Time-Frequency Analysis of Strong Ground Motions from the Mw 6.8 1991 Uttarkashi Earthquake

Abstract

The stress build-up along the plate boundary of the Indian and Asian plates is a source of major destructive earthquakes in the state of Uttarakhand. The high seismic hazard in the region has led the Indian Standard Code 1893:2016 to place the entire state of Uttarakhand in seismic zone IV and V, corresponding to seismic intensity levels of severe and very severe, respectively. On 20^th October 1991, a strong earthquake having a moment magnitude Mw 6.8, occurred along the Main Central Thrust in the Uttarkashi and Garhwal regions. Field observations indicated that the maximum damage intensity corresponded to a level of IX as per the modified Mercalli Intensity scale. Reconnaissance studies performed in the aftermath of the earthquake indicate that the high lateral forces generated by the horizontal shaking associated with the strong ground motions caused severe destruction of stone masonry and concrete block structures prevalent in the region. Strong ground motions from the 1991 Uttarkashi earthquake are obtained from 13 stations with hypo-central distances varying from 22 to 155 km. The 26 horizontal waveforms are analyzed and their key characteristics, such as Peak Ground Acceleration (PGA), Peak Ground Velocity (PGV), Arias Intensity (AI), and Predominant Period (Tp) and significant duration are tabulated. The frequency content is analyzed using fast Fourier transforms and the time-frequency analyses are performed using continuous wavelet transforms. The results indicate that the arrival of high-amplitude waves with high frequencies corresponding to the resonant frequency range of low-rise structures at several sites corresponds to observed enormous damage to the structures.