Intensity-based seismic hazard assessment and zone map of India using SASHA approach

The peak ground acceleration (PGA)-based zone map based on Probabilistic Seismic Hazard Assessment (PSHA) faces challenges in regions like South India, the Himalayas, and Northeast India due to a lack of strong-motion data that adequately covers the full range of magnitudes and distances. In contrast, the Site Approach to Seismic Hazard Assessment (SASHA)-based computationally straightforward approach utilizes spatially abundant MMI data and offers a more practical and reliable method for generating zone maps. With widespread MMI data available across the country, this approach provides a robust first-hand hazard assessment. In this paper, an intensity-based seismic hazard map of India is developed using the SASHA approach utilizing a comprehensive dataset of recent and historic intensity catalogue. An intensity-based zone map of India with smoothed boundaries is also plotted based on this hazard map. A critical assessment of the intensity-based zone map based on the SASHA approach and the PGA-based zone map based on the PSHA approach is next presented. The assessment highlights key differences between the two approaches, revealing that while the PGA-based map better captures future hazards and the associated risk for the regions abundant in the recorded ground dataset, the intensity-based map more accurately reflects earthquake damage patterns based on past damage distribution. The study underscores the importance of combining both approaches to create a more comprehensive seismic hazard assessment, enhancing our understanding of seismic risk in the Indian subcontinent. Additionally, the paper discusses the limitations of using global intensity scales (e.g., EMS-98, MMI) in the Indian context, suggesting the need for a localized intensity scale to better account for India's unique structural and built environment conditions.