Spectral Characterization of Himalayan Near-Fault Ground Motion

Near-Fault Ground Motion (NFGM) spectral characteristics of three moderate-sized Himalayan earthquakes, viz., the 1986 Dharamsala earthquake (Mw = 5:5), the 1991 Uttarkashi earthquake (Mw = 6:8), and the 1999 Chamoli earthquake (Mw = 6:5) have been studied from the 33 available strong ground motion recordings. Pulse characteristics of fault-normal components in terms of pulse-periods and pulse-indicators have been extracted adopting wavelet analysis. Seven mother wavelets were used in the analysis, and it was found that db4 and db7 mother wavelets were more efficient in extracting the pulse-type characteristics. NFGM spectra, at Bhatwari and Gopeshwar stations, showed higher spectral amplitudes in the velocity-sensitive and acceleration-sensitive regions compared to Indian codal response spectra. This is attributed to high PGV/PGA ratios. The study shows that NFGM leads to widening of acceleration-sensitive region, and the structures that are designed according to the Indian seismic code as flexible structures shall behave as stiff structures when subjected to NFGM.