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Himalayan Geology, Vol. 29 (2), 2008, pp.109-117 Printed in India

Active fault and paleoseismic investigation: evidence of a historic earthquake alongChandigarh Fault in the Frontal Himalayan zone, NW India

J.N. MALIK1*, T. NAKATA2, G. PHILIP3, N. SURESH3, N.S. VIRDI3

1Department of Civil Engineering, Indian Institute of Technology-Kanpur, Kanpur - 208 016, India2Faculty of Environmental Studies, Hiroshima Institute of Technology, Hiroshima, Japan.

3Wadia Institute of Himalayan Geology, Dehra Dun - 248 001, India*Email: javed@iitk.ac.in

Abstract: The Himalaya is seismically one of the most active terranes in the world. Seismic and related hazards affectnot only the Himalaya but also the adjacent alluvial plains. Convergence of Indian and Eurasian plates and ongoingactivity along three major faults viz. MCT, MBT and HFT often generate large and destructive earthquakes. We haveattempted to demarcate active faults using high resolution CORONA photographs and a DEM of the area adjacent to theHimalayan Frontal Thrust (HFT)near Chandigarh. The studies have been complemented by trenching along the HFTand by OSL dating of sediments in the trench. The HFT has repeatedly ruptured in the past as is evident from faultscarps with heights varying from 15-38 m. The OSL dates further suggest the occurrence of a major earthquake around1300-1400 A.D. and corroborate the report of such an event from the Himalayan Front about 600 years back.

INTRODUCTION

Seismic hazard evaluation is one of the most crucial problemsin a tectonically active region like the Himalaya. The historicand instrumental earthquake data available till date from theHimalayan belt is not so comprehensive hence actual appraisalof hazard from this data set is difficult (Iyenger & Sharma1999; Bilham et al. 2001). In the recent past, the Himalaya hasexperienced three major large-magnitude earthquakes during

1905 (Kangra Ms 7.8), 1934 (Bihar M 8.1) and 1950 (UpperAssam M 8.4) (Seeber & Armbruster 1981; Yeats et al. 1998;Ambraseys & Bilham 2000; Ambraseys & Douglas 2004) (Fig.1). These major earthquakes are the largest known that haveoccurred on thrust faults, however, it is believed that none ofthese events was accompanied by any coseismic surfacerupture (Yeats and Lillie 1991) though definite evidences aboutHFT being not blind have now been published (Philip & Virdi2006). Also it is suggested that less than 45% of the Himalayan

Fig. 1. Historical and modern seismicity along eastern and northwestern Himalaya and Tibet since 25 AD. Epicentres of earthquakesfrom India Meteorological Department, New Delhi; NEIC USGS; ISC UK; ANSS Berkeley; Harvard CMT catalogue. Bold blacklines mark the fault traces (after Nakata 1972; Nakata et al. 1990; Yeats et al. 1992; Powers et al. 1998; Wesnousky et al. 1999;Lavé & Avouac 2000; Tapponnier et al. 2001; Kumar et al. 2001; Malik et al. 2003; Malik & Nakata 2003); HFT – HimalayanFrontal Thrust, MBT – Main Boundary Thrust, MCT – Main Central Thrust, ITSZ – Indus Tsangpo Suture Zone, JFZ – Jialifault zone, KF – Karakoram fault. Map prepared using SRTM 30 data. Hexagon – historic earthquakes ≥M 7.

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arc has ruptured during these major earthquakes in the last200 years (Valdiya 2001). The remaining undeformed portionscategorized as seismic gaps have high probabilities of largemagnitude earthquakes in the future (Bilham et al. 2001).Therefore, identification of paleoearthquakes and related activefeatures in the seismic gaps, as well as the areas which havealready experienced major earthquakes in the recent past isvery important.

The studies carried out during several decades haveprovided important data on the ongoing crustal deformationin the Himalaya. However, not much effort is directed to site-specific studies (Nakata 1989; Valdiya 1992; Yeats et al. 1992;Wesnousky et al. 1999). Investigations along Sirmuri Tal faultin Dehra Dun valley based on the colluvial wedges in a trenchsuggests that two major earthquakes have struck Dehra Dunregion in the last 1000 years (Oatney et al. 2001). Paleoseismicinvestigations along Black Mango (Kala Amb) tear fault alongthe HFT in the NW Himalaya have revealed evidence of two

large earthquakes with surface ruptures during the past 650years; subsequent to 1294 AD and 1423 AD and yet anotherat about 260 AD (Kumar et al. 2001). In continuation of ourwork and earlier reports (Malik et al. 2003; Malik & Nakata2003) we document here the evidence of one single largemagnitude earthquake from a trench excavated nearChandigarh city, northwest Himalaya (Fig. 2a&b).

