Subsurface Shear wave velocity (Vs) mapping in Srinagar region of Kashmir Basin using microtremor data

Abstract:

In this study, we present a reconnaissance based very first high resolution 1-D Vs mapping over one of the most populated cities of Kashmir Himalayas, Srinagar city. The city is exposed to more than 2 million inhabitants and growing very fast in terms of infrastructure and urbanization. Due to its location in one of the most seismogenic zones, the city falls under high seismic hazard ans risk. The shear wave velocity (Vs) profiling for an urbanized city is one of the most critically essential requirements to characterize its subsurface sediment and thus to assess site specific seismic hazard too. To produce Vs profile, Horizontal to Vertical Spectral Ratio (HVSR) inversion approach based on guided Monte Carlo method is applied to single station microtremor data recorded at 429 locations of the city. Results from the obtained HVSR curves and their inversion reveal the presence of two well-defined impedance contrasts, existing at two different depth scales of the study region. Deeper contrasts delineate the sediments-bedrock interface at a depth ranging between 303m and 1205m in contrast to the shallow reflector with a depth ranging between 3m and 60m. We interpret the shallow sediment contrast as (238 ± 46) m/s as possible lithological separation between the unconsolidated Quaternary sediments while deeper contrasts as (1084 ± 108) m/s mark the passage from unconsolidated Quaternary sediment and consolidated Tertiary sedimentary sequence basement (hard rock) as Quaternary-Tertiary (Q-T) interface. Our extensive result also shows the efficiency of single station microtremor data in terms of bothVs profiling and site-specific earthquake hazard assessment and mitigating seismic risk in case of probable big earthquakes.