Paleoseismic evidence from trench investigation along Hajipur fault, Himalayan Frontal Thrust, NW Himalaya: Implications of the faulting pattern on landscape evolution and seismic hazard

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Abstract

The study area falls within the mesoseismal zone of 1905 Kangra earthquake (Mw 7.8). Two parallel NNW–SSE striking active fault scarps named as Hajipur Faults (HF1 and HF2) along the northwestern end of the Janauri anticline in the foothill zone, have displaced floodplain sediments of the Beas River. The HF1 and HF2 represent the imbricate faults of the Himalayan Frontal Thrust (HFT), and are the result of lateral propagation of deformation from two fold segments i.e., JF1 and JF2 respectively in northwest direction along the strike. Ground Penetrating Radar (GPR) profiles and trenching across the HF2 reveal two low-angle thrust fault strands (F1 and F2). Displacements of ∼7.5 m on F2 and ∼1.5 m on the associated branching faults (fa, fb and fc) were observed. Total four stratigraphic units: unit A (gravel) – with a lens of medium sand (unit A′) is the oldest; overlain by units B – medium to coarse sand; unit C – with fine to medium sand; and unit D – fine to medium sand with scattered gravel were observed in trench. Radiocarbon ages of the charcoal samples from unit B and unit D, optical ages of sediments from units A′, B and C, GPR data and trench log, suggest two major events along F1 and F2 strands. Event I along F1 occurred during 2600–800 yr BP and Event II along F2 around 400 yr BP and before 300 yr BP. Given the uncertainty in dates it is suggested that the latest event occurred during 1500–1600 AD. Considering the oldest unit (unit A) exposed in trench with vertical displacement of 7.5–8 m, age of 2600 ± 500 yr BP and net displacement of ∼9 m during single event along low-angle fault (θ = 25°), implies slip rate = 7.6 ± 1.7 mm/yr, uplift rate = 3.2 ± 0.6 mm/yr, shortening rate = 6.9 ± 1.4 mm/yr and recurrence interval = 1160 ± 250 yr for large-magnitude event with Mw >7.0. With the recurrence of 1100 yr, the penultimate event probably occurred at around 1400–1500 yr BP. Given the recent GPS based slip rate of 14 ± 1 mm/yr in Kangra reentrant (Baneerjee and Burgman, 2002), the present study suggests that about half of this slip is consumed along the HFT and that this fault is more active compared to those in the hinterland.

Introduction

The continued convergence between Indian and Eurasian plates has made the Himalayan arc a seismically active region that experienced moderate to large earthquakes. In last 100 years the Himalaya has experienced three major large-magnitude events during 1905 Kangra (Mw 7.8), 1934 Bihar (Mw 8.1) and 1950 Upper Assam (Mw 8.4) earthquakes (Seeber and Armbruster, 1981, Yeats et al., 1997, Ambraseys and Bilham, 2000, Ambraseys and Douglas, 2004) (Fig. 1a). The recent October 8, 2005 Muzaffarabad earthquake (Mw 7.6) occurred along an earlier identified active fault named Balakot–Bagh fault causing extensive damage in Pakistan as well as in Indian side (Nakata, et al., 1991; Kaneda et al., 2008). Seismic hazard evaluation in Himalaya is one of the most crucial problems. Historic records and instrumental data available so far is not so comprehensive and also little or no written records are available from much of the Himalayan belt, hence actual appraisal of hazard from this dataset is difficult (Iyengar and Sharma, 1999, Bilham et al., 2001). For proper seismic hazard estimation, identification of active faults bears significant importance, it is also necessary to know the accurate locations and geometry of active faults (Gross et al., 2002). Paleoseismic investigation is one of the most commonly adopted techniques towards identification/cataloguing the historic and pre-historic earthquakes in tectonically active regions of the world (McCalpin, 1996).

The studies carried out during several decades have provided important data on the ongoing crustal deformation in the Himalaya. However, not much effort is directed to site-specific studies (Nakata, 1989, Valdiya, 1992, Yeats et al., 1992, Wesnousky et al., 1999, Malik et al., 2003, Malik and Nakata, 2003). Recent paleoseismic investigations from India, Pakistan and Nepal along the Himalaya arc have added valuable information towards the occurrence of large-magnitude earthquake during recent historic past. Investigations along Sirmuri Tal fault in Dehra Dun valley along Main Boundary Thrust (MBT) suggests two major earthquakes have struck Dehra Dun region in the last 1000 years (Oatney et al., 2001). Paleoseismic investigations along Black Mango (Kala Amb) tear fault along the Himalayan Frontal Thrust (HFT) in the NW Himalaya have revealed evidence of two large earthquakes with surface ruptures during the past 650 years; subsequent to 1294 AD and 1423 AD and yet another at about 260 AD (Kumar et al., 2001). Also studies from NW Himalaya have indicated major event at around ∼1400–1500 AD along HFT (Kumar et al., 2006, Malik et al., 2008). Paleoseismic evidence from east central Nepal reveals a single earthquake rupture along the Frontal Thrust during ∼1100 AD (Lavé et al., 2005). Trench investigation performed along Balakot–Bagh fault after the 2005 Muzaffarabad earthquake suggests recurrence interval of 1000–3300 yr and shortening rate of 1.4–4.1 mm/yr (Kaneda et al., 2008).

