Site characterisation in Kangra Valley (NW Himalaya, India) by inversion of H/V spectral ratio from ambient noise measurements and its validation by multichannel analysis of surface waves technique
A.K. Mahajan and P. Kumar
Journal name: Near Surface Geophysics
Issue: Vol 16, No 3, June 2018 pp. 314 - 327
Special topic: Urban Geophysics: New Developments and Research
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The ambient noise measurements were performed at 200 sites in and around the upcoming urban centre of Kangra Valley to derive the predominant frequency of soil. The shear-wave velocity of the soft soil cover is obtained by joint-fit inversion modelling of the horizontal-to-vertical spectral ratio and the dispersion curves derived from multichannel simulation with one receiver survey. Simultaneously, shear-wave velocity investigations were also performed employing an active 24-channel engineering seismograph using multichannel analysis of surface waves. Finally, the derived one-dimensional shear-wave velocity profiles were compared between these two different approaches, which were found to be in good agreement. The shear-wave velocity investigations of the study area have indicated that the majority of the sites either fall in soil class D (Vs = 180–360 m/s, stiff soil) or class C (Vs = 360–760 m/s, very stiff soil) as per NEHRP classification. The microtremor data also suggest high fundamental frequency (4 to >20 Hz) within and on the fringes of the basin, thus covering 80% of the study area, which is in agreement with the known shear-wave velocity variation in the Kangra Valley. The large variation in high frequencies cannot be attributed to the presence of thick loose alluvial sediments (gravels and sand) but can be related to the presence of moraine deposits or bedrock (upper conglomerates) underneath the basin. However, a few isolated locations in the northern and southeastern parts of the basin are characterised by a low predominant frequency (2–3 Hz) or frequency less than even 2 Hz. The results further suggest that both multichannel analysis of surface waves and horizontal-to-vertical spectral ratio methods, in combination with multichannel simulation with one receiver, are complementary to each other and are suitable for estimating the shear-wave velocity structure for hilly urban regions, where exploring a large area is a major challenge. The analysis further reveals that Kangra Valley can have a major devastation from a near-source earthquake rather than from a far-source event.