Site Characterization of Mohand Region along Himalayan Frontal Thrust (HFT) using passive HVSR method

  Bhopya K., Ghosal D., Verma S., Srivastav H., Verma S., Vikas

 The ongoing collision between the Indian and Eurasian plates has been accumulating strain along the Main Himalayan Thrust (MHT) causing the Sub-Himalaya highly vulnerable to Earthquakes. To examine seismic vulnerability in a hazard-prone area,  a complete understanding on its lithological properties like resonance frequency, shear wave velocity, bedrock depth, and thickness of overlying soft sediments is essential. As the study area spans from Madarsa Darul Quran to Mohand along the Himalayan Frontal Thrust (HFT) and lies in an earthquake-prone area, we record seismic ambient noise over twelve measuring points using a three-component portable seismograph (Tromino) with the natural frequency of 0.1 Hz. We carry out the HVSR (horizontal to the vertical spectral ratio) study on the recorded data using the Nakamura Method which is a technique for estimating the resonance frequency and site amplification caused by different stratigraphic units underlain by the top of the bedrock. The variable resonance frequency has been identified in this region in the range of 0.42 to 4.8 Hz, which indicates this region is prone to site amplification as overlain by Doon fan deposits. We invert the P-velocity (Vp), S-velocity (Vs), and density (ρ) by using Monte Carlo Inversion Method and identified three different stratigraphic units in this region. The top has a thickness of 3 meters with a mean Vs, Vp, and ρ as 218 m/s, 385 m/s, and, respectively. The second layer has a thickness of 6 meters with a mean Vs, Vp, and ρ of 406 m/s, 725 m/s, and 1.7 g/cm³, respectively. The bedrock depth in this region is 127 meters with a mean Vs, Vp, and ρ of 582 m/s, 1238 m/s, and 1.8 g/cm³ , respectively. This study will further help in response analysis up to a depth of bedrock.