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The influence of source and epicentral distance on the local seismic response in the Kolkata city is investigated by computing the seismic ground motion along 2-D geological cross-sections in the Kolkata city for the earthquake that occurred on 12 June 1897 ( M = 8.1; focal mechanism: dip = 57°, strike = 110° and rake = 76°; focal depth = 9 km) in Shillong plateau. For the estimation of ground motion parameters, a hybrid technique is used, which is the combination of modal summation and finite difference method. This technique allows the estimation of site specific ground motion for various events located at different distances from Kolkata city, taking into account simultaneously the position and geometry of the seismic source, the mechanical properties of the propagation medium and the geotechnical properties of the site. The epicenter of the Shillong earthquake is about 460 km away from Kolkata. The estimated peak ground acceleration (PGA) varies in the range of 0.11-0.18 g and this range corresponds to the intensity of IX to X on the Mercalli-Cancani-Sieberg (MCS) scale and VIII on the Modified Mercalli (MM) scale. The maximum amplification in terms of response spectral ratio (RSR) varies from 10 to 12 in the frequency range 1.0-1.5 Hz. These amplifications occur in correspondence to low-velocity shallow, loose soil deposit. The comparison of these results with earlier ones obtained considering the Calcutta earthquake that occurred on 15 April 1964 ( M = 6.5; focal mechanism: dip = 32°, strike = 232° and rake = 56°; focal depth = 36 km) shows that the source parameters (magnitude and focal mechanism) and epicentral distance play an important role on site response but the variation in the frequency of the peak values (RSR) is negligible. The obtained results match with observed reported intensities in Kolkata region. [ABSTRACT FROM AUTHOR]

Kolkata, one of the oldest cities of India, is situated over the thick alluvium of the Bengal Basin, where it lies at the boundary of the zone III and zone IV of the seismic zonation map of India. An example of the study of site effects of the metropolitan Kolkata is presented based on theoretical modeling. Full synthetic strong motion waveforms have been computed using a hybrid method that combines the modal summation and finite difference techniques. The 1964 Calcutta earthquake, which was located at the southern part of Kolkata, is taken as the source region, with the focal mechanism parameters of dip = 32°, strike = 232° and rake = 56°. Four profiles are considered for the computation of the synthetic seismograms from which the maximum ground acceleration ( A) is obtained. Response spectra ratios (RSR) are then computed using a bedrock reference model to estimate local amplifications effects. The A varies from 0.05 to 0.17 g and the comparison of the A with the different intensity scales (MM, MSK, RF and MCS) shows that the expected intensity is in the range from VII to X (MCS) for an earthquake of magnitude 6.5 at an epicentral distance of about 100 km. This theoretical result matches with the empirical (historical and recent) intensity observations in Kolkata. The RSR, as a function of frequency, reaches the largest values (largest amplification) in the frequency range from 1.0 to 2.0 Hz. The largest site amplification is observed at the top of loose soil. [ABSTRACT FROM AUTHOR]