Crustal structure across Himalayan Frontal Thrust (HFT) using high-resolution seismic datasets

The Himalayan fold-thrust belt has been developing due to the northward convergence of the Indian plate against the Eurasian plate since ~55 Ma. Three major thrust systems: Main Central Thrust (MCT), Main Boundary Thrust (MBT), and Himalayan Frontal Thrust (HFT) are distinctly observed in the Himalayan orogeny from north to south indicating southward propagation of active deformation. These active thrust systems produced several devastating earthquakes in the past such as 1905 Kangra (Mw 7.8), 1934 Nepal-Bihar (Mw 8), and 1950 Assam (Mw 8.6) earthquakes. Presently HFT is found to be the tectonically very active zone that accommodates a strain rate of ~10-15 mm/year and is a zone for great threats in near future to the societies residing over the Himalayan foothills. The present study carried out in the lower Siwalik Himalaya near Pawalgarh in Nainital District of Uttarakhand, India with an objective to estimate the velocity model across HFT in the locality. To accomplish the objective, seismic data were acquired along three profiles of a cumulative length of ~13 km using a seismic thumper as a source and 96 vertical component geophones with the natural frequency of 5 Hz and Remote Acquisition Unites (RAUs) as sensors and data loggers, respectively, and with a group and shot interval of 20 m and near offset of 100 m. Highly uneven Himalayan terrain causes large static errors. In order to overcome this challenge, we used Real Time Kinematics (RTK) to estimate more precise source and receiver surface elevation. In the pre-processing phase of acquired seismic data, three different shots taken at the same location are vertically stacked to eliminate random non-coherent noises and improve the SNR of the data. We then applied a low-frequency array filter (LFAF) to suppress the ground roll using velocity estimates from the ambient noise tomography (ANT). We process the data by implementing conventional seismic processing techniques including normal move-out (NMO) correction, velocity analysis followed by stacking. In the stack section, we observe a northward dipping reflector extending from the surface to ~ 1- 1.25 s TWT indicating evidence of HFT. Another reflector observed at ~3-4 s TWT demarcating the extent of overlying sedimentary deposits on the top of the under-thrusting lithosphere. Rocks of the Siwalik Himalaya mainly composed of sedimentary deposits of sandstone mudstone, and alluvial deposits. Average velocity obtained from the refraction tomography ~ 2900 m/s matches well with rock type in the region. Thus, the high-resolution crustal structure across the highly active HFT can be crucial to understand the earthquake mechanism in the locality and for a better hazard assessment.