On the damage caused by the Chamoli earthquake of 29 March, 1999

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1 Journal of Asian Earth Sciences 19 (2001) 129±134 On the damage caused by the Chamoli earthquake of 29 March, 1999 I. Sarkar*, A.K. Pachauri, M. Israil Department of Earth Sciences, University of Roorkee, Roorkee , India Abstract The moderate magnitude Chamoli earthquake, which occurred on 29 March, 1999 in the Alkananda river valley region of Garhwal Himalaya, caused considerable loss to human life and property. From a systematic eld survey of the damage, conducted for ve continuous days immediately after the earthquake, the following could be noted: (i) The consequences of the damage are most pronounced near the Main Central Thrust (MCT). (ii) The maximum damage is between Chamoli and the 40 km 2 area to the its north and was probably caused by major uplift. (iii) Distinct radial cracks accompanied by considerable ground subsidence due to this earthquake, could be observed north of the completely devastated village of Mawana, situated 4 km northwest of Chamoli. (iv) The intensity of damage in the meisoseismal zone, covering approx. 580 km 2 area, suggests a value of 0.2 g for the ground acceleration. q 2001 Elsevier Science Ltd. All rights reserved. Keywords: Chamoli earthquake; Garhwal Himalaya 1. Introduction In the early hours of 29, March, 1999, at 00:35 IST, the Chamoli district of Garhwal Himalaya and its adjoining area was severely rocked by a devastating earthquake. According to information provided on the Internet by the US Geological Survey (USGS), this moderate magnitude earthquake (m b ˆ 6.3, M S ˆ 6.6, M W ˆ 6.4, M 0 ˆ Nm) occurred at ( N, E, 12.0 km). The details of the two nodal planes (NP1 and NP2) as speci ed by USGS fault plane solution are as follows: NP1 with strike ˆ 2828, dip ˆ 98, slip ˆ 958; NP2 with strike ˆ 978, dip ˆ 818, slip ˆ 898. We prefer to consider the gently dipping plane NP1 as the fault plane of this earthquake for the following aspects of regional geology. The great, moderate and small earthquakes of Himalaya, all occur due to the relative movement of the converging Indian and Eurasian lithospheric plates along the Himalayan tectonic zone. However, the great (M. 8.0) earthquakes, generally located beneath the Sub and Lesser Himalaya, occur by thrusting on a major, sub horizontal intra-crustal basal detachment surface representing the upper surface of the Indian lithospheric plate. The moderate magnitude (5.0, m b, 7.0) earthquakes generally occur near the northern edge of the Lesser Himalaya, closer to the surface trace of the MCT, at 10±20 km depth and are mostly due to reactivation of low angle, detachment parallel, smaller thrust planes dipping gently towards the north. The smaller * Corresponding author. magnitude (m b, 4.0) earthquakes of the region generally occur in the upper crust due to reactivation of high angle reverse thrust faults and generally do not represent the main process of seismic energy release that is associated with the great and moderate earthquakes of the region (Seeber and Armbruster, 1981; Lyon-Caen and Molnar, 1983; Ni and Barazangi, 1984; Khattri, 1987, 1999; Chander, 1988; Molnar, 1990; Sarkar et al., 1993). We systematically surveyed the earthquake damage in Chamoli, and its adjoining areas in the Alkananda and Mandakini river valleys, between 1±5 April, 1999 over an area covering about 3000 km 2. The main focus of the survey was to identify any plausible patterns in the damage, which could provide observational constraints for enhanced understanding of the earthquake and its related processes. In the article we report the details of our observations on the geology and damage of the area and later provide possible inferences. 2. Geology of the area The Chamoli region lies in the vicinity of the Main Central Thrust (MCT) (Gansser, 1964). This thrust locally strikes NW±SE and dips 158±208N. The quartzites are well exposed at Chamoli and extend 2-3 km to the northeast and are replaced by limestone and slate sequences of the Pipalkoti window. The area falls within a high landslide hazard zone. Several landslides have resulted from the Chamoli earthquake. At Gopeshwar, less than 2 km northwest /01/$ - see front matter q 2001 Elsevier Science Ltd. All rights reserved. PII: S (00)

2 130 I. Sarkar et al. / Journal of Asian Earth Sciences 19 (2001) 129±134 Fig. 1. Map locations of villages and towns where earthquake damage was surveyed are marked with dots. The dot indicated by G denotes the location of Gopeshwar. The four dots between Mawana and Mandal denote villages, which are identi ed in more detail in Fig. 2. Fig. 2. Geological map of the area around Chamoli. The locations of surveyed villages and towns are marked with dots. The Main Central Thrust (MCT) passes through Maithana and later takes a northeast turn towards Sagar. The lineaments 1 and 4 denote parts of MCT while the fracture lineaments 2 and 3 locally cut across MCT. Close to Mawana, the Sagar±Mawana lineament (1) and the Mandal±Mawana lineament (3) nearly intersect. The lineaments 2 and 4 are nearly parallel to Mandal±Mawana lineament (3), perhaps marking a shear zone. Chamoli is situated in quarzites while Birahi and Pipalkoti are situated in Pipalkoti limestone and slate sequence.

