Shallow subsurface imaging across the Himalayan Frontal Thrust (HFT) analysing ambient noise data from a riverine source

Abstract

Modelling subsurface geology across the Himalayan Frontal Thrust (HFT) is an essential task as it plays a significant role in the transfer of stresses from infrequent major earthquakes, which influence site characterization, groundwater estimates, and environmental engineering. But it is challenging to carry out a controlled-source seismic survey across HFT due to the difficult terrain. Interestingly, the Himalayan rivers flow downhill across the HFT and generate seismic noise that can be useful in extracting information about the subsurface geology. In this study, we record the noise generated by one such river, the Dabka, near Pawalgarh in the Nainital district of Uttarakhand, India, to understand the shallow subsurface geology across this region of the HFT of the Siwalik Himalayas. We use a network of 23 geophones of 5 Hz natural frequency, deployed for 46 days, to record the noise datasets. We cross-correlate these time series for the station pairs and extract the noise correlation functions (NCFs). Dispersion analysis of the NCFs provides the Rayleigh wave group velocity dispersion curves for the station pairs, using which we created 2D Rayleigh wave group velocity and shear wave velocity maps at different depths. These velocity-depth models demarcate the location of the HFT where there is a jump in the Rayleigh and shear wave velocities, with average standard deviations of ∼50 m/s and ∼ 93 m/s, respectively, across the HFT. The obtained shallow subsurface velocity models provide crucial information for geotechnical assessment and site characterization of this earthquake-prone region and thus can be useful for future hazard assessment.