Estimation of Arias intensity and peak ground acceleration (PGA) using probabilistic seismic hazard assessment of Uttarakhand state (India)

Uttarakhand, a state located in the seismically active western Himalayas (India), has witnessed several high magnitude earthquakes in the past. It falls under zones IV (severe) and V (very severe) as per the seismic code of India. The region has several major hydroelectric and infrastructural projects that are presently operational and in the planning stages. Therefore, it is necessary to determine the area expected to be affected by the seismic activity in this region. The seismic hazard assessment of the study area was conducted using the Cornell–McGuire approach. A comprehensive earthquake catalog was prepared considering all the earthquakes occurrences within a 300-km radius of the study area. The catalog was declustered and homogenized into a single-moment magnitude scale considering different region-specific conversion relationships. The study area was divided into ten discrete seismogenic source zones. These source zones were modeled as separate area sources. For each of these zones, seismicity parameters were evaluated using the earthquake catalog and tectonic framework. The hazard assessment at the bedrock level was carried out by combining regional and global attenuation models in a logic tree architecture. The seismic hazard at the surface level was evaluated based on suitable site amplification factors determined for different site classes using shear velocity in the top 30 m (VS30) obtained from the topographic gradient. The results were presented in terms of Arias intensity and peak ground acceleration (PGA) hazard maps for 2% and 10% probability of exceedance in 50 years. The analysis observed that 80 % of the study area is highly vulnerable to seismic hazards, including landslides/rockslides. The predicted ground motion intensity in this study will aid in the safe design of structures and the proper planning of important infrastructure projects.