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Geomatics, Natural Hazards and Risk, 2015 Vol. 6, No. 3, 224–242, http://dx.doi.org/10.1080/19475705.2013.839483 H.N. SRIVASTAVAy*, MITHILA VERMAz, B.K. BANSALx and ANUP K. SUTARx y128, Pocket A, Sarita Vihar, New Delhi 110076, India zCentre for Seismology, Ministry of Earth Sciences, New Delhi 110003, India xGeoscience Division, Ministry of Earth Sciences, Lodi Road, New Delhi 110003, India (Received 23 April 2013; accepted 27 August 2013) Contrary to most of the earlier theories that great earthquakes (M 8.5 or even larger) may occur anywhere along the Indian plate boundary assuming uniform strain accumulation, this paper proposes two types of gaps with discriminatory characteristics. The new gaps were initially identiﬁed from earthquakes of magnitude 6, whose return periods in Himalaya vary between 20 and 30 years and are well within the period of reliable instrumental data of about 100 years. These gaps were then integrated with the largest magnitude event in instrumental era and historical times; information on paleoseismicity, micro-seismicity data, GPS-based geodetic observations and the tectonic features. The regions where great/major earthquakes (Mw 8 or larger) have occurred in the past are classiﬁed as seismic gap of category 1, namely Kashmir, west Himachal Pradesh (Kangra), Uttarakhand to Dharachulla, central Nepal to Bihar, Shillong, Arunachal gap including Assam–Tibet–Myanmar syntaxis. On the other hand, the second category of seismic gap includes Jammu– Kishtwar block, east Himachal Pradesh, western Nepal (excluding Dharachulla region) and Sikkim–Bhutan where history of large earthquakes is not available. In these gaps, the largest earthquake magnitude is smaller (7–7.5) and the recurrence interval for earthquakes of same magnitude is larger as compared to category 1 gaps. 1. Introduction Himalayan seismic belt is one of the most seismically active regions of the world result- ing from the collision of Indo-Eurasian plates, where four great earthquakes, namely Shillong (1897), Kangra (1905), Bihar–Nepal (1934) and Assam–Tibet (1950), occurred during a short span of 53 years only. Detailed source characterization studies in respect of these earthquakes were handicapped due to sparse and limited instrumen- tal records with poor time keeping, which raised many questions about the location accuracy, source mechanism and their rupture zones (Srivastava et al. 2010). Since, no great earthquake (Mw 8 or larger) has occurred after 1950 in the region, many seis- mologists anticipate such earthquakes to recur in the Himalayan foot hills, which could be far more catastrophic than the earlier earthquakes due to rapid growth of population, buildings and other infrastructural facilities (Bansal & Verma 2013). Spatio-temporal variations in seismicity patterns of earthquakes in the Himalayan region have shown seismic quiescence preceding earthquakes of magnitudes ranging *Corresponding author. Email: firstname.lastname@example.org 2013 Taylor & Francis Discriminatory characteristics of seismic gaps in Himalaya 225 from 4.8 to 6.8 (Srivastava 2004). An alternate approach using clusters also observed seismic quiescence preceding earthquakes (Srivastava 1992). For large or great earth- quakes, such quiescence may last for a century or more, which is then more com- monly referred to as seismic gap. The initial success in earthquake prediction in Paciﬁc region (Fedotov 1965) accelerated search for potential seismic gaps along plate boundaries. McCann et al. (1979) found that most of the great earthquakes reported during 15-year period ruptured such seismic gaps. The ﬁrst seismic gap in Himalaya was delineated in east Himachal Pradesh (Srivastava & Chaudhury 1979; Srivastava et al. 1984) with prognosticated magnitude of 7–7.5 highlighting that all the areas in Himalaya may not be equally seismogenic to generate great earthquakes. Ignoring this result Khattri (1987) delineated three seismic gaps in whole of Himalaya using four earthquakes during 1897, 1905, 1934 and 1950. However, a basic question against postulation of these gaps arises due to inclusion of data only for 53 years, while the recurrence interval for great earthquakes in Himalaya is found to be 300–500 years (Molnar & Pandey 1989). Also, these gaps do not fulﬁl the crite- ria of occurrence of at least one great/large earthquake (magnitude 8). Further, due to the presence of tear faults between main central thrust (MCT) and MBT (main boundary thrust) / HFT (Himalayan frontal thrust) in the Himalayan tectonic zone (Valdiya 1976; Acharya & Narula 1998; Mukhopadhyay 1984; Dasgupta et al. 2013a), all segments along the plate boundary may not move