Strong motion envelope modelling of the source of the Chamoli earthquake of March 28, 1999 in the Garhwal Himalaya, India

Garhwal Himalaya has been rocked by two major earthquakes in the span of just eight years, viz. Uttarkashi earthquake of 20th Oct, 1991 and Chamoli earthquake of 28th March, 1999. Chamoli earthquake of March 28, 1999 was recorded at 11 different stations of a strong motion array installed in the epicentral region. The maximum peak ground acceleration (353 cm/s²) was recorded at an accelerograph located at Gopeshwar. The data from eleven stations has been used for comparison with the simulated acceleration envelopes due to a model of the rupture responsible for this earthquake. For simulation of acceleration envelope the method of Midorikawa (1993) has been modified for its applicability to Himalayan region. This method has earlier been used by Joshi and Patel (1997) and Joshi (1999) for the studyof Uttarkashi earthquake of 20th Oct, 1991. The same method has been used for study of Chamoli earthquake. Layered earth crust has been introduced in place of homogeneous one in this method. The model of rupture is placed at a depth of 12 km below the Munsiari thrust for modelling Chamoli earthquake. Peak ground acceleration was calculated from simulated acceleration envelope using layered as well as homogeneous earth crust. For the rupture placed in a layered crust model peak ground acceleration of order 312 cm/s² was simulated at Gopeshwar which is quite close to actually recorded value. The comparison of peak ground acceleration values in terms of root mean square error at eleven stations suggests that the root mean square error is reduced by inclusion of a layered earth crust in place of homogeneous earth crust.