Chamoli Earthquake of Magnitude 6.6 on 29 March 1999

The paper presents the results of damage survey for the Chamoli earthquake of Ms 6.6 (USGS) on March 29,1999. Fifteen strong aftershocks of M≥4.0 (including 5 of M≥5.0) were recorded within 20 days of the mainshock. The hypocentre given by US Geological Survey (30.55°N; 79.424E; focal depth 15 km) is south of the Main Central Thrust (MCT) and north of the meizoseismal area. The fault plane (strike 282°; dip 9° N; slip 95°) indicates thrusting along the detachment surface at a depth of 15 km below the MCT. The rupture is inferred to have started near the hypocentre and progressed southward giving maximum moment release in the meizoseismal area. The maximum damaged area of MM Intensity VIII assigned from field observations is about 10 km x 5 km, extending in WNW-ESE direction from Charnoli to Mandal. The other isoseismals also trend WNW-ESE. The areas covered by isoseismals VII, VI and V are 120 km x 70 km, 280 km x 190km and 525km x 670 km respectively, which yield M_w oF6.3 using the empirical relations given for stable continental regions. This M_w is same as 6.3 obtained from moment-tensor analysis. However, Intensity V area (525 km x 670 km or less) is observed to be less as compared to that in SCR regions for such earthquakes as it yields a smaller value of M_w. From the list of 13 earthquakes in the past 97 years in Kumaon-Garhwal region, M≥6.0 earthquakes seem to repeat every eight years in this region.