Surface-consistent Probabilistic Seismic Hazard, Microzonation and Damage Potential Modelling for the City of Nashik, Maharashtra, India

Abstract

The Koyna-Warna seismic region located in western Indian state of Maharashtra, encompassing the south-western part of the Deccan Volcanic Province, reveals prolific seismicity attributed to both intraplate tectonism and reservoir-triggered activities. The 1967 Koyna earthquake of MW 6.3 marked the largest reported instance of Reservoir-Induced Seismicity, resulting in the destruction of the town of Koynanagar; subsequently, the 1993 Killari earthquake of MW 6.2 claimed thousands of lives with enormous structural damages in Latur district with maximum intensities of IX and VIII respectively. This, therefore, underscores the imperative need for seismic hazard assessment to enhance earthquake disaster preparedness and risk mitigation. Both areal and tectonic sources in two hypocentral depth ranges of 0-25 km and 25-70 km along with 15 Ground Motion Prediction Equations including 6 Site-specific Next Generation Spectral Attenuation models pertaining to Koyna-Warna, Kutch and Central India seismogenic sources have been incorporated to deliver Probabilistic Peak Ground Acceleration (PGA) of Koyna-Warna region at firm rock condition varying from 0.05-0.48g for 10% probability of exceedance in 50 years. Extensive Geotechnical and Geophysical investigations combined with Topographic Gradient data in high-altitude areas have provided the effective shear wave velocity distribution, classifying the region into ten site classes viz. E/F (≤180m/s), D4 (180-240m/s), D3 (240-280m/s), D2 (280-320m/s), D1 (320-360m/s), C4 (360-440m/s), C3 (440-520m/s), C2 (520-620m/s), C1 (620-760m/s) and B (760-1500m/s), leading to a detailed seismic hazard modelling of the ancient holy city of Nashik, the fourth largest city in the state of Maharashtra. 2D nonlinear site response analysis using PLAXIS 2D has been performed for the city, which amplified the bedrock PGA by a factor ranging from 1.75 to 3.18 times, thus generating surface-consistent hazard in the range of 0.14-0.25g. The estimated PGA has further been used for the holistic microzonation integrating multiple geo-hazard themes viz. Surface-consistent Probabilistic PGA, Liquefaction Potential Index (LPI), Site Class, Geomorphology and Geology which categorized the city into five zones based on Seismic Hazard Index (SHI) namely ‘low (0.00<SHI≤0.20)’, ‘moderate (0.20<SHI≤0.40)’, ‘high (0.40<SHI≤0.60)’, ‘very high (0.60<SHI≤0.80)’ and ‘severe (0.80<SHI≤1.00)’. Structural damage potential modelling through “Capacity Spectrum Method”-based SELENA considering ten model building types has yielded Damage Probability in terms of five damage states which predicted that the majority of Unreinforced Masonry type buildings (URML) will experience ‘complete’ damage and others (A1, RS2, C1L, C1M, C1H, C3L, C3M, C3H and HER) will sustain ‘slight to moderate’ damage levels when exposed to the estimated surface-consistent probabilistic seismic hazard scenario of the city. Human casualties has also been speculated for three distinct periods of a day viz. “Night”, “Day” and “Commuting” times thereof. This model is believed to contribute significantly to the seismic-resilient urbanization process by providing precise disaster management and mitigation recommendations for the city of Nashik.