coherently and could be attributed to the crustal variations or lithospheric fragmentation contrary to assumption of uniform seismic slip. Additional evidence in support of fragmentation of the lithosphere (Oxburgh 1972 also comes from the deeper focus earthquakes and strike slip mechanism of August 1988 (Bihar–Nepal) and September 2011 (Sikkim), suggesting complexities in the regional plate tectonic model (Tandon & Srivastava 1975). Also, Uttaranchal earthquake of 1803, with its revised magnitude (Rajendran & Rajendran 2005) comparable to that of Kangra earthquake (1905), cannot be excluded as a plate boundary earthquake. The inclusion of Shillong earthquake, which is located away from Himalayan thrusts, raises concern to use it for deﬁning Assam seismic gap (Dube et al. 1986; Bilham & England 2001; Rajendran et al. 2004; Rajendran & Rajendran 2011). The suggestion of Gahalaut and Chander (1992) about the seismic gap in eastern Himalaya between 1934 and 1950 earth- quakes also needs to be reviewed due to seismically active Kopili lineament, where an earthquake of magnitude 7.5 occurred in July 1943. Since a few subduction regions have largest earthquakes of magnitude less than 8 (Kanamori 1987),itisof interest to examine whether in some sectors in Himalaya, similar seismic potential exists. It may also be interesting to examine some diverging views expressed in the past about occurrence of large earthquakes in Himalaya. Many authors including Bilham et al. (2001) have given importance to three seismic gaps in Himalaya rather than the places where earthquakes have occurred in the past. Surprisingly, Bilham and Wallace (2005) later modiﬁed their own theory after the occurrence of 26 December 2004 Sumatra Andaman earthquake that ruptured plate boundary upto the epicentre of 1881 Andaman earthquake. On the analogy with Andaman region, they con- cluded that strains insufﬁciently released in 1905 would be released again, generating a 7.5 magnitude earthquakes in Kangra region. It is also interesting to note that Bilham and Wallace (2005) excluded Bihar–Nepal region as a possible region from their recurrence theory by suggesting that 1833 and 1934 earthquakes had overlap- ping rupture zone and accumulated strain was already released. This is not 226 H.N. Srivastava et al. corroborated by the epicenters of these two earthquakes that occurred on two dis- tinct faults, namely MCT and HFT, as discussed later. Such interpretational ambigu- ities have been addressed in this paper. The pattern of seismicity, micro-earthquakes, historical and paleoseismicity, GPS observations, local tectonics and other related data sets were analysed to examine whether all regions along Himalayan plate boundary are equally seismogenic. Integration of these observations allows us to delineate two types of seismic gaps having discriminatory characteristics, enabling removal of interpretational ambiguities in earlier studies. 2. Seismotectonics Plate motion models using seismic slip vector and recent GNSS geodetic measure- ments suggest that Indian plate is moving in a north easterly direction with velocities ranging between 40 and 50 mm per year. This convergence of Indian and Eurasian plates is manifested through seismic slip associated with great earthquakes (Ni & Barazangi 1984). On account of underthrusting of Indian plate below Eurasian plate, three main thrusts, namely the MCT, MBT, and HFT, developed in Himalayan region (ﬁgure 1). All the three thrusts appear to merge with the common decollment called main Himalayan thrust (MHT). The four large/great earthquakes in Uttarak- hand (1803, M 7.8), Kangra (1905, M 8.0) and Bihar–Nepal (1934, M 8.1) w w occurred near MBT/HFT, while Assam (1950, M 8.4 – 8.6) was located between Lohit Thrust and Pochu fault (Tibet) close to eastern syntaxial bend (Mukhopad- hyay et al. 2011a). Transverse structural features in the under thrusting Indian plate between MCT and MBT (Valdiya 1976; Mukhopadhyay 1984; Dasgupta et al. 1987; Dasgupta et al. 2013a) divided the detachment surface (upper surface of the under thrusting Indian plate based on steady state model of Himalaya) into different blocks and some of these blocks might be locked and others creeping or lie in transition zone. The Himalayan region is also deeply dissected by transverse rivers and further com- plicated by valleys, ridges, mountains, basins and plateau that generate varying earthquake effects (Seeber & Gornitz 1983). Towards west, the portion of MCT Figure 1. Generalized tectonic map of Himalaya with various geological and tectonic features (after Ganser, 1964 modiﬁed). Discriminatory characteristics of seismic gaps in Himalaya 227 north of the Panjal imbricate zone is called Panjal