Arora and Malik Geosci. Lett. (2017) 4:19 DOI 10.1186/s40562-017-0083-6 REVIEW Open Access Overestimation of the earthquake hazard along the Himalaya: constraints in bracketing of medieval earthquakes from paleoseismic studies Shreya Arora and Javed N. Malik* Abstract The Himalaya is one of the most seismically active regions of the world. The occurrence of several large magnitude earthquakes viz. 1905 Kangra earthquake (Mw 7.8), 1934 Bihar–Nepal earthquake (Mw 8.2), 1950 Assam earthquake (Mw 8.4), 2005 Kashmir (Mw 7.6), and 2015 Gorkha (Mw 7.8) are the testimony to ongoing tectonic activity. In the last few decades, tremendous eforts have been made along the Himalayan arc to understand the patterns of earthquake occurrences, size, extent, and return periods. Some of the large magnitude earthquakes produced surface rupture, while some remained blind. Furthermore, due to the incompleteness of the earthquake catalogue, a very few events can be correlated with medieval earthquakes. Based on the existing paleoseismic data certainly, there exists a com- plexity to precisely determine the extent of surface rupture of these earthquakes and also for those events, which occurred during historic times. In this paper, we have compiled the paleo-seismological data and recalibrated the radiocarbon ages from the trenches excavated by previous workers along the entire Himalaya and compared earth- quake scenario with the past. Our studies suggest that there were multiple earthquake events with overlapping sur- face ruptures in small patches with an average rupture length of ~300 km limiting Mw 7.8–8.0 for the Himalayan arc, rather than two or three giant earthquakes rupturing the whole front. It has been identifed that the large magnitude Himalayan earthquakes, such as 1905 Kangra, 1934 Bihar–Nepal, and 1950 Assam, that have occurred within a time frame of 45 years. Now, if these events are dated, there is a high possibility that within the range of 50 years, they may be considered as the remnant of one giant earthquake rupturing the entire Himalayan arc. Therefore,± leading to an overestimation of seismic hazard scenario in Himalaya. Keywords: Recalibration, Radiocarbon age, Seismic hazard scenario, Medieval earthquakes, Himalayan Frontal Thrust Introduction systems along the Himalayan arc. Tectonic deformation Te Himalaya is a 2500 km long belt of mountains, has resulted in the formation of four major south-verging which are the result of the progressive under thrusting thrust systems viz. South Tibetan Detachment (STD), of the Indian Plate beneath Tibetan Plate along Main which separates Tibetan sedimentary and Higher Hima- Himalayan Trust (MHT) (Zhao et al. 1993; Molnar layan meta-sedimentary sequences, Main Central Trust and Tapponnier 1977). Global Positioning System (GPS) (MCT), which bounds the Higher Himalayan Crystal- measurements indicate 4–5 cm/year of convergence line rocks, and Upper Precambrian to Paleozoic rocks of rate between these two plates (Banerjee and Bürgmann Lesser Himalaya. MCT has not been observed to rupture 2002) of which 18 mm/year is accommodated by thrust the Quaternary deposits and is considered to be inac- tive (Nataka 1989; Valdiya 1992). Main Boundary Trust (MBT) marks the southern edge of Lesser Himalaya. Te *Correspondence: [email protected] Department of Earth Sciences, Indian Institute of Technology Kanpur, southernmost thrust is Himalayan Frontal Trust (HFT), Kanpur 208016, India which has displaced Tertiary and Quaternary alluvial © The Author(s) 2017. This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. Arora and Malik Geosci. Lett. (2017) 4:19 Page 2 of 15 and molasse sediments and is expressed as discontinu- et al. 2016; Jayangondaperumal et al. 2016; Kumar and ous range front scarps (Nakata 1972; Valdiya 1980, Mahajan 2001; Kumar et al. 2006, 2010; Kumahara and 1992). Tese fault systems are the result of the southward Jayangondaperumal 2013; Kondo et al. 2008; Lavé et al. propagation of the Himalayan Front through time and 2005; Meigs et al. 2012, Madden et al. 2011; Malik and each fault merges into the basal detachment fault, i.e., Nakata 2003; Malik et al. 2008, 2010a, b, 2015, 2016; Main Himalayan Trust (Zhao et al. 1993; Pandey et al. Mishra et al. 2016; Sapkota et al. 2013; Yule et al. 2006; 1995). Based on the active fault topography, the MBT Vassallo et al. 2015). An attempt has been made to high- and HFT are considered as active (Nakata 1989; Valdiya light the problems of bracketing the ages and assigning 1980; Malik and Mohanty 2007). Historical archives a specifc event to the surface ruptures along the Hima- indicate that Himalaya has experienced several damag- layan front, where other earthquake events with similar ing earthquakes of intensity >XI in the last millennium time frame