Variation in Seismic Response of Basin Sediments in the Kathmandu Valley

Abstract

The Kathmandu Basin, located within the Lesser Himalaya, consists of Plio-Pleistocene fluvio-lacustrine sediments overlying Precambrian-Paleozoic bedrock of the Kathmandu Complex. Its complex basement topography, variable sediment thickness, and contrasting geological conditions significantly influence seismic ground response. The basin contains loose sand and gravel along its margins, while silt and clay dominate the central areas. Sediment thickness reaches approximately 650 m, creating considerable variations in subsurface conditions. Historical earthquakes in 1934, 1989, and 2015 demonstrated differences in structural damage associated with local ground conditions and basin effects. Soft and unconsolidated sediments can amplify seismic waves, producing stronger ground shaking than nearby hard rock sites. This study examines variations in seismic response through subsurface velocity characterization and ground motion analysis. Borehole information and earthquake records from strong-motion stations installed on rock and sediment sites were used to develop shear wave velocity profiles. Modelling of an Mw 7.3 earthquake demonstrated differences in sediment response. Additionally, microtremor array measurements conducted at over 200 stations provided estimates of Vs30 and predominant frequency. The findings emphasize the importance of understanding subsurface geological variations and seismic amplification for assessing earthquake effects across the Kathmandu Basin and supporting infrastructure planning.