thrust, which comprises of phyl- lites, slates and limestone and is seismically active. Towards north-west of Himachal Pradesh lies the Kishtwar window, which is a large anti-formal fold. Jammu and neighbourhood, which is a zone of diffuse seismicity, where two earthquakes of 23 August 1980 (M 5.5) were associated with MBT and Surinmustgarh anticline (Dube & Srivastava 1983). In Himachal Pradesh, the prominent tear faults include Chamba (which is source zone of M 6.5, 1945 earthquake), Ropar tears, eastnortheast (ENE)-oriented strike slip fault near Dharmasala (Srivastava et al. 1987) and Kaurik faults (which gener- ated M 7, 1975). Three blocks, namely Kangra re-entrant (associated with M8, 1905), Shimala block and Dehradun re-entrant, are also detached by strike slip movement and therefore, present different conﬁguration of the detachment surface. In the adjacent Kumaon and Nepal regions, the tear faults have caused predominant right lateral shear movements (Valdiya 1976). These transverse faults are parallel to the great faults discovered in the Ganga basin and South Indian block, suggesting genetic connection (Acharya & Narula 1998). Two such transverse lineaments, i.e. Gangtok and Tista (source zone of M 6.9, 2011 earthquake), are seismically active. Many transverse folds and faults in Nepal, Bhutan and Assam (Dhubri, M 7, 1930) are found besides seismically active Kopili lineament (1943, M 7.5 earthquake). 3. New classiﬁcation of seismic gaps in Himalaya Study of global catalogues of earthquakes (USGS, ISC) and paleoseismic data shows that large/great earthquakes tend to recur in the same region with varying time inter- vals (Yeats et al. 1997; Sykes & Menke 2006). The regions where such earthquakes have been located in the past, therefore, assume more importance in contrast to pla- ces where history of large earthquakes is not available. Preliminary search for seismic gaps along Himalayan foothills was based on earthquakes of magnitude 6 using the catalogues of Tandon and Srivastava (1974) and India Meteorological Department. This criteria was adopted because, the return period of earthquakes of magnitude 6 in different sectors was found as 20–30 years (Srivastava & Dattatrayam 1986), which is well within the period of reliable instrumental data of about 100 years. Thereafter, these regions were integrated with the largest earthquakes, historical, paleoseismic and micro-seismicity data, tectonics and GPS-based geodetic observa- tions (Mahesh et al. 2012). This analysis (table 1) led us to deﬁne seismic gaps into two categories. Category 1 seismic gaps are classiﬁed as those where largest earth- quakes of magnitude 8 or larger occurred and may be expected to recur, while those in category 2 could experience earthquakes of magnitude less than 8. Closer insight also brings out difference in the recurrence interval of the earthquakes of the same magnitude; being lesser in the category 1 as compared to category 2 seismic gaps. ﬁgure 2 shows the delineation of 10 seismic gaps in the Himalaya. Data on earth- quakes, tectonics, micro-seismicity, paleoseismicity and GPS used to demarcate these seismic gaps are given in table 1. 3.1. Kashmir seismic gap This gap lies between the epicenters of Muzaffarabad earthquake of 2005 to Kishtwar (up to Ravi tear) excluding the Jammu region. Historical and instrumental data during 228 H.N. Srivastava et al. Table 1. New categories of seismic gaps in Himalaya. Earthquakes of Earthquakes of M and above 7 M 6 (IMD (IMD catalogue catalogue until Sl No Seismic gap Tectonic feature until 2012) 2012) Micro-seismicity Paleoseismicity GPS data 1. Kashmir HFT/MBT/ MCT 1500(X), 1552 2 No data 13 to 17 mm/yr@ In close (7.5), 1554/55 proximity; (7.7), 1662 (7.5), Panjal thrust 1737(7.5), 1778 active (7.7), 1784(7.3), 1863(7.0), 1885 (7.3) 2. Jammu Between Chamba Nil Nil Moderate Nil nil and Ravi tears, MBT/ MCT 3. Kangra Strike slip fault 1905 (7.8–8.0) 6 Concentration 1400 AD# 14 mm/yr; 100 km triggering on locking MBT, HFT 4. East Himachal Transform motion Nil Nil Marked gap 1423,260 AD# 10 km locking Pradesh on HFT 5. Uttarakhand- Tear faults, HFT/ 1505(8), 1803(7.8), 10 Clusters 800 130 AD$ 10–18 mm per Dharachulla MBT/MCT, 1916(7.5) year 6. Western Nepal MBT/HFT Nil Nil Moderate Nil Locked zone 100 km 7 Central Nepal- Tear faults, MCT/ 1833(7.7), 1934 7 Clusters near 1100 ADV Locked zone less Bihar (central MBT/ HFT (8.1) MCT/ MBT/ than 100 km Nepal) HFT (continued) Discriminatory characteristics of seismic gaps in Himalaya 229 8. Sikkim- Bhutan Transverse 2011 (6.9) 2 Scattered 3 episodes, M6.9 10–12 mm per tectonics in Bhutan year (Pleistocene – Holocene)< 9. Arunachal MCT, MBT, 1697, 1713/14, Many Syntaxial bend 500 yrs ago£ 15–20 mm per HFT, Lohit and 1950(8.4-8.6) year Mishmi Thrust. 