could be ftted. Along with this, a scenario for (Iyengar et al. 1999; Chitrakar and Pandey 1986). Instru- past 100 years of Himalayan earthquakes has been com- mental records and geodetic data indicate that several pared to the earthquake scenario of the medieval times earthquakes have occurred along the basal detachment and the spatial extent of few medieval earthquakes has and propagated southward to produce surface rupture been debriefed. at Himalayan Front such as 1934 Nepal–Bihar (Sapkota et al. 2013) (Fig. 1c), but few nucleate below the Higher Seismicity Himalaya and produced ruptures in the higher reaches During past few centuries, Himalaya has witnessed sev- only, 1905 Kangra Earthquake is one such example which eral moderate and large earthquakes with magnitudes produced surface rupture along Kangra Valley Fault 7.8 ≤ Mw ≤ 8.4 from these, and fve events are consid- (KVF) (Fig. 2a) (Malik et al. 2015). Te variable nature of ered as great earthquakes viz. 1934 Bihar–Nepal earth- the rupture pattern raises several questions: (a) does all quake (Mw 8.2), 1950 Assam earthquake (Mw 8.4), 1951 stored strain developed due to India–Tibet convergence (Mw 8) after shock of Assam earthquake, 1905 Kangra is released during a single event? (b) Is the strain accu- earthquakes (Mw 7.8), and 2015 Nepal earthquake (Mw mulation similar along the entire Himalaya, or (c) how 7.8) (Seeber and Armbruster 1981; Pandey and Mol- does the variation in the Himalayan strike and segmenta- nar 1988; Ambraseys and Bilham 2000; Ambraseys and tion efects the variable rupture pattern? Douglas 2004; Martin and Szeliga 2010; Bollinger et al. In the last few decades, several paleo-seismological 2016) (Fig. 1a). Some of these earthquakes seem to have investigation has been carried out along the active HFT ruptured the upper brittle portion of the locked Main which has provided signifcant data to understand the Himalayan Trust (MHT) (Sapkota et al. 2013; Malik occurrence of large magnitude earthquakes in the Hima- et al. 2015, 2016) and have produced surface ruptures layan region (Wesnousky et al. 1999; Malik and Nakata along HFT. Te location of rupture areas of these large 2003; Lavé et al. 2005; Malik et al. 2008, 2010a, b, 2015, earthquakes exhibits a Seismic Gap and one such exam- 2016; Kumar and Mahajan 2001; Kumar et al. 2006, 2010; ple is between 1905 Kangra and 1934 Bihar-Nepal earth- Kumahara and Jayangondaperumal 2013; Sapkota et al. quakes which is termed Central Seismic Gap (Khattri 2013; Yule et al. 2006; Bollinger et al. 2014, 2016; Jayan- 1987; Khattri and Tyagi 1983; Bilham et al. 1998; Bilham gondaperumal et al. 2016; Mishra et al. 2016). However, and Wallace 2005; Gupta and Gahalaut 2014) (Fig. 1a). there exist constraints in terms of their period of occur- Tis region has not experienced any great magnitude rence and in determining the extent of surface rupture. earthquake since 1344 event (Kumahara and Jayan- In this paper, we have divided Himalaya into three gondaperumal 2013; Rajendran et al. 2015; Malik et al. zones, i.e., Northwestern Himalaya, Central Himalaya, 2016) or probably the event has not been precisely and Eastern Himalaya to understand the pattern of occur- documented in historical chronicles. Few large mag- rence of paleo-earthquakes (Figs. 1, 2, 3). Te Northwest- nitude earthquakes 6.6 ≥ Mw ≤ 7.8 have occurred in ern Himalaya is further categorized into Kashmir sector, this region such as 1991 Uttarkashi earthquake Mw 6.9, Kangra reentrant and Dehradun sector, Central Himalaya 1999 Chamoli earthquake Mw 6.6, 1803 Mw 7.2 in Cen- into Ramnagar sector, Western Nepal and Central Nepal tral Himalaya, and 1833 Mw 7.8 with an epicenter in the and Eastern Himalaya into Eastern India (Figs. 1a, 3). inferred rupture zone of 1934 Bihar–Nepal earthquake Succession of earthquakes along these Himalayan zones (Bilham 1995) (Fig. 1a). It has been postulated that a from historical archives of (Oldham 1883; Iyengar et al. great earthquake can follow a major earthquake in the 1999; Pant 2002; Ambraseys and Douglas 2004) and the Himalayan domain. Tis was suggested that in case of paleo-seismological data including timing, displacement AD 1833 (Mw 7.8), a major earthquake was followed by of the latest event, and slip rates from the trench studies 1934 AD (Mw 8.2) great earthquake in Nepal (Feldl and has been compiled (Bollinger et al. 2014, 2016; Gavillot Bilham 2006). Arora and Malik Geosci. Lett. (2017) 4:19 Page 3 of 15 a 1555(7.7) 2005 (7.6) 1778(7.7) 1774(7.5) 1884(7.3) 1905(7.8) N o A’ r t h w e A st e 1991(6.9) rn Fig. 2a H im 1999(6.6) 1505(?) al ay Cen 1803 (7.2) 1950(8.6) a tral Seismic >8 1833(7.5) Y >7.5≤7.8 2015(7.8) >7≤7.4 C Gap >5.8≤6.9 en 2015 (7.3) >5≤5.7 tra l H 1255 (?) ima 1934(8) ya laya 1897(8.7) ala Him X Eastern 300 km Fig.