Near-surface seismic site characterization using Nakamura-based HVSR technique in the geological complex region of Kangra Valley, northwest Himalaya, India

Abstract

The two major towns, Kangra and Dharamshala, located in the northwest Himalaya have been a subject of great interest for the first-level microzonation due to their high seismic hazard potential. Tectonically, the region is geologically complex and lying between two major thrusts, i.e., the Main Boundary Thrust (MBT) to the northeast and the Jawalamukhi Thrust (JMT) to the southwest. Therefore, the valley is highly deformed and fractured and has developed several small and major faults longitudinally and transversely. The Kangra town is underlain by the Upper Siwalik Boulder Conglomerates, whereas Dharamshala town by the sandstone, claystone, and mudstone with variable thickness of soft soil cover above the bedrock. These regions faced devastating damage from the 1905 Kangra earthquake (Ms 7.8). So, the ambient noise measurement has been performed using a dense network of 200 free-field stations to measure the fundamental frequency of each site. The analysis revealed a high variation in the typologies of HVSR peaks. The iso-frequency map of the study region indicated a wide variation within and on the peripheries of the basin in a frequency ranging from 0.26 to 12 Hz. Some isolated locations in the north-eastern and north-western parts of the study area also showed a dominance of the high resonance frequency component (12-20 Hz). The study further indicated the amplification factor varying from 1.67 to 7.68, reflecting the impact of impedance contrast between the soft soil and the bedrock.