10. Shillong Chedrang, Dauki, 1897(8.2) 4 Concentrated 500-1200 years 11 mm per year/ Haﬂong and ago€ entry> Disang thrust. Data from Verma and Bansal (2012), except that at Sl No.1; @Bilham et al. (2011); # Kumar et al. (2001); $ Joshi et al. (2009); V Lave et al. (2005); < Dasgupta et al. (2013b); £ Rajendran and Rajendran (2011); € Rastogi et al. (1993); Sukhija et al. (1999); Rajendran et al. (2004). 230 H.N. Srivastava et al. Figure 2. New categories of seismic gaps in Himalaya. the period of 1500–1900 show that several destructive earthquakes (M 7 or more) occurred in Kashmir (table 1). Of these, the last earthquake along the Himalayan plate boundary in Kashmir occurred 120 years prior to the 2005 Muzaffarabad earth- quake of M 7.6 (Singh et al. 2006). Bilham et al. (2011) inferred 13–17 mm conver- gence rate with maximum southsouthwest (SSW) velocity gradient, northeast of Kashmir valley beneath the Zanskar range. Although these results are based on limited data, they are comparable to that reported for Kangra region (Banerjee & Burgmann 2002), where 1905 (Mw 7.8–8) earthquake occurred. The north-west part of Kashmir lying near the syntaxial bend, where all the major tectonic units, namely MCT, MBT and HFT are in close proximity and the stresses built up due to the plate movement are localized, causing high seismicity and large magnitude earthquakes near Panjal thrust. Shah (2013) identiﬁed active northeast dipping thrust faults in Kashmir Basin using sat- ellite imagery and geomorphology. The frequency of large/great earthquakes in this region and tectonics designate it as seismic gap of category 1 with the largest expected magnitude 8. 3.2. Jammu seismic gap The area between Chamba and Ravi tears is included in this gap. Earthquake cata- logues show that only a few earthquakes of moderate intensity (M less than 6) have occurred in this region. Micro-earthquake activity is also found to be moderate as recorded by local network of stations at Jammu, Udhampur, Ramban and a few other stations around Pong and Pondoh dams in Himachal Pradesh (ﬁgure 3). Due to non-availability of history of large/great earthquakes for such earthquakes as Discriminatory characteristics of seismic gaps in Himalaya 231 Figure 3. Micro-seismicity during October 1977 to September 2002 in Himachal Pradesh and neighbourhood. Himachal Pradesh seismic gap between 1905 Kangra and 1975 Kinnaur earth- quakes marked as A, ﬁrst identiﬁed by Srivastava and Chaudhury (1979). (Courtesy; India Meteorological Department.) compared to Kashmir, this region is placed in seismic gap of category 2 with the larg- est magnitude earthquake expected as 7–7.5. 3.3. West Himachal Pradesh seismic gap (Kangra gap) Towards east of Chamba tear lies the west Himachal Pradesh (Kangra gap) enclos- ing primarily main meizoseismal area of Kangra earthquake of 1905 (M 7.8–8). There is a concentration of micro-earthquake activity delineated by close network of seismological stations (ﬁgure 3). In this region, the presence of Chamba tear and the ENE-oriented strike slip fault close to Dharamshala generate high micro-seis- micity (Srivastava et al. 1987;Kumar et al. 2009), where stresses are accumulated along MCT, MBT and HFT, caused by the plate movement resulting in fault interaction. This is evident from the pattern of earthquakes on tear fault and occur- rence of large earthquake near HFT/MBT. During 1974 and 1978, two earthquakes of magnitude about 5 occurred near Dharamsala, with strike slip fault mechanism (Srivastava et al. 1987). The earthquake of magnitude 5.6 in 1986 near Dharma- sala, however, suggested thrust faulting with one of the nodal planes oriented along MBT. Extending this analogy, the stresses accumulated along the strike slip fault near Dharamsala may trigger large earthquakes near MBT/MCT in Kangra region, where ambient strain is accumulated due to plate motion. Malik et al. (2010) identi- ﬁed Hajipur fault from Corona satellite in the mesoseismal area of 1905 earthquake along the north-west of Janauri anticline. As mentioned in table 1, previous large earthquake in 1400 AD (Kumar et al. 2001), and high seismicity classify this region as seismic gap of category 1 with the largest magnitude 8. 