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Instituto de Geofísica, U.N.A.M.
Ciudad Universitaria
04510 México, DF, Mexico
krishna{at}ollin.igeofcu.unam.mx
(S.K.S.)
Department of Civil Engineering
Indian Institute of Technology
Hauz Khas, New Delhi 110016, India
(W.K.M.)
Department of Science and Technology
Government of India
New Mehrauli Road
New Delhi, 110016, India
(B.K.B.)
Centre for Geosources
University of Delhi, South Campus
Benito Juarez Road
New Delhi 110021, India
(G.S.R.)
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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