Study of ambient noise for seismic-microzonation of the Eastern Part of Hetauda Dun Valley, Central Nepal

The experimental study of the horizontal-to-vertical spectral ratio utilizing ambient noise is an effective technique for determining the seismic microzonation of an area. This study performed seismic microzonation using the resonance frequency and estimated the amplification factor and seismic vulnerability index for the eastern half of the Hetauda Dun Valley. Analysis of the horizontal-to-vertical spectral ratio of the ambient noise revealed that the resonance frequency varies from 0.65 to 4.64 Hz. The peripheral sites of the valley exhibit a higher resonance frequency, which gradually decreases towards the valley center. The low resonance frequency in the valley center indicates the possibility of long-period shaking during overdue earthquake events. A northwest-southeast (NW-SE) trending low-resonance frequency zone was observed through Sangam Chok, Chisapani, north of Hatiya, Harnamadi, and west of Phurkechaur, indicating a depression in the basement rock topography. The amplification factor in the valley ranges from 1.96 to 11.65. Higher amplification factors are associated with areas containing fine Quaternary and floodplain deposits of Karra Khola. The seismic vulnerability index is higher in the area characterized by fine and unconsolidated Quaternary deposits, particularly in the northern and core areas of Hetauda city. The estimated resonance frequencies show the risk of resonance of the buildings at lower frequency at the center and at higher frequency at the periphery of the valley. Overall, the results clearly indicate that the rapidly urbanizing Hetauda Dun Valley is prone to localized seismic effects. The obtained results are useful for site-specific seismic hazard assessment, building code implementation, risk-sensitive land-use planning, and urban earthquake risk assessment and management.