The main aim of this paper is to document the geomorphicexpression of active faulting based on remote sensingapproach and supplement the occurrence of one single largemagnitude event 600 years or so ago along the HFT asdeduced from paleoseismic investigation in the field and byOSL dates of sediments exposed in the trench.

TECTONIC SETTING

The mighty Himalaya stretching east to west over a length of2500 km formed due to continental convergence and collision

Fig.2a. CORONA satellite photo of the study area around Chandigarh along Himalayan foothill zone. The photo shows thickly populated cityof Chanidgarh located in the Indo-Gangetic Plain. Black dotted line represents the Himalayan Frontal Thrust (HFT), which marks theboundary between the Upper Siwalik Hill Range and the Indo-Gangetic Plain. Active fault traces are marked by white arrows. The boxwith dotted line marks the location of figure 2b.

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of alluvial fans comprising conglomerates, sand and siltylayers. The rivers draining through the Outer Himalaya havecut 2-3 levels of terraces.

METHODOLOGY

Application of Remote Sensing Techniques for Active Faults

Satellite remote sensing has emerged as a powerful tool forearth scientists in various geological investigations. Remotesensing techniques and a DEM based studies have beensuccessfully employed in the field of tectonic geomorphologyand active faults by many workers (e.g. Nakata 1982, 1989;McCalpin 1996; Keller & Pinter 1996 ; Krishnamurthy &Srinivas 1996; Philip & Sah 1999; Novak & Soulakellis 2000;Ganas et al. 2001, 2005; Bishop et al. 2003; Malik & Nakata2003; Kaya et al. 2004; Lorenz 2004; Gupta 2005; Jordan et al.2005; Walker 2007; Malik & Mohanty 2007; Dashora et al.2007). In the present study, high resolution declassifiedCORONA KH-4B satellite data (stereo pair) with groundresolution of approximately 6 meter acquired during 28September 1971, LANDSAT-TM (MrSid–Multi-resolutionSeamless Image Database) image with raster cell resolution of28.5 m and Shuttle Radar Topographic Mission (SRTM) 3 arcsecond data with resolution of about 90 m were used (Figs. 2aand 3a). The LANDSAT-TM image was resized as per the

between India and Eurasia. Since the collision at around ~ 50Ma along the Indus-Tsangpo Suture Zone, the successivezones of deformation have progressively shifted southward,resulting in folding and faulting along the prominent structuralfeatures of the Himalayan orogenic belt (Gansser 1964; Seeber&Armbruster 1981; Lyon-Caen & Molnar 1983). These majortectonic features: Indus-Tsangpo Suture Zone (ITSZ); MainCentral Thrust (MCT), Main Boundary Thrust (MBT) andHimalayan Frontal Thrust (HFT) have contributed insculpturing the structural and topographic architecture of theHimalaya (Fig. 1). The MCT and MBT are considered to bethe sites of Cenozoic shortening along the entire length of theHimalaya (Gansser 1964; Valdiya 1984), but the occurrence ofyoung fault scarps along the MBT and the HFT suggestsmovement during Holocene-Recent period (Nakata 1975, 1989;Valdiya 1992; Yeats et al. 1992, 1997).

The study area is located along the Himalayan Front,east of Chandigarh, where the HFT brings the Siwaliksediments over the piedmont zone consisting of fan deposits.The HFT on the surface is a moderately north dipping (25-30o)thrust. The fault zone marked by a prominent topographicalscarp is easily identified on air photos, satellite images andeven 1:50,000 scale topographic sheets. The Siwalik exposuresin the area and surroundings are Upper Siwaliks consisting ofsandstone and conglomerates. The piedmont zone consists

Fig.2b. Stereo-pair of CORONA satellite photo showing active fault traces (marked by arrows) along the Himalayan Front south east ofChandigarh on the right bank of Ghaggar river (refer to Figure 2a for location). Two active fault traces (Chandigarh fault) running parallelto one another between Panchkula and Mansadevi with strikes N25°W to N15°E have vertically displaced and warped the Ghaggar, Kalka,Pinjore and Koshallia terraces comprising an alluvial-fan sediment succession of late Pleistocene to Holocene age. Warping is clearly seennear the fault line. Vertical uplift along the faults has resulted in 15 m to 38 m high southwest facing fault scarplets. These scarps aretransverse to the course of the Ghaggar River channel, representing the younger branching out faults of the HFT system.

