Seismic Site Characterization of the Ganderbal District, Kashmir Valley

Seismic site characterization is the process of categorizing a site based on the dynamic properties of the soil deposit at the site and is vital for understanding site-specific seismic behaviour as well as mitigating earthquake hazards. The current study focuses on the Ganderbal district in the seismically active Kashmir Himalayas, employing Multichannel Analysis of Surface Waves (MASW) and Microtremor Horizontal-to-Vertical Spectral Ratio (MHVSR) techniques to determine essential dynamic soil parameters viz., time-averaged shear wave velocity (V_s30) and peak HVSR frequencies respectively. The geophysical tests were performed at about 35 sites in the main town area of the district, covering major landforms and geological deposits. The results facilitated the determination of seismic site classes at the testing locations using the methodology established by Zahoor et al. (2023) for the Kashmir Valley. This classification system, adapted from Di Alessandro et al. (2012), incorporates peak H/V amplitudes and frequencies, the HVSR curve shape, and V_s30 as proxies for site amplification. Field experimental data, combined with topographical and geological information, identified four distinct zones in the study area showing distinct site response namely, Zone A, characterized by alluvial deposits from the Sind and Jhelum rivers; Zone B, consisting of the Karewa highlands; Zone C, comprising marshy lands; and Zone D, representing hilly terrains. V_s30 estimates from MASW testing revealed varying stiffness in the zones, with average values of ~210 m/s in Zone A, ~400 m/s in Zone B, ~100 m/s in Zone C, and ~516 m/s in Zone D. H/V amplitude as high as 6.0-15.0 at frequencies of 1.0-5.0 Hz were obtained in Zone A, indicating significant impedance contrast within the deposit or trapping of seismic waves. Zone B showed peaks with H/V amplitude 2.0-3.0 at frequencies < 1 Hz indicating deep sedimentary depth, along with secondary peaks at higher frequencies signifying a multi-layered subsurface. Zone C on the other hand exhibited clear peaks in the range of 1.0-3.0 Hz with H/V amplitude of 6.0-11.0. and smaller peaks at higher frequencies (>10 Hz). In Zone D, broadband peaks in HVSR curves were attained, implying complexity of subsurface conditions, probably due to lateral variations or sloping underground layers. Using the computed values of these amplification proxies, seismic site characterisation for the study area was conducted. The results align closely with the geology and topography of the area and demonstrate a clear connection to factors such as proximity to rivers. This study offers insights into the seismic behavior of soils in the Ganderbal district, aiming to support seismic microzonation and risk assessment efforts in the region. The results will contribute to the understanding of local site effects in the region, such as ground motion amplification and the potential for seismic hazards like liquefaction and landslides. Given the critical seismotectonic setting of the Himalayas, the findings are crucial for informing town planning and enhancing disaster risk reduction initiatives in the area.