232 H.N. Srivastava et al. 3.4. East Himachal Pradesh seismic gap This gap lies between the eastern boundary of primary meizoseismal area of Kangra earthquake, 1905 (M 7.8) and Kinnaur earthquake (M 6.8, 1975), i.e. roughly between longitude 77 E and 78 E. No earthquake of magnitude 6 or more has been recorded in this gap. This gap ﬁrst identiﬁed by Srivastava and Chaudhury (1979), on the basis of long-term micro-seismicity data, suggested the possibility of an earth- quake of magnitude 7–7.5 only. The region showing gap in seismicity in Himachal Pradesh (ﬁgure 3) is considered as an anomalous zone, in terms of rapid attenuation of seismic intensity mainly due to Kangra earthquake of 1905 and restraining its frac- ture length. Malik et al. (2008) found terraces of active faults near Chandigarh close to HFT and suggested that a major earthquake occurred during 15th or 16th century but could not be speciﬁc whether this was due to manifestation of one of the two 1505 earthquakes with epicenters far away in Kabul and central Himalaya (magni- tude 8–8.2) or some other earthquake during 1400 AD. History of earthquakes recorded during medieval period (Iyengar et al. 1999) does not mention any such earthquake and therefore, the effects were not local but distant effects of the two great earthquakes during 1505. Paleoseismic investigations about 150 km to the southeast around Kala Amb suggests two large rupture earthquakes during past 650 years around 1423 AD and 260 AD (Kumar et al. 2001). It was found from trenching that Black Mango (Kala Aam) fault has transform motion between the two main thrust segments of the HFT. It is obvious that in the regions, where such strike slip faults occur, the stresses cannot accumulate to the same extent as in a thrust fault and accordingly the estimates of the magnitude of the largest earth- quakes in this region would be less than that for the Kangra region. This was corrob- orated by Kumar et al. (2001) who found smaller slips than expected from magnitude 8 or larger earthquake. Also (Malik and Nakata (2003) found a lower slip rate of about 6.3 mm/y near Chandigarh. GPS observations suggested only 10 km locking in the region (Banerjee & Burgmann 2002). Near its eastern boundary deﬁned by Kinnaur earthquake (1975), the recurrence interval of earthquakes has been inferred as 500 years near Sumdo area (Lower Spiti) and 148–1000 years in Baspa valley (Kinnaur) near Kaurik–Chango fault zone for earthquakes of magni- tude more than 7. However, further investigations using 14C and TL dating were suggested (Bagati 2001) to conﬁrm the results. Due to transform faulting on HFT, absence of large earthquakes, micro-seismicity gap and small locking, this region is designated as seismic gap of category 2 with the largest possible earthquake magni- tude as 7–7.5. 3.5. Uttarakhand–Dharachula seismic gap The Uttarakhand–Dharachulla seismic gap lies between Kaurik fault (HP) and MCT near Dharachulla, Nepal. Besides many earthquakes of large magnitude (M > 6) during 1900–2000 (table 1), micro-seismicity in this region is high (Pandey et al. 1999). Rajendran and Rajendran (2011) suggested that the last great earthquake in the region occurred during 1119–1292 instead of 1505 or 1803. The magnitude of 1119–1292 event cannot be quantiﬁed and could be the site response of a more dis- tant central Nepal earthquake of 1100. We estimate that the earthquake of 1505 was slightly larger than 1803 since liquefaction features due to former extended to larger distance up to Agra instead of Mathura during the later event. Also, considering the Discriminatory characteristics of seismic gaps in Himalaya 233 signature of 1505 event in several trenches in the secondary meizoseismal area in sedi- ments (Kumar et al. 2010), it may be more appropriate to assign its epicenter near Dharachulla, where large earthquake (M 7.5) occurred in 1916. This would support the steady state model (Seeber et al. 1981), which suggested that MBT and MCT are contemporaneous features and MCT is still active as supported by clusters of micro- seismicity and geomorphological evidences (Seeber & Gornitz 1983). Corroborative observation about magnitude of 1803 earthquake was based on similarity of damage between 1803 Uttarakhand (above sixth ﬂoor in Kutub Minar) and 1985 Mexico earthquakes (maximum damage above sixth ﬂoor) at about 300 km epicentral dis- tance in Delhi and Mexico cities. Similar values for estimated (Singh et al. 2002) and recorded (Singh et al. 1988) acceleration were reported in respective soft ground con- ditions. Thus, in spite of difference in tectonics in the source regions, the site effect predominates in secondary meizoseismal areas (Srivastava et al. 2010). The magni- tude of 1803 earthquake may, therefore, be similar to that of Mexico earthquake 1985 and would be closer to 8. On the western side near Dehradun, signiﬁcant dam- age may also occur due to site response from a distant earthquake similar to that observed during Kangra earthquake 1905 (Srivastava et al. 2010), which was errone- ously interpreted and attributed to a large local earthquake of magnitude 7 based on meagre data (Bilham & Wallace 2005). Paleoseismic investigations along Sirmuri Tal fault in Dehradun valley based on colluvial wedges in a trench suggested two major earthquakes in the region during the last 1000 years (Oatney et al. 2001). Joshi et al. (2009), based on paleo liquefaction features near north–south-oriented Yamuna tear fault suggested an earthquake during 800 130 years with magnitude greater than 7. They pointed out that if the epicenter of this event was near MCT, its magnitude would be 8 or more. The convergence rate of 10–18 mm year (Jade 2004) is in agreement with that reported in Kangra region. The seismotectonic model in this zone is almost similar to Kangra gap characterized by strike slip faults interacting with MBT and MCT. High seismicity and recurrence of large earthquakes suggest this region to be classiﬁed as seismic gap of category 1 with the largest earthquake magnitude 8. 3.6. Western Nepal seismic gap This gap lies between Uttarakhand and central Nepal excluding Dharachulla region. The cluster of micro-seismicity suggests segmentation of the Himalayan arc and two major discontinuities demarcate the micro-seismicity belt at 82.5 E and 86.5 E (Pandey et al. 1999). In western Nepal, strain accumulation of more than 30 10 per year is estimated in south of the higher Himalayas in a zone where an intense micro-seismicity reﬂects a stress build-up. Based on the measured and simulated dis- placement ﬁeld from GPS data, Jouanne et al. (2004) inferred that the width of the locked zone between main frontal thrust and the creeping zone is of the same order but rather slightly more in western Nepal than in central Nepal. Therefore, earth- quake of magnitude M8.5 could occur in seismic gap in western Nepal. The conver- gence between India and southern Tibet occurs at rate of 20.5 1 mm per year in western Nepal compared with 18.8 0.5 mm per year in central and eastern Nepal (Ader et al. 2012). They suggested large deﬁcit of seismic slip over 500 years and prognosticated earthquakes of magnitude 8.5 and 9 with return periods of about 1000–3000 years. The manner in which slip is partitioned in Himalayan region with 234 H.N. Srivastava et al. complex fault pattern does not enable us to give correct estimate of seismic slip and needs further investigations. Their estimate of 8.5–9 magnitude earthquake is, there- fore, ruled out in central and western Himalaya (Srivastava et al. 2013). Since, there is no history of large/great earthquake in this region; it is placed in seismic gap of category 2 with the largest earthquake of magnitude 7.5. 3.7. Central Nepal–Bihar seismic gap Historical seismicity in Nepal is generally based on report of damage in Kathmandu valley during earthquakes of 1255, 1408, 1681, 1810, 1833, 1866 and 1934. Three largest magnitude earthquakes in central and eastern Nepal and adjoining north Bihar occurred in 1100 (Lave et al. 2005), 1833 (M 7.7) and 1934 (M 8.1), respec- tively. In eastern Nepal, dense clusters of seismicity in the vicinity of MCT and HFT close to the epicenter of 1934 earthquake are attributed to the stress concentration zones (Monsalve et al. 2006). A large number of transverse lineaments exist in west- ern Nepal as well as Patna tear fault in Gangetic plain (Dasgupta 1993). Fault mod- els for the east and west Nepal also show fault planes dipping at different angles (3 – 8 ), locking at different depths (15–25 km) and different seismicity patterns. Chen and Molnar (1977) revised the epicenter of 1934 earthquake and placed it within Nepal close to MCT based on sparse seismological stations and poor time-keeping. The recent 1988 earthquake (M 6.8) whose epicenter was well determined occurred south of Himalaya in Indo-Gangetic plains in Bihar. Since, this earthquake also gen- erated two meizoseismal areas similar to 1934 earthquake, the inference of Geologi- cal Survey of India about the location of 1934 and 1988 at the same place gets corroborated (Dasgupta et al. 1993; GSI 1993). Both these earthquakes are attrib- uted to Patna tear fault. Assuming that 1934 earthquake is attributed to a thrust in Himalaya and its epicenter associated with HFT, the 1833 earthquake could be related with MCT based on ﬁeld observations (Bilham 1995). The inference by Bil- ham and Wallace (2005) that stresses due to 1833 and 1934 earthquakes have been released due to overlapping fracture zones is, therefore, ruled out. The recurrence of large earthquakes and tectonic features (table 1) qualify this region as seismic gap of category 1 with the largest earthquake of magnitude 8. 