S T G Raghukanth

In this article, the spatial variation of ground motion in Imphal City has been estimated by the finite-fault seismological model coupled with site response analysis. The important seismic sources around Imphal City have been identified from the fault map and past seismicity data. The rock level acceleration time histories at Imphal City for the 1869 Cachar (Mw 7.5) earthquake and a hypothetical Mw 8.1 event in the Indo-Burma subduction zone have been estimated by a stochastic finite-fault model. Soil investigation data of 122 boreholes have been collected from several construction projects in Imphal City. Site response analysis has been carried out and the surface level ground motion has been determined for Imphal City for these two earthquake events. The results are presented in the form of peak ground acceleration (PGA) contour map. From the present study it has been ascertained that the maximum amplification for PGA over Imphal City is as high as 2.5. The obtained contour maps can serve as guidelines for identifying vulnerable areas and disaster mitigation in Imphal City. © Birkhäuser Verlag, Basel, 2009.

In the present study, strong motions are estimated at 17 stations in Southern Peninsular India (SPI) for the 7 February 1900 Coimbatore earthquake (Mw 6) using the empirical Green's function (EGF) method. The broadband recordings of three small earthquakes of ML 3.5, 2.9 and 3.0 respectively, are taken as EGFs to simulate ground motion. The slip distribution of the main event is considered as a von Karman random field. The stress drops of the three small events estimated from finite fault stochastic seismological model lie between 130 and 140 bars. The peak ground acceleration (PGA) values, an ensemble of acceleration time histories and response spectra, are estimated at all the 17 stations using corresponding EGFs, and the mean response spectra are reported. Another estimate of PGA is also obtained using the stochastic seismological model. The estimated PGA values from the two methods are compared to check the consistency of the results. It is observed that the mean PGA values are within the bounds of the maximum and minimum PGA values obtained from the EGF method, while the differences at some stations can be attributed to the local site conditions. The ground motions simulated in the present study can be used to perform nonlinear dynamic analysis for future and existing structures in the SPI region for any event of magnitude Mw 6.

In this article, a study on development of ground motion prediction equations (GMPEs) is undertaken for seismically active regions in India. To derive the equations, the seismically active regions are divided into four units based on seismotectonic setting and geology. Due to lack of strong motion data, a stochastic finite-fault simulation method is used for generating a complete synthetic database with respect to magnitude and distance. The input parameters in the stochastic seismological model, such as site amplification and stress drop, are first derived from the past strong-motion data. A total of 236 three-component records from 62 earthquakes with magnitudes ranging from Mw 3.4 to 7.8 are used to calibrate the seismological model. The obtained stress drops of these 62 events lie in between 60 and 165 bars. With the help of a large synthetic database generated from the calibrated seismological model, ground motion relations for 5 % damped spectral acceleration are obtained by regression analysis. The developed ground motion relations are compared with the existing GMPEs of the other active regions in the world. Although the proposed equations have trends similar to those of the existing relations, there are some differences attributed to stress drop and the quality factor of active regions in India. These relations will be useful to prepare spectral acceleration hazard maps of India for a given annual probability of exceedance.

In this paper, the seismic susceptibility of Imphal city with respect to ten synthetically generated samples of the historic 1869 Cachar (Mw 7.5) earthquake that occurred in the Kopili fault is presented based on the finite-fault seismological model in conjunction with nonlinear site response analyses. For all the synthetic sample earthquake events, the mean and standard deviation of surface level spectral ground acceleration at peak ground acceleration (PGA) and natural periods of 0.3 and 1 s have been reported in the form of contour maps. These contour maps can serve as guidelines for engineers and planners to identify vulnerable areas for possible seismic disaster mitigation of Imphal city.

In this paper, ground motion during six past devastating earthquakes and one possible future event in the northeastern part of India is estimated by seismological approaches. Considering uncertainty in the input source parameters, a series of ground motions have been simulated. The peak ground acceleration (PGA) and response spectra at important cities and towns in the epicentral regions of these events are obtained. The PGA distribution over the entire northeastern region of India, encompassing the epicenter, is presented in the form of contours. The obtained results can be used for the seismic analysis and design of structures in this region. © 2009 Springer Science+Business Media B.V.