Research Roundup
Seismic evidence for the Main Himalayan Thrust and the crust-mantle boundary in the Kumaon-Garhwal Himalaya.
Mandal P, Raghavan R, Saha S, Prathigadapa R.
JOURNAL OF EARTH SYSTEM SCIENCE
The study shows that the Himalayan crust in Uttarakhand is not uniform. There is a shallow, high-velocity, stiff upper crust on top of a low-velocity, fluid-rich lower crust, and a complicated, twofold Moho beneath parts of the Main Himalayan Thrust (MHT).Local earthquake tomography shows that the upper crust (0-5 km) has a 10-20% increase in velocity and a low Vp/Vs (1.5-1.7), which is what you'd expect from compact felsic to metamorphic rocks. The lower crust (20-40 km) has a 15-30% decrease in velocity and a higher Vp/Vs (≈1.8–2.0), which means that it is fractured and hydrated with partial melts or high pore fluid pressure. The MHT is a moderately north-dipping, mid-crustal low-velocity zone (8-20 km) with a higher Vp/Vs.
(a) Map and tomographic sections of the Uttarakhand Himalaya showing broadband stations, relocated earthquakes, major thrusts, and profiles (b-d) B1B2 and (e-g) C1C2 with dVp, dVs and Vp/Vs structures, highlighting hypocenters of the 1803 Garhwal, 1991 Uttarkashi and 1999 Chamoli earthquakes, and interpreted thrust geometries from Srivastava and Mitra (1994) projected along the profiles.