3.8. Sikkim–Bhutan seismic gap This gap extends from Bihar–Nepal border to Bhutan. An earthquake of magnitude 6 occurred in Sikkim close to MBT on 19 November 1980. Prior this event, damaging earthquakes occurred in 1849, 1852 and 1899 rarely exceeding seismic intensity of VIII MM. The focal mechanism of earthquakes in Sikkim shows that the tectonics of Himalaya in this region is dominated by large-scale strike slip motion (Hazarika et al. 2010; Kayal et al. 2010; Dasgupta et al. 2013a; Pradhan et al. 2013). The recent Sikkim earthquake (M 6.9) of September 2011 with strike slip mechanism on a NNE-oriented fault showed its extension down to a depth of about 50 km implying deeper extension of lineaments and suggesting fragmentation of detachment surface in this region (Harward CMT solution). Micro-earthquake activity is scattered between MCT and MBT and also along lineaments (Paul 2010). Three episodes of uplift since 16,000 years were interpreted to correspond to three morphogenic earth- quakes of magnitude 6.9 in Bhutan rupturing the frontal back thrust during Discriminatory characteristics of seismic gaps in Himalaya 235 Pleistocene–Holocene period (Dasgupta et al. 2013b). Further eastwards, Dhubri fault was associated with an earthquake of magnitude 7 in 1930 (Gee 1934). Due to non-availability of history of any large earthquake (M 8) and dominance of strike slip motion, this zone is placed in seismic gap of category 2 with the largest earth- quake magnitude 7.5. 3.9. Arunachal seismic gap This gap lies between eastern Bhutan border and the Himalayan syntaxis. The source mechanism of 1950 earthquake that occurred near the syntaxis is complex (Tandon 1955; Ben-Menahem 1974; Chen & Molnar 1977). This region is characterized by major earthquakes in 1697 (Sadiya, upper Assam), 1713/1714 (southeast of Sibsagar), along Kopili fault (1943, M 7.5) and several earthquakes of magnitude 6 and larger. Near the north-eastern syntaxial bend in Himalaya, where the largest earthquake of 1950 (M 8.4–8.6) occurred, the high seismicity is attributed to Mishmi block and transverse mountain range. Rajendran and Rajendran (2011) sur- mised that the last penultimate earthquake in the 1950 upper Assam sources may have occurred 500 years ago. Mukul et al. (2010) estimated that the convergence is being accommodated in the NE Himalayan wedge at a rate of about 15–20 mm per year. The largest earthquake (1950, M 8.4–8.6) occurred near syntaxial bend (Mukhopadhyay et al. 2011a). The recurrence of the largest earthquake in this zone is attributed to dominance of thrusting and remnant subduction along the eastern boundary and larger component of stress vector due to north-east movement of Indian plate (Srivastava et al. 2013). The region is, therefore, placed in seismic gap of category 1 with the largest earthquake magnitude as 8.5. A question arises whether in view of high seismicity and history of large earth- quake (1943, M7.5) along Kopili lineament, new seismic gap could be demarcated along its 300-km length. The north-western end of this fault extends into Bhutan (Kayal et al. 2010), which is characterized as seismic gap of type 2. However, the remaining portion in Assam characterized by higher seismicity and large earthquake (1943) could be placed under category 1. Considering the complexities involved and different tectonic features of Arunachal gap, the classiﬁcation of Kopili region needs to be addressed in future when more geophysical data are available. 3.10. Shillong seismic gap Shillong plateau experienced a few earthquakes of magnitude 6 or more. The great Shillong earthquake (1897) occurred near the northern margin of this plateau, which is considered as a detached portion of Archean shield. It is bounded by Chedrang fault in the north and Dauki fault (right lateral strike slip) towards south. Several north–south trending faults have been identiﬁed in the west of the plateau. On south- east side, the old rocks of the plateau are thrust over upper tertiary and Cretaceous formations along the Haﬂong–Disang fault zone. Micro-earthquake activity is very high (ﬁgure 4) over the plateau (Srivastava et al. 1996). Banerjee and Burgmann (2002) estimated the convergence rate of 11 mm per year near the plateau and suggested southward motions at a rate of 4–7 mm per year for the sites on the Shillong plateau. Since Shillong plateau is bounded by faults on all sides, it is more vulnerable for recurrence of a great earthquake due to stress focusing. Limited 236 H.N. Srivastava et al. Figure 4. Micro-seismicity in north-east India during 2001–2010. (Annual Seismological Bulletin, NEIST-Jorhat/ NGRI-Hyderabad publication, 2001–2010.) paleoseismological data (Rastogi et al. 1993, Sukhija et al. 1999; Rajendran et al. 2004) suggest earthquake recurrence intervals ranging from 500 to 1200 years around Shillong. This region falls in seismic gap of category 1, where seismic quies- cence of 116 years may be noted. The largest earthquake in this gap could be of magnitude 8.2. 