Abstract

We estimate ground motions in Delhi from possible future large/great earthquakes in the central seismic gap in the Himalayan arc. The closest distance from the rupture areas of such postulated earthquakes to Delhi may be about 200 km. We have used two methods to synthesize the expected ground motions. In the first, recordings in Delhi (three on soft sites and one on a hard site) of the 1999 Chamoli earthquake (Mw 6.5; epicentral distance, ∼300 km), which was located in the gap, are used as empirical Green's functions (EGFs). The ground motion during the target event is synthesized by random summation of the EGFs. In the second, the stochastic method, the motions have been estimated from the expected Fourier spectrum of the ground motion in Delhi through the application of Parseval's theorem and results from random vibration theory. We apply two versions of the stochastic method: the first assumes a point source while the second considers the source to be finite. The predictions from the two methods are in reasonable agreement for Mw ≤ 7.5. For Mw > 7.5 events, the finiteness of the source becomes important. Several rupture scenarios are considered in the application of the finite-source stochastic method. The largest ground motions are predicted in Delhi for rupture occurring between the main boundary thrust and main central thrust and the hypocenter located at the northeast edge of the fault. For this rupture scenario and a postulated Mw 8.0 earthquake, the maximum expected horizontal acceleration (Amax), and velocity (Vmax) at soft sites in Delhi range between 96 and 140 gal and 8 to 19 cm/sec, respectively. For Mw 8.5 event, the corresponding values range between 174 and 218 gal and 17 to 36 cm/sec. Amax at the hard sites are 3 to 4 times less than at the soft sites. The differences are somewhat smaller for Vmax, which are roughly 2 to 3 times at soft sites as compared to the hard site. The horizontal Amax and Vmax estimated by Khattri (1999) for Mw 8.5, using a composite source model, are remarkably similar to those estimated here. The seismic hazard in Delhi may be especially high to the east of Yamuna river because the area is underlain by recent fluvial deposits. More extensive earthquake recordings, microzonation studies, research on liquefaction potential of the fluvial deposits, and further work on the estimation of expected ground motions in Delhi area are urgently needed.

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