Modeling of strong ground motions for 1991 Uttarkashi, 1999 Chamoli earthquakes, and a hypothetical great earthquake in Garhwal-Kumaun Himalaya

In this study, stochastic finite fault modeling is used to simulate Uttarkashi (1991) and Chamoli (1999) earthquakes using all available source, path, and site parameters available for the region. These two moderate earthquakes are recorded at number of stations of a strong motion network. The predicted peak ground accelerations at these stations are compared with the observed data and the ground motion parameters are constrained. The stress drop of Uttarkashi and Chamoli earthquakes is constrained at 77 and 65 bars, respectively, whereas the quality factor Q (C) is 112 and 149 for these two regions. The high-frequency attenuation parameter Kappa is in the range 0.04-0.05. The constrained ground motion parameters are then used to simulate Mw 8.5 earthquake in central seismic gap region of Himalaya. Two scenarios are considered with epicenter of future great earthquake at locations of Uttarkashi and Chamoli earthquakes using above constrained parameters. The most vulnerable towns are the towns of Dehradun and Almora where expected PGA is in excess of 600 cm/s(2) at V-S30 520 m/s when the epicenter of the great earthquake is at the location of Uttarkashi (1991) earthquake. The towns of Shimla and Chandigarh can expect PGA close to 200 cm/s(2). Whereas when the epicenter of the great earthquake is at the location of Chamoli (1999) earthquake, the towns of Dehradun and Almora can expect PGA of around 500 and 400 cm/s(2), respectively, at V-S30 620 m/s. The National Capital Region, Delhi can expect accelerations of around 80 cm/s(2) in both the cases. The PGA contour maps obtained in this study can be used to assess the seismic hazard of the region and identify vulnerable areas in and around central Himalaya from a future great earthquake.