4. Discussion Micro-seismicity, instrumental records, paleoseismic and historical data as well as variation in local tectonics show that the whole of Himalayan plate boundary extending for 2900 km may not be equally seismogenic to produce the largest earth- quake of M 8.5 anywhere. The analysis of seismic gaps has revealed that Jammu, east Himachal Pradesh, west Nepal and Sikkim placed in category 2 may experience the largest magnitude of earthquake as 7–7.5 instead of magnitude 8 or more in seis- mic gaps of category 1. The classiﬁcation of seismic gaps into two categories, there- fore, appears to be more realistic from the point of view of earthquake hazard assessment due to shorter recurrence period for earthquakes of same magnitude in seismic gap of category 1 as compared to gaps of category 2. This also results into Discriminatory characteristics of seismic gaps in Himalaya 237 reduction of the size of Central seismic gap of 800–900 km length (Khattri 1987) into two smaller seismic zones around Uttarakhand to Dharchulla and central Nepal to Bihar only. This result, therefore, reduces alarm raised by Bilham et al. (2001) sug- gesting that several earthquakes of 8.5 magnitudes would occur in Himalayan plate boundary. It may be interesting to compare these results with those based on spatio- temporal clusters, which suggested that Kangra, east Nepal, Garhwal and Kumaun, west Nepal clusters have decreasing order of threat for large (M 7.7 and above) earthquakes (Mukhopadhyay et al. 2011b). Seismic quiescence of 108, 79 and 210 years has occurred since the last 3 major/great earthquakes (1905, 1934 and 1803) in the respective gaps, giving a different result. Accordingly, recurrence of major/great earthquake could be earlier in Garhwal Himalaya rather than in Kangra region. The quiescence of 200 years in Garhwal agrees with similar recurrence inter- val of 193 years in Kutch region in the failed rift, away from Indian plate boundary. These inferences would be helpful in reﬁnement of the current practices of seismic hazard assessment (Nath et al. 2008; Pal et al. 2008; Verma and Bansal 2013a, 2013b). However, the complexity of fault pattern resulting in nonlinear build-up of stresses in the Himalayan region calls for a comprehensive geophysical research. Although information from seismic tomography and MT surveys is scanty in the Himalayan region, inference about a low velocity layer at a depth of about 12 km in the Kangra region (Kumar et al. 2009); a conductive zone below about 25 km in Uttarkashi region from MT survey and conductive zone in northern parts of central Nepal (Lemonnier et al. 1999) suggest that a plausible model for seismic gap of cate- gory 1 could be based on the role of ﬂuids in the region. Accordingly, in the ﬁrst phase stresses caused by plate movement after reaching a certain value pressurize the trapped ﬂuids. In the second phase, they start inﬁltrating the upper crust and acceler- ate the release of seismic energy through major earthquakes. This is somewhat simi- lar to well-known diltatancy diffusion model of earthquakes. In the category 2 seismic gaps, if the ﬂuids have already inﬁltrated the crust, such as hot springs in Mannikaran near Kulu in east Himachal Pradesh, the stress accumulation would be slightly less resulting in earthquake of lesser magnitude. Also in western Nepal, lack of evidence about the absence of such conductive layer suggests longer recurrence interval supported by the absence of great earthquake. Whether, this model can be extended to other regions of Himalaya needs further data. 5. Conclusions The limitations in the three seismic gaps in the Himalaya (Khattri 1987) brought out the need to re-examine earthquake potential based on larger data spanning back to 1100 AD, now available, besides detailed tectonics, GPS and micro-earthquake observations. The results show that all the regions in Himalaya are not equally seis- mogenic to produce magnitude M 8.5 earthquakes contrary to the suggestions made by the earlier workers on the assumption of uniform seismic slip. The new seismic gaps delineated in this study have been placed into two categories. The regions, where great earthquakes have occurred in the past have been placed in seismic gap of category 1, while those portions of the Indian plate boundary, where history of large earthquakes is not available, are designated as seismic gap of category 2. The distinc- tive criteria between two types of gaps are based on magnitude of largest earthquakes and their return period. 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