3 I. Sarkar et al. / Journal of Asian Earth Sciences 19 (2001) 129± Vairagna, northwest of Mawana, demonstrate the extent of this zone. 3. Damage survey of the area Our survey was conducted in four continuous phases. During the initial phase, we surveyed from Lachauli, 10 km south of Srinagar via Rudraprayag, Nandaprayag, Maithana to Chamoli, along a generally northeast direction stretching approx. 60 km. During the intermediate phases, we surveyed an approx. 12 km stretch NNE of Chamoli along the Chamoli± Birahi±Pipalkoti section and an approx. 10 km stretch NW of Chamoli along the trend Chamoli±Gopeshwar±Sagar± Gawar±Deoldhar±Mawana±Vairagna±Mandal. In the nal phase, a survey was generally conducted in the Mandakini river valley from Mandal to Chopta, Ukhimath, Chandrapuri, Kuonja, Agastmuni, Tilwara to Srinagar for a distance of approx. 100 km (Fig. 1). 4. Inferences The following inferences could be drawn on the basis of this study: Fig. 3. Photograph of the fault gauge found exposed at Mawana village whose location is indicated in Figs. 1 and 2. of Chamoli (Fig. 1), a huge landslide caused by the present earthquake was active for several days. The landslide is a failed ridge and now forms a cliff. It shows that the high degree of slope and active faults in the region form high hazard zones and are likely to fail when triggered by an earthquake (Pachauri and Pant, 1992; Pachauri et al., 1998). Within the context of the occurrence of the present earthquake, the local structural features have possibly played a major role. There are two lineaments that virtually intersect at Mawana (Figs. 1 and 2). These are the (i) Mandal± Mawana Lineament which trends NW±SE and follows an internal structural extension of the Mawana±Mandal zone of the MCT, and (ii) Sagar-Mawana lineament which follows the boundary of the MCT closely up to Mawana where it intersects the Mandal±Mawana lineament. The basic contention of the geological work is the presence of the intersection of these two lineaments at Mawana and the NW±SE zone along the Mawana±Mandal tract within the MCT zone. This is evidenced in the fault gauge exposed at Mawana village (Fig. 3). The bold scarps exposed at 1. Fig. 4 is the isoseismal map drawn on the basis of our eld data and modi ed Mercalli intensity (MMI) scale. The meisoseismal zone extends over a 580 km 2 area which commensurate with the areal extent of rupture for an earthquake with seismic moment M 0 ˆ Nm (Kanamori and Anderson, 1975). The intensity VIII assigned to this zone implies a ground acceleration of approx. 0.2 g (Richter, 1958). Because of obvious drawbacks in directly correlating ground acceleration to intensity, this value is only an initial estimate. However it may be mentioned that for the Uttarkashi earthquake (m b ˆ 6.6) of 19 October, 1991, the peak horizontal acceleration overlying the aftershock zone was instrumentally estimated as nearly 0.3 g (Yu et al., 1995). 2. Our observations reveal that, in the meisoseismal zone, maximum damage in terms of building collapse leading to greater loss of human life and property is generally con ned to south of Mawana (Fig. 4). Gahalaut et al. (1994) provide estimates of permanent horizontal and vertical ground displacement at different points of the ground lying directly above and around a large rupture area due to a shallow, gently dipping thrust fault along which slip varies only in the dip direction. Their calculations indicate that the up-dip section of the ground, immediately above the rupture zone, is elevated several times more than the surrounding region. In contrast, the down dip section of the ground indicates considerable subsidence. The comparatively high damage observed in the up dip section of the meisoseismal zone from Mawana to Chamoli has been possibly caused by similar

4 132 I. Sarkar et al. / Journal of Asian Earth Sciences 19 (2001) 129±134 Fig. 4. The isoseismal map of the Chamoli earthquake. The intensities assigned to the different isoseismal zones are shown with Roman letters. The star (*) marks the USGS located epicentre (see text). The locations of some important towns of Garhwal Himalaya are identi ed. uplift. In the Chamoli area, there are several buildings that appear to have been uplifted resulting in their total collapse. The best example of this is seen in the Jail area of Upper Chamoli (Fig. 5). Again just north of Mawana, in the down-dip direction, there are extensive ground cracks almost radial in nature where there is notable subsidence (Fig. 6). We are of the opinion that Mawana village and its immediate neighborhood lies on the Fig. 5. Photograph of the Jail building (A type) at Chamoli. The walls of this old building have caved in possibly due to major ground uplift. Note that the strongly built beams and windows have withstood the earthquake motion.

