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The seismic hazard estimation for Northern Indian region has been attempted by many researchers in the past using Probabilistic Seismic Hazard Assessment (PSHA) methodology to estimate the hazard parameters and various seismic hazard maps have been proposed (Khattri, 1992; Sharma et al., 2003; Iyengar & Ghosh, 2004; Agrawal & Chawla, 2006; Mahajan et al., 2009). The varying assumptions and use of different strong motion prediction relations have resulted in vast differences in the final products. For calculating the PGA value Sharma et al. (2003) used the attenuation model by Abrahamson and Lithehiser (1989) and computed the value of PGA for 20% exceedance value in 50 years considering all the zones as 0.34g. Khattri (1992) (using velocity–density-Q model) and Iyengar and Ghosh (2004) (using Sharma, 1998 attenuation model with added new data) reported PGA value at the centre point of Delhi of the order of 0.2g. Agrawal and Chawla (2006) have used two International attenuation models viz. Abrahamson and Lithehiser (1989) and Joyner and Boore (1981), and two National attenuation model Sharma (1998) and Iyengar and Ghosh (2004) and evaluated PGA value to be 0.17g. Although the studies differ a lot in the end results, none of the studies have reported and quantified the uncertainties. It is in this context important to estimate the uncertainties in the results which in turn help the end user to put confidence levels for their use.
The seismic hazard assessment is generally carried out using deterministic approach or probabilistic approach. Although, the approaches like Extreme value statistics, use of distributions like Gamma, Weibul or lognormal etc, or sometimes when we are unable to fit in the distribution new methodologies like ANN may be used for estimation of seismic hazard (Sharma & Arora, 2005), probabilistic seismic hazard assessment has its own advantages over the others (Joshi, 2009). While Deterministic Seismic Hazard Assessment (DSHA) provides a scenario like results, Probabilistic Seismic Hazard Assessment (PSHA) yields a framework in which the uncertainties in the size, location and rate of recurrence of earthquakes and in the alteration of ground motion characteristics with earthquake size and location could be explicitly taken into account. The information needed to develop a PSHA or DSHA is essentially the same for both, except the concept of ‘recurrence relationships’, their evaluation and/or their process before providing them as ‘inputs’ to the scientists and/or engineers differs. In this paper an attempted has been made to estimate seismic hazard at bed rock level in terms of peak ground acceleration (PGA) and its uncertainty in terms of coefficient of variations (COV) using probabilistic seismic hazard Assessment (PSHA).
The study area for the estimation of strong ground motion and its uncertainty has been chosen as one of the important areas from the point of view of its development and current seismicity and the various hazard assessment studies being carried out without reporting the uncertainties in usable form, viz, northern part of India adjacent to the Himalayas. The Himalayas are a result of Continent-Continent Collision which dates back to the early Tertiary, or about 60 million years ago. After the initial collision, the convergence rates have slowed down and are presently driving the Indian subcontinent northward into Eurasia (Press & Siever, 1982). The present convergence rate is estimated to ~20 mm/ year between India and southern Tibet (Jackson & Bilham, 1994), which is higher than the 10-15 mm/year assumed earlier. The Himalayas are created through this collision and under-plating of the Indian subcontinent under the Asian plate combined with a slight counter-clockwise rotation of India which has experienced many disastrous earthquakes during historical times.