Earthquakes along active faults are periodic (Yeats et al., 1997, McCalpin, 1996) and a proper seismic hazard assessment needs an active fault map with paleoseismic history. The Muzaffarabad 2005 event has raised more concerns towards the seismic hazard assessment in areas along Himalayan foothills, where practically no historic or no active fault-paleoseismic data is available. In this paper we document the geomorphic manifestation of newly identified active faults, named as Hajipur faults in the mesoseismal zone of 1905 Kangra earthquake (Mw 7.8) along the northwestern end of the Janauri anticline (Fig. 1a). These faults along the Himalayan front were identified using CORONA satellite (stereo pair) photos, Digital Elevation Model (DEM) generated from SRTM data followed by detail field investigations (Fig. 2, Fig. 3, Fig. 4). GPR (Ground Penetrating Radar) profiling was conducted to understand the geometry of the faulting and to locate the appropriate site for further paleoseismic investigation. Paleoseismic investigation revealed occurrence of two major events during 2600–800 yr. BP and another at around 400 yr BP. The 1905 Kangra and 2005 Muzaffarabad events are the only well documented events from NW Himalaya (Iyengar and Sharma, 1999, Ambraseys and Jackson, 2003, Malik et al., 2007, Kaneda et al., 2008). And also up to now, surface ruptures associated with the 2005 event (Yeats and Hussain, 2006, Kaneda et al., 2008) and historic events during 1100 AD and 1400–1500 AD are the only coseismic features known so far along the Himalaya thrust (Lavé et al., 2005, Kumar et al., 2006, Malik et al., 2008). It has been suggested that the 1905 earthquake rupture corresponds to the Jawalamukhi Thrust (JMT, Fig. 1a) and that the slip was never transferred to the HFT (Wallace et al., 2005), this is consistent with our trench results. Moreover, the historical records of pre-1905, major earthquake in Kangra are not available (Bilham, 2001). The historical catalogue from this area and further northwest reports the occurrence of two earthquakes in 1555 AD (Mw 7.6) and in 1885 AD (Mw 6.4) in the Kashmir valley. It is quite possible that the latest event in our trench represents the event occurred during 1500–1600 AD. The information generated by us will be of great importance since no paleoseismic information is available from this region of NW Himalaya.

Section snippets

General seismotectonic background

Since the collision (∼50 Ma) along the Indus-Tsangpo Suture Zone (ITSZ), successive deformation zones have progressively advanced southward towards foreland, causing faulting and folding along the south verging prominent structural features [Gansser, 1964, Seeber and Armbruster, 1981, Lyon-Caen and Molnar, 1983]. These principal intracrustal thrusts are the Main Central Thrust (MCT), the Main Boundary Thrust (MBT), and the Himalayan Frontal Thrust (HFT) (Fig. 1a and b). These thrusts with an

General geomorphological setting

In this study we use high resolution CORONA KH-4B satellite data (stereo pair, ground resolution ∼6 feet) of September 1971, Shuttle Radar Topographic Mission (SRTM) 3 arc second data (resolution ∼90 m) and Survey of India (SOI) topographic maps (scale 1: 50,000) in order to map active faults and landforms. Field investigations allow us to identify two active fault traces on the left bank of the Beas River (Figs. 1a, 2a, 3a and 4).

The Himalayan belt in northwestern portion of the arc is widely

Georadar survey and paleoseismic investigation

With an aim to identify most recent events in the area, the Ground Penetrating Radar (GPR) survey was carried out across the HF2 fault scarp. GPR survey using a 200 MHz shielded antenna with SIR 3000 system (Geophysical Survey System Inc.) was carried out to confirm shallow subsurface faulting and to locate appropriate trenching sites along HF2 (Fig. 3, Fig. 5a). Several profiles helped optimize acquisition parameters (Table 1), and finally a 32 m long profile with 7–8 m penetration depth was

Chronology of events

Optically Simulated Luminescence (OSL) of sediments and radiocarbon Accelerator Mass Spectroscopy (AMS) of detrital charcoal were used for the chronology of stratigraphy and related faulting events (Table 2, Table 3). OSL dated the depositional event of the sediment and radiocarbon ages dated charcoal associated with the sediment. OSL ages are calendar ages whereas radiocarbon ages refer to 1950 base line and hence in comparing OSL and radiocarbon ages, ∼50a should be added to radiocarbon ages.

Discussion and conclusion

Any displacement along normal or reverse faulting or on elastic crack will propagate laterally as well as vertically with a reduction towards the tips of the faults along the strike (e.g., Gudmundsson, 1987a, Gudmundsson, 1987b, Gudmundsson, 2000, Cartwright et al., 1995, Walsh et al., 2002, Davis et al., 2005). It has been suggested that as displacement occurs during major earthquake – fault grows, propagates laterally by acquiring more length along the strike (Walsh et al., 2002, Champel

Acknowledgments

Financial support provided to JNM by DST, New Delhi (vide project No. DST/23(411)/SU/2003, now Ministry of Earth Sciences) is duly acknowledged. We are thankful to M.Tech and B.Tech students of IITK for their help in field. We are grateful to Prof. Gudmundsson for providing valuable comments and suggestions which helped in bringing more clarity to our paper. We are also thankful to Prof. Joao Hipprett for his suggestions and comments which added in improving our manuscript. We are also thankful

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    1

    Present address: Reliance India Limited, Mumbai, India.

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    Present address: School of Earth and Environmental Sciences, James Cook University, Townsville, Queensland 4811, Australia.

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