3-D seismic velocity structure of Kumaon-Garhwal Himalaya: Insight in to Main Himalayan Thrust and earthquake occurrence

English Abstract:

We present a detailed three-dimensional P- wave velocity (V_p) and Vp/Vs structure for the Kumaon-Garhwal Himalaya generated by inverting arrival times of 1180 local earthquakes recorded by 50 broadband stations operated during April 2005 - June 2008. The upper crust (< 20 km) show heterogeneous structure in this region. In the near surface (upper 5 km), south of Main Frontal Thrust (MFT) is characterised by low V_P (~5.2 - 5.4 km/s) and low V_P/V_S (~1.67 - 1.72) that could be impression of unconsolidated sediments. We observe that the earthquakes are localized in the regions of low V_P (~ 5.3 - 5.7 km/s) and low V_P/V_S (~1.6 - 1.72) in the upper crust (< 20 km) possibly due to the presence of quartz-rich rocks. To the north of Main Boundary Thrust (MBT) a low velocity (V_P ~5.7 km/s) layer is observed at 10-15 km depth. This could be signature of the Main Himalayan Thrust (MHT) in this region. At the depth of 18 - 25 km beneath the Main Central Thrust (MCT) zone an isolated zone of anomalous V_P ~5.7-6.0 km/s and V_P/V_S ~1.75.1.85 is observed. This is just below the maximum concentration of seismicity and coincides with the low resistivity zone reported by magneto telluric studies. This could be interpreted as possible presence of fluids due to metamorphic dehydration reaction during underthrusting of the Indian crust. The presence of fluids around and below the seismogenic zone can influence the activity of the fault system; it can affect the long-term structural and compositional evolution of the fault zone, and can alter the fault-zone strength and local stress regime. As fluids enter the active faults in the crust (such as the MCT zone), fault zone friction decrease facilitating the stress concentration in the seismogenic layer leading to mechanical failure. We, speculate that the earthquake occurrence in the Kumaon-Garhwal Himalaya is not only due to stress caused by collision tectonics, but also associated with the lithology of the region.