Simulation of strong ground motions of 1991 Uttarkashi (M 7) and 1999 Chamoli (M 6.6) earthquakes using modified hybrid technique

In the present study, 1991 Uttarkashi (M 7) and 1999 Chamoli (M 6.6) earthquakes that occurred on October 19, 1991, at 21:23:14 h and March 28, 1999, at 19:05:11 h, respectively, have been simulated using the modified hybrid technique. Hybrid technique is the combination of two existing techniques, i.e., envelope technique and composite source model technique. In the present modified technique, site amplification functions and kappa factor have also been incorporated. The simulated waveforms and their corresponding response and Fourier spectra for each site have been generated. In this study, simulation has been done at 11 and 9 recorded stations of Uttarkashi and Chamoli earthquakes, respectively. Important frequency- and time-domain parameters, i.e., Fourier spectra, response spectra, peak ground acceleration (PGA) and duration at stations, have been estimated and compared with the observed accelerograms. It has been observed that the simulated PGA (231 cm/s2) at the closest distance Bhatwari (22 km) matched with the observed one (248 cm/s2) for the Uttarkashi earthquake. The same has been observed at the nearest most station Gopeshwar (19 km) of the Chamoli earthquake. The simulated PGA (347 cm/s2) for this station has been found well matched with the observed PGA value (352 cm/s2). Similar matching has been observed for other stations also. The present technique is independent of velocity-Q structure of earth’s layered model and past events data of small earthquakes. This study brings light on the site effect and high-frequency decay parameter. This study can be very helpful in the estimation of seismic hazard in a specific region and designing earthquake-resistant buildings.