and delivered a recurrence interval of fore, structural as well as seismic discrepancies ∼679 years for great earthquakes in the central in this area require more detailed palaeoseismic Himalaya. investigations at closer intervals. This would help In the eastern Himalaya, the study of Roux- to determine the past activity on the poten- Mallouf et al. (2016) throws light on the youngest tial active faults along the Himalayan front and rupturing earthquakes in the region. They pro- hence, estimate their slip and recurrence rate. posed at least two palaeoearthquakes in this region. It may also throw light over the rate of ongo- The earlier event was constrained between AD 1140 ing collision between India and Asia, providing and 1520, and the later event between AD 1642 a blueprint for the strain release pattern in the and 1836. The penultimate and the recent events region. generated vertical offsets of ∼8.5 and 0.5 m, respectively. Recent GPS measurements in the Bhutan Himalayas by Vernant et al. (2014) suggest that Acknowledgements convergence rates in the Bhutan Himalayas ranges from 16.5±1.5 mm/yr in the western Bhutan, AP and IS thank the MoES for receiving their 15±1.5 mm/yr in the central portion and reaches to fellowships from the MoES sponsored research 17±1 mm/yr in the eastern Bhutan. They inferred project vide grant number (MoEs/P.O/(Geosci.)/ that in absence of a foredeep or a correspond- 11/2013, dated 31/3/2014) awarded to RJ. RJ ing flexural bulge with no aseismic creep on the thanks the Ministry of Earth Sciences (MoES) for Himalayan decollement south of the locking line, financial support through the project. AP thanks this segment is having a seismic slip deficit which is the Department of Science and Technology, Gov- capable of generating great earthquakes in future. ernment of India, for the INSPIRE Fellowship However, they have also suggested that if AD (IF160527) for pursuing the PhD degree. RLM 1714 earthquake has released all the accumulated thanks the Department of Science and Technology, strain, then at the present day this segment is Government of India, for the INSPIRE Fellowship capable of generating an earthquake of Mw 8.2 (IF120844). This work is a part of AP’s PhD the- 2 considering a rupture zone of 150 × 90 km .This sis. Lastly, the authors thank the Director, Wadia value closely matches with that of Mishra et al. Institute of Himalayan Geology, for providing nec- (2016a), who suggested that the rupture zone of essary facilities to carry out this work. 66 Page 16 of 17 J. Earth Syst. Sci. (2018) 127:66

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