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study area using ENVI 3.6 and reproduced using RGB filtersto obtain the standard false colour composite (FCC) image.Finally, the LANDSAT-FCC was draped on the SRTM data toobtain the digital terrain model of the area (Fig. 3a). This datahas helped us in delineating the prominent tectonic featureslike uplifted fluvial terraces, alluvial fan surfaces and back-tilting of the surfaces etc. along the active fault (Fig. 3a&b).

GEOMORPHIC EVIDENCE OF ACTIVE FAULTING

The study area around Chandigarh city marks thesouthernmost fringe of the Himalaya, located southeast ofKangra, which falls within the meizoseismal area of 1905Kangra earthquake (Fig. 1). Here the HFT system defines theboundary between the undeformed alluvial successions ofthe Indo-Gangetic Plains and the detached and strongly folded-faulted Siwalik Hills comprising Siwalik molasse of MidMiocene to late Pleistocene age (Fig. 3a-c). Most prominentdeformation in the frontal portion along the HFT was observedon the right bank of the Ghaggar River, northeast of Chandigarh(Nakata 1989). In addition the deformation identified by Nakata(1989), we have reported the preliminary observations aboutthe existence of two active fault traces striking N25°W in thefoothill zone with the average length varying from 2 to 10 km(Figs. 2a&b and 3a&b). The surface expressions of the faultsare not well marked and masked due to rapid deposition andpoor preservation of coarse-grained alluvial deposits alongthe dynamic range-front. Besides these, straight linearmountain-fronts marked by well-developed triangular facetsalso indicate the ongoing tectonic activity in this region (Fig.3a). These two traces of active faults running parallel to each

other between Panchkula and Mansadevi have verticallydisplaced and warped the Ghaggar, Kalka, Pinjore andKoshallia terraces (Nakata 1972, 1989) comprising latePleistocene to Holocene alluvial-fan sediments (Figs. 3a-c).The occurrence of fault scarps with height varying from 15 to38 m is indicative of long-term uplift/deformation along thesefaults through late Pleistocene to Recent times. These scarpsare transverse to the course of the Ghaggar River channel.The first fault trace exhibits a 20 m high fault scarp along itsnorthwest fringe, which was responsible for uplifting the Kalkaand Pinjore terraces near Mansadevi, and the scarp heightreduces up to 6 m towards southeast where it has displacedthe Koshallia terrace.

PALEOSEISMIC INVESTIGATION ACROSSCHANDIGARH FAULT AND THE HFT

A 14 m long, 2 to 4 m deep and 4.5 m wide E-W trench was dugat the base of a 16-20 m high fault scarp across the Chandigarhfault. The succession in the trench was divided into 7 differentlithounits, from A (oldest) to G (topsoil) (Figs. 4a & b). A andF units are made up of loose matrix (very fine gravel + medium-coarse sand + silt) – supported pebble-cobble-boulder clastsof sandstone derived from Dagshai, Kasauli and Subathu (?)formations of Lower Tertiary age. These were transported anddeposited by the river from the north debouching into theGangetic Plains. The external geometry of unit F shows aslightly erosive contact with respect to the adjacent (to theleft) units C and D and with the underlying unit B and concave-up lower-bounding surface representing shallow channel-fillgeometry. Units B and D represent massive medium-coarsesandy facies. The unit C comprises fine gravel clasts as seenin A and F units and shows pinching out morphologyrepresenting a channel bar. The unit E is made up of medium-

Fig.3a. 3-D perspective view of the terrain of the study area aroundChandigarh, NW Himalaya (LANDSAT TM data overlain onDEM prepared from USGS SRTM 3 arc seconds data).Continuous black line shows the trace of “Chandigarh Fault”.Uplifted alluvial surface are seen to east of the ChandigarhFault (CF). Further to the northwest CF along HFT the frontis marked by well developed triangular facets. The HFT marksthe boundary between the Sub-Himalaya (Upper Siwalik Hills)and the Indo-Gangetic Plains. Note the location of ChandigarhCity, which is one of the major cities in Gangetic Plains

Fig.3b. 3A view of Chandigarh fault scarp, fault trace shown by whitearrows and location of the trench.

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fine gravel similar to unit C, which represents slope deposit,deposited after the seismic event. Finally the succession iscapped by top soil cover (unit G).

