Subsurface Site Characterization of Donga Fan, Northwest Himalaya using Multichannel Analysis of Surface Waves and Response Analysis

The characterization of sediments in a tectonically complex region is important from the seismological point of view to study possible earthquake effects due to the presence of soft sediments. Multichannel analysis of surface waves (MASW) method was used to acquire seismic data from 87 sites for estimating shear wave velocity (Vs) of near-surface materials beneath the Donga Fan. The majority of the Donga Fan is underlain either by alluvial fan or river terrace deposit. About 80% of the Donga Fan has an average shear wave velocity ranging from 180 to 360 m/s, whereas 20% of the area has high stiffness values (Vs ú 360 m/s). The estimated Vs values are higher in the northern part of the study area due to thin sediment (150 m) above bedrock. The response analysis suggests that peak spectral acceleration varies from 0.49 g at 1.61 Hz to 1.69 g at 3.22 Hz with variation in amplification ratio from 4 to 11 times. The spectral acceleration computed for two-storey buildings also varies from 0.10 to 0.40 g (at 5% damping), whereas in the case of singlestorey buildings it varies from 0.12 to 0.22 g (at 5% damping). The predominant frequency estimated using ambient noise measurements varies from 0.84 to 5 Hz, indicating variation in the thickness of sediments and is in good agreement with the Vs values estimated using the MASW technique.