Seismic Imaging of the Kumaon-Garhwal Himalaya: Insights into the Main Himalayan Thrust and the Crust-Mantle Boundary

Abstract

Abstract The seismic velocity tomography of reveals a high-velocity (5–20% increase in dVp, 4–16% increase in dVs, low Vp/Vs (~ 1.5–1.8)) upper crust between 0 and 20 km depth and a low-velocity (10–20% drop in dVp, 4–12% drop in dVs, high Vp/Vs (~ 1.65-2.0)) lower crust between 20 and 40 km depth. Our modelling detects a low-angle north-dipping low-velocity layer (10–20% drop in Vp and Vs, 10–15% increase in Vp/Vs) at depths of 8–20 km, which may be the Main Himalayan Thrust (MHT). The presence of low-velocity pockets in the lower crust may be indicative of partial melts, while those in the upper and middle crust may be indicative of aqueous or metamorphic fluids. The Common Conversion Point stacking (CCP) of radial P- Receiver Functions (PRFs) detects a double Moho structure below the rupture zone of the 1803 Mw7.8 Garhwal, the 1991 Mw6.8 Uttarkashi, and the 1999 Mw6.5 Chamoli earthquakes, which might be accumulating marked crustal stresses on the MHT. Our modelling also suggests that fluids or high pore-fluid pressure caused the above-mentioned three Mw≥6.5 earthquakes, which occurred in low-velocity zones on the MHT. Our CCP imaging also suggests a north-dipping crust-mantle boundary (with larger positive impedance contrast) at depths of 30–55 km while these CCP images also detect a north-dipping zone with larger negative impedance contrast at depths of 8–20 km, possibly representing the low-velocity MHT. Our joint inversion of radial PRFs and group velocity dispersion data of Rayleigh waves has also mapped an MHT and Moho that dip to the north.