The fault strand identified, displaces units A, B and F andextends for about 6.5 m with variable dips ranging from 20° to46° due northeast. The fault is marked by steeper dip of about46° near the surface in the western portion of the trench, whichdecreases to about 20° towards east. In the central segment,

the dip of the fault is low (4° to 8°), and becomes steeper in thelower part. The fault plane is distinctly marked by 10 to 12 cmthick crushed zone of angular gravel where it passes throughthe B unit (Figs. 4a&b and 5a). Further up, the fault contactbetween B and F shows prominent shear-fabric marked bydiscoidal pebbles aligned along their long axes oriented parallelto the fault-plane (Figs. 4a&b and 5b). The internal arrangementof the gravelly clasts in unit F and gravelly lens in unit B areindicative of slight folding-related to faulting, which resembles

Fig. 3c. Distribution of terraces and active fault traces along northwestern Himalayan Foot Hill Zone around Chandigarh along rightbank of the Ghaggar River. White box shows location of the trench site. CF- Chandigarh Fault; HFT- Himalayan Frontal Thrust(after Malik & Nakata 2003).

Fig. 4a.Northwall view of E-W trench at the base of Chandigarh fault scarp near Mansa Devi (refer to figure 2 for location). Thread on the wallshows grid on 1 m x 1 m.

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the folding near the tip of the fault.Palinspastic reconstruction of various displaced units in

the trench suggests that one major large magnitude earthquakehas struck this region in the historic past, which post-datesthe channel-fill deposit – unit F (Fig. 6). A total displacementof about 3.5 m was measured in the field, taking into accountthe lower-bounding surface between B and F units, whichmarks the prominent piercing point and contact that has beendisplaced along the fault. The maximum displacement (To =3.5 m) and average angle of the fault (θ=25°) were used to

Fig. 4b.Detailed trench log of northwall. Bold line marks the trace of low-angle thrust fault, which has displaced A, B and F units. Unit A – olderchannel deposit; B and D – massive sand deposit; C – channel bar; E – fine gravel slope deposit; F – channel deposits and G – topsoil. Thetotal amount of displacement (3.5 m) has been measured between the displaced B and F units taking into consideration the lower-boundingsurface between them. The boxes with white dotted line show location of Figure 5a and b.

Fig.5a. Close-up of the fault plane. The fault contact between unit Band F is marked by prominent shear-fabric defined by aligneddiscoidal pebbles along their long axes oriented parallel to thefault-plane contact (for location refer to Fig. 4a).

Fig.5b. Close-up of the fault plane distinctly marked by 10-12 cmthick crushed zone comprising angular gravel where it passesthrough the B unit (for location refer to Fig. 4a).

determine other parameters like vertical slip (Vm) and horizontalshortening (Sh). This gives Vm = 1.47 m and Sh = 3.17 m forone event. The vertical displacement measured directly in thetrench from the top of unit B gives Vm = 1.33 m, which is moreor less consistent with the value calculated from palinspasticconstruction. This vertical displacement of 1.33 m for oneevent should have been reflected by the scarp height. However,the present scarp height of 1.2 m is a little less than theexpected/calculated displacement, indicating erosion of thescarp after faulting.

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yielded ages ranging from 3000 yr. to 600 yr. (Table 1). SamplesCH1 and CH3 from the unit B, gave similar ages of 1393+165yr. and 1338+197 yr. respectively. The sample CH2 with age of2864±165 yr. suggests that massive sand unit B was depositedbetween 2800 and 1400 yr. (Fig. 4a&b). The sample CH4 fromunit D, gave an age of 609+66 yr. The depositional-erosionalsequence and stratigraphic relationship of the exposedsuccession, clearly reveals that the unit F (gravel) wasdeposited under channel-fill environment after the depositionof unit D and also the eroded surface of units B, C and D.Therefore, the stratigraphic position of the event horizon (unitF) and the age of unit D suggest that the seismic event occurredaround 600 year ago.

DISCUSSION AND CONCLUSIONS

Earthquake hazard is one of the most common problems in theHimalaya. Since little or no written history of past events isavailable from much of the belt (Iyenger & Sharma 1999),identification of active fault/paleoseismic events is of immenseimportance.

