Site-specific Seismic Hazard Assessment of Uttarakhand, India with special emphasis on Liquefaction Potential modelling of the terrain and Seismic Hazard Microzonation of Dehradun City

Abstract: The incidences of earthquakes in the north Indian state of Uttarkhand are broadly associated with the presence of active fault viz. Main Central Thrust and Alaknanda Fault in the north, Moradabad Fault and Himalayan Frontal Thrust in the southern margin, Martoli Thrust and Indus Suture in the eastern, Mahendragarh Dehrdun Fault in the west. Uttarakhand falls under Seismic Zone IV and V and has been struck by several devastating earthquakes viz. 1905 Kangra earthquake of M_W 7.8, 1991 Uttarkashi earthquake of M_W 6.8 and 1999 Chamoli earthquake of M_W 6.5 with maximum MM Intensity of IX observed in near-source region causing widespread damage and destruction in the study region. Uttarakhand region has undergone unprecedented development and population growth, emphasizing the importance of analysis of Seismic Hazard to ensure safe and secure progress in this seismically vulnerable region. Consideration of seismicity patterns, fault networks and similarity in the style of focal mechanisms yielded 10 areal seismogenic sources with additional active tectonic features in 0-25km, 25-70km, and 70-180km hypocentral depth ranges, along with 15 Ground Motion Prediction Equations for the tectonic provinces of Uttarakhand region yielding Probabilistic Peak Ground Acceleration (PGA) at engineering bedrock  seen to vary from 0.36g to 0.63g for 475years of return period which places the region in the moderate to high hazard zone necessitating a case study for site-specific seismic characterization of the region. Seismic site classification has been done based on an enriched geophysical, in-situ downhole, geotechnical database and surface geoscience attributes comprising of Geology, Geomorphology, Landform and Topographic Gradient derived shear wave velocity categorizes the region into Site Classes E, D4, D3, D2, D1, C4, C3, C2, C1, B and A. Using the input ground motion at bedrock level obtained from stochastic simulation for the near-source earthquakes, nonlinear site response analyses have been performed using PLAXIS-2D software package wherein site amplification has been mapped which is seen to vary in the range of 1.02 to 2.86. Surface-consistent probabilistic seismic hazard in terms of Peak Ground Acceleration (PGA) for a return period of 475 years has been assessed for the study region by convolving site amplification with bedrock hazard thus predicting a variation of PGA in the range of 0.51-1.61g. Additionally, assessment of liquefaction potential of the terrain and seismic hazard microzonation have been done for Dehradun city to identify areas with varying level of ground shaking and its associated liquefaction phenomenon during earthquakes, enabling the development of site-specific building codes and land-use regulations. The results of this investigation are expected to play vital roles in the earthquake–related disaster mitigation and management of the region.