Macroseismic ®eld generated by 29 March, 1999 Chamoli Earthquake and its seismotectonics A.K. Mahajan * , N.S. Virdi Wadia Institute of Himalayan Geology, Dehradun 248001, Uttar Pradesh, India Received 23 February 2000; revised 21 July 2000; accepted 4 August 2000 Abstract A damaging earthquake of magnitude 6.4 mb USGS) rocked the Garhwal region on 29 March, 1999 at 00:35:00 hours IST). Maximum intensities of VIII and VII on the Medvedev±Sponheaer±Karnik MSK-64) scale and the European Macroseismic Scale-98 EMS-98) were observed in the affected region. Extensive damage was noticed both in the Chamoli and Rudraprayag regions of the Garhwal Himalaya. In the Chamoli region, damage of intensity VIII was con®ned to the right bank of the river Alaknanda. In the Rudraprayag region, the damage was widespread between Chandrapuri and Cantha villages. An isoseismal map prepared on the basis of a damage survey shows a NW±SE trend with its major axis in the NW±SE direction and minor axis in the NE±SW direction in the Rudraprayag region. The major axis is 11 km long and minor axis 8 km long. Thus the VIII isoseismal zone covers an area of approximately 88 km 2 . The damage survey revealed that maximum damage has been either along major faults or to adobe houses, locally called Pathal houses. The damage was also higher than expected because of the use of undressed stones and heavy Pathal slate) roo®ng. The villages of Kansali, Pingalpani, Akhori, Kiyunja, Sena, Cantha, Tewri, Talsari, and Chamoli were the most affected in the Rudraprayag and Chamoli regions. Grade 4 and Grade 3 damage has been observed in many adobe houses in the intensity zone VII. The study of damage distribution and intensity attenuation has demonstrated an asymmetric distribution and heterogeneous behaviour. The heterogeneous pattern could be related to rupture asperity distribution. A noteworthy feature of this earthquake was that it has produced a lot of ground ®ssures, some traceable for many tens to hundreds of meters affecting both rocks and overburden. One such ®ssure was traced for nearly 500 m trending NW±SE with a downthrow of 20±25 cm. This ®ssure shows thrust faulting with the foot wall having moved down towards the NE and the hanging wall upwards toward the SW side. This is in conformity with the fault plane solution given by the USGS using the moment tensor technique, which indicates that the earthquake originated by slip along a low angle thrust fault dipping NE. Changes in hydrological conditions have been noticed in a number of villages, for example Kiyunja, Kansali, Talsari, Pingalpani, Mukku, Sena, Akhori, Tewri, Gurpunga and Bhatwari etc. q 2001 Elsevier Science Ltd. All rights reserved. Keywords: Macroseismic; Earthquakes; Himalaya; Seismotectonics 1. Introduction At about 00:35 hours on 29 March, 1999 an earthquake rocked the northern parts of India and frightened the people as far away as Pune in Maharashtra in the southwest and in Punjab, Himachal Pradesh and Haryana states in the north- west as well as in Uttar Pradesh UP) and Bihar states in the east. The basic source parameters of this earthquake, as determined by different organizations, are shown in Table 1. People in Delhi and Punjab said that it was the second most frightening earthquake since 1991. Most people reported horizontal movement during the event. People at Dehradun reported that their beds swayed in an E±W direction. The damage was widespread in the Chamoli, Rudraprayag and Tehri districts. The earthquake killed about 64 people and injured 500 and also caused extensive damage to property. In all 74,000 people residing in about 500 villages were affected. The maximum damage occurred to traditional Pathal houses, which were located on the phyllitic or calcareous formations and especially near major weak zones. The development of wide cracks and continuous aftershock activity has continued to damage the already affected houses, thus rendering them unsafe. People in many villages are living in tents or makeshift accommodation. New Delhi and Dehradun suffered struc- tural damage to many buildings. The Wadia Institute of Himalayan Geology WIHG) team reached the affected region the day after the main shock in order to assess the damage pattern and seismogenic effects Journal of Asian Earth Sciences 19 2001) 507±516 1367-9120/01/$ - see front matter q 2001 Elsevier Science Ltd. All rights reserved. PII: S1367-912000)00049-3 www.elsevier.nl/locate/jseaes * Corresponding author. Tel.: 191-135-625952; fax: 191-135-625212. E-mail address: wihg@nde.vsnl.net.in A.K. Mahajan).