Traces of active faults near Chandigarh, in the northwestHimalaya, indicate that the frontal portion has ruptured duringthe historic past. The occurrence of two active fault traces areindicative of imbricated fault splays branching out from themain HFT system, suggestive of propagation of activitytowards the foreland (Fig. 3b & c). The variable heights ofscarps ranging from 15 to 38 m suggest continued long termdeformation, which is also supported by 3.5 m displacementrecorded in a trench near Chandigarh. The OSL age from unitD (609+66 yr.) is the deposition age, where this unit was erodedby later depositional phase marked by unit F – which gotdisplaced during the latest event. Since we do not have theage from the uppermost lithounit, it is rather difficult to pin-point the event age. However, the available dates suggestthat the event postdates the age of unit D. Thus we are of theopinion that the event must have occurred some time duringthe 15th or 16th century. Paleoseismic investigation ~ 150 km tothe southeast from the present site around Kala Amb suggestsoccurrence of two large rupture earthquakes during the past650 years at round 1423 A.D. and 260 A.D. (Kumar et al. 2001).The surface rupture of large magnitude event during 1400-

Due to non-availability of charcoal, samples from sandyunits were collected for dating using OSL technique. Foursamples, CH1, CH2, CH3 and CH4 were collected. For OSLmeasurements quartz grains were extracted and multiplealiquots were prepared for each sample for obtainingequivalent dose (ED) by additive dose method. The samples

Fig. 6. Stage I: Simplified version of the trench log showing thestratigraphic position of lithounits before faulting aftereliminating the displacement along the fault. Fault pre-existedbefore it displace the unit F - represents the channel troughand marks erosive contact with the underlying sandy unit Band other units like C and D on the left hand side of thesuccession. Stage II: Trench log showing stratigraphicrelationship of the lithounits after faulting. Where the B andF units are faulted and moved up. Unit A must have beenupthrusted from down. Stage III: Final picture of the trenchafter the scarp was subjected to erosion and then followed byphase of deposition of slope derived material.

Table 1. OSL dates of the samples from trench (refer Fig. 4b for sample location).

Sl . Sample U(ppm) Th(ppm) K (%) H2O (%) Dose E.D. Gy AgeNo. No. Rate Gy/Ka (Yr B.P.)

1 CH1R 4.6+ 1 9.5+1 1.31 2.53 3.2+0.2 4.58+ 0.41 1393 +1652 CH2R 5.1+1 9.8+1 1.51 3.17 3.6+0.25 10.31+ 0.59 2864+2604 CH3R 4.3+1 10.2+1 1.30 1.73 3.282+ 258 4.39+ 0.55 1338+ 1973 CH4R 5.1+1 10.2+1 1.32 2.66 3.46+ 0.25 2.11+ 0.17 609+ 66

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1500 A.D. extended for about ~150-200 km towards NW ofKala Amb, where displacement of 3.5 m was registered alongthe Chandigarh fault. Available historic data does not haverecord of such an event from this area, however, several eventsincluding the 1505 (M=8) Kabul and the 1505 Mw=8.2 Kumaonearthquakes have been reported from the western Himalaya(Ambraseys & Jackson 2003). Therefore, the question ariseswhether we are looking at one of these events whose locationis uncertain as mentioned in the present earthquake data-set,or a major earthquake that occurred during 1400 A.D. butremained unrecorded during historic times.

The available instrumental and historic seismicity datashows uneven distribution of earthquakes with 6<M≤7.5 (Fig.1). The map suggests that apart from the 1905 Kangra eventlarge number of major earthquakes have occurred in the areanorthwest of Dharmsala during historic times, along with therecent 2005 Muzaffarabad event of Pakistan. Some eventsoccurred during 1505, 1803 and 1816 in the Central SeismicGap. Other than these some events with 6>M≤ 7.5 (e.g. 1991 &1999) have been reported from the region around Nainital andNepal Himalaya, these too having been located between MCTand MBT. If we consider that the existing data-set is completethen based on the distribution of seismicity there are severalseismic gaps located between the great Himalayanearthquakes. If not then we are missing some events whichremained unrecorded in the past, and hence such events andalso those that struck this region during the last 10,000 yearsshould be identified to get a better picture of distribution andrecurrence of earthquakes and to understand the behaviourof individual fault.

The active fault scarps along the HFT in Frontal Himalayasuggest that many large magnitude events in the past haveruptured up to the surface (Philip & Virdi 2006, 2007). This is atestimony to the occurrence of 15-38 m high fault scarps,suggestive of ongoing long-term deformation along the HFT.Also the young deformation recorded in trench suggestsrecent movement along the front. This very important evidencewill be helpful in evaluating the seismic hazard in the region,specially for major cities in the foothill zone.

Acknowledgements: JNM is thankful to Department of Science andTechnology, New Delhi for financial support. The authors are gratefulto the Director, Wadia Institute of Himalayan Geology, Dehra Dun forproviding facilities for OSL dating and permission to publish the dates.

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