5 I. Sarkar et al. / Journal of Asian Earth Sciences 19 (2001) 129± Fig. 6. Photograph of distinct ground subsidence of more than 15 cm as observed at Mawana. A part of the extensive radial cracks in the ground can also be seen here. boundary of the updip section of the causative fault of the Chamoli earthquake. 3. The damage to the buildings in the villages of Vairagna and Mandal, situated at northern end of the meisoseismal zone and closer to the USGS located epicenter, show that greater damage was caused here by vertical overloading. Similar instances of prominent vertical components of earthquake motion, rather than the horizontal component near the epicentral zone, has been reported for other earthquakes (Richter, 1958). 4. There were several instances of increased ow or drying up of streams in the meisoseismal zone. This seems a natural consequence of a shallow thrust fault regime where the minimum principal stress is nearly vertical while the maximum and intermediate principal stresses are near horizontal. Large horizontal compression of the rocks can result in release of pore water from freshly fractured rocks. This may result in the creation or reactivation of streams having a relatively increased ow or the drying up of already existing ones. 5. There are con icting views about which of the two major thrusts of the Himalayas viz. Main Boundary Fault (MBF) and Main Central Thrust (MCT) is active at present (Valdiya, 1980; Seeber and Armbruster, 1981; Seeber and Gornitz, 1983; Molnar, 1990). Our study and observations reveal that the consequences of the Chamoli earthquake is more pronounced near the MCT, rather than the MBF. Acknowledgements We gratefully acknowledge all help provided by Professor N.C. Nigam, Vice Chancellor and Professor A.K. Awasthi, Head, Department of Earth Sciences, University of Roorkee, to conduct the survey. We also wish to put on record the support of Mr Uma Kant Pawar, District Magistrate, Chamoli and his administrative staff during our eld programme. References Chander, R., Interpretation of observed ground level changes due to the 1905 Kangra earthquake, Northwest Himalaya. Tectonophysics 149, 289±298. Gahalaut, V.K., Gupta, P.K., Chander, R., Gaur, V.K., Minimum norm inversion of observed ground elevation changes for slips on the causative fault during the 1905 Kangra earthquake. Proceeding Indian Academy of Sciences (Earth and Planetary Sciences) 103, 401±411. Gansser, A., Geology of the Himalayas. Interscience, New York. Kanamori, H., Anderson, D.L., Theoretical basis of some empirical relations in seismology. Bulletin of the Seismological Society of America 65, 1073±1075. Khattri, K.N., Great earthquakes, seismic gaps and potential for earthquake disaster along Himalayan plate boundary. Tectonophysics 138, 79±92. Khattri, K.N., An evaluation of earthquake hazard and risk in northern India. Journal of Himalayan Geology 20, 1±46. Lyon-Caen, H., Molnar, P., Constraints on the structure of the Himalaya from an analysis of gravity anomalies and a exure model of the lithosphere. Journal of Geophysical Research 88, 8171±8191. Molnar, P., A review of the seismicity and rate of active underthrusting and deformation at the Himalaya. Journal of Himalayan Geology 1, 131±154. Ni, J., Barazangi, M., Seismotectonics of the Himalayan collision zone: Geometry of the underthrusting Indian plate beneath the Himalaya. Journal of Geophysical Research 89, 1147±1163. Pachauri, A.K., Pant, M., Landslide hazard mapping based on geological attributes. Engineering Geology 32, 81±100.

6 134 I. Sarkar et al. / Journal of Asian Earth Sciences 19 (2001) 129±134 Pachauri, A.K., Gupta, P.V., Chander, R., Landslide zoning in a part of Garhwal Himalaya. Environmental Geology 36, 325±334. Richter, C.F., Elementary Seismology. Freeman, San Francisco. Sarkar, I., Chander, R., Khattri, K.N., Sharma, P.K., Evidence from small magnitude earthquakes on the active tectonics of the Garhwal Himalaya. Journal of Himalayan Geology 4, 279±284. Seeber, L., Armbruster, J.G. Great detachment earthquake along the Himalayan Arc and long-term forecasting. Earthquake prediction Ð An International Review, Maurice Ewing Series 4, American Geophysical Union Seeber, L., Gornitz, V., River pro les along the Himalaya arc as indicators of active tectonics. Tectonophysics 92, 335±367. Valdiya, K.S., Geology of the Kumaon Himalaya. Wadia Institute of Himalayan Geology, Dehradun. Yu, G., Khattri, K.N., Anderson, J.G., Brune, J.N., Zeng, Y., Strong ground motion from the Uttarkashi, Himalaya, India, earthquake: comparison of observations with synthetics using the composite source model. Bulletin of the Seismological Society of America 85, 31±50.