Integrated geophysical techniques for subsurface imaging of active deformation across the Himalayan Frontal Thrust in Singhauli, Kala Amb, India

摘要

Non-invasive geophysical techniques employed across the Himalayan Frontal Thrust (HFT) at Singhauli in the north-western Frontal Himalaya to understand shallow subsurface geological structures and their nature in the context of active tectonics. The combination of high-resolution Ground Penetrating Radar (GPR), Electrical Resistivity Tomography (ERT) and Multi-channel Analysis of Surface Waves (MASW) techniques with the different acquisition parameters found to be beneficial in gaining high-resolution images of shallow subsurface geological structures. The contact zone of resistivity obtained between 134 and 134 Omega m suggests the juxtaposition of dissimilar lithological units explained as two faults dipping to the northwest related to the HFT. The average separation between Fault 0 and Fault I is 12m which in general agreement with the trench excavation survey. The GPR radargram over a length of 45m show offset reflectors at a horizontal distance of similar to 12-15m close to the active deformation zone. The horizontal reflectors abruptly truncate across two northerly dipping planes. The 2D Vs section (MASW1&2) exhibited dipping attitude of the litho units (with different velocity zones) in the northeast direction which are also consistent with the local geological setting. Prominent liquefaction feature identified in the ERT section specifically with the 1 and 1.5m electrode spacing show explicit expression of its existence due to contrasting resistivity ranging between 100 and 450 Omega m. A strong correlation has been established between the field based geological observations, carried out earlier in this area through a trench excavation survey, with the present work using geophysical techniques. The integrated approach found to be highly beneficial in those areas where contrasting sub lithological units exist in terms of their physical properties. This study argues for judicious use of the ERT, GPR and MASW techniques to delineate shallow subsurface geology across various active faults in the Himalayan terrain to comprehend the active deformation related to large magnitude paleoearthquakes and earthquake hazard assessment.