Modelling of Strong Ground Motions from 1991 Uttarkashi,India, Earthquake Using a Hybrid Technique |
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| Authors: | Dinesh Kumar S S Teotia V Sriram |
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| Institution: | (1) Department of Geophysics, Kurukshetra University, Kurukshetra, 136 119, India;(2) Wadia Institute of Himalayan Geology, Dehradun, 248 001, India |
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| Abstract: | We present a simple and efficient hybrid technique for simulating earthquake strong ground motion. This procedure is the combination
of the techniques of envelope function (Midorikawa
et al. Tectonophysics 218:287–295, 1993) and composite source model (Zeng
et al. Geophys Res Lett 21:725–728, 1994). The first step of the technique is based on the construction of the envelope function of the large earthquake by superposition
of envelope functions for smaller earthquakes. The smaller earthquakes (sub-events) of varying sizes are distributed randomly,
instead of uniform distribution of same size sub-events, on the fault plane. The accelerogram of large event is then obtained
by combining the envelope function with a band-limited white noise. The low-cut frequency of the band-limited white noise
is chosen to correspond to the corner frequency for the target earthquake magnitude and the high-cut to the Boore’s f
max or a desired frequency for the simulation. Below the low-cut frequency, the fall-off slope is 2 in accordance with the ω2 earthquake source model. The technique requires the parameters such as fault area, orientation of the fault, hypocenter,
size of the sub-events, stress drop, rupture velocity, duration, source–site distance and attenuation parameter. The fidelity
of the technique has been demonstrated by successful modeling of the 1991 Uttarkashi, Himalaya earthquake (Ms 7). The acceptable
locations of the sub-events on the fault plane have been determined using a genetic algorithm. The main characteristics of
the simulated accelerograms, comprised of the duration of strong ground shaking, peak ground acceleration and Fourier and
response spectra, are, in general, in good agreement with those observed at most of the sites. At some of the sites the simulated
accelerograms differ from observed ones by a factor of 2–3. The local site geology and topography may cause such a difference,
as these effects have not been considered in the present technique. The advantage of the technique lies in the fact that detailed
parameters such as velocity-Q structures and empirical Green’s functions are not required or the records of the actual time
history from the past earthquakes are not available. This method may find its application in preparing a wide range of scenarios
based on simulation. This provides information that is complementary to the information available in probabilistic hazard
maps. |
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