ESTIMATION OF SITE-SPECIFIC DESIGN PARAMETERS BASED ON SEISMIC HAZARD ANALYSIS FOR KOYNA DAM

Abstract

In this research, a comprehensive evaluation of the seismic threat was conducted for the Koyna Dam situated in Maharashtra, India. Both the Probabilistic Seismic Hazard Analysis (PSHA) and Deterministic Seismic Hazard Analysis (DSHA) methodologies were employed. The Koyna region in western India is renowned for its seismic activity and has a history of noticeable seismic events, especially the earthquake of Koyna region in 1967 marked 6.3 in Richter Scale. According to the seismic code of India, it falls under zone IV, which signifies a severe hazard level. The presence of a major fault beneath the Koyna Dam increases the region's vulnerability to earthquakes. As part of the Probabilistic Seismic Hazard Assessment (PSHA), an extensive earthquake catalog was compiled, including all seismic events occurring within a 300km radius of the study area. To ensure data completeness, the earthquake data were analyzed statically, considering spatial and temporal declustering to remove foreshocks and aftershocks. Based on the various seismic parameters such as fault rupture mechanisms, faults systems and seismic events the area is divided into four seismic source zones. The relevant parameters 'a' and 'b' were computed with the aid of the Gutenberg-Richter relationship. The determination of the PGA and PSA are very important for this study for which the study region is divided into 0.5°×0.5°size grids for various time periods. By In the DSHA, a scenario earthquake representing the 1967 Koyna earthquake with a magnitude of 6.3 was considered. The worst-case ground motion was determined, and PGA and SA values were computed using OpenQuake software. These values were then compared with the PSHA results to evaluate the disparities in hazard estimation between the two approaches. The study provides comprehensive results, including PGA, hazard curves, and hazard spectra, with probabilities of exceedance of 2% and 10% over a fifty-year period. The results obtained from the Probabilistic Seismic Hazard Assessment (PSHA) revealed substantial spatial variations in seismic hazard within the Koyna region, which contradicted the uniform hazard values proposed by the Indian standard seismic code. The hazard maps developed in this research showcased enhancements compared to previous probabilistic hazard maps, offering a more precise depiction of the seismic threat in the area.