Tomographic evidence for segmentation of the Main Himalayan Thrust in the Uttarakhand Himalaya

Abstract

Imaging the Main Himalayan Thrust (MHT) along and across the strike of the Himalaya is critical to understand seismogenesis and earthquake hazard. This study investigates the 3D P and S velocity structure of the Garhwal-Kumaon Himalaya (GKH) using travel times of 846 local events recorded at 54 broadband stations. Depth sections of the velocity anomalies capture the flat-ramp-flat structure of the MHT. The MHT is expressed as a low-velocity layer, possibly due to dehydration of sediments. Also, the geometry of the MHT seems distinctly different in the Garhwal and Kumaon segments. It is steeper with a narrow width in the former and gentler with a larger width in the latter. The width of the MHT is ∼60 km in Garhwal and ∼ 85 km in Kumaon. However, if the whole MHT is assumed as a locked zone, its width from the Main Frontal Thrust to the 3.5 km elevation contour in the Higher Himalaya is ∼115 in the former and ∼ 130 km in the latter. The MHT in Garhwal has a 30 km wide shallow ramp (∼2°) in the upper flat attached to a ∼ 30° steeply dipping 10 km wide ramp, which merges with a lower flat at a depth of ∼20 km beneath the Main Central Thrust(MCT). Such a flat-ramp-flat geometry is not evident in Kumaon, with the MHT remaining flat. Also, the subsurface structures and earthquake depths beneath the MCT differ in these segments, with a varying thickness of the low-velocity layer. Relocation of earthquakes using the 3D tomographic model reveals a dominant (85–90 %) occurrence in the upper crust. The seismicity is found to straddle within a narrow zone of 30–50 km around the MCT. While the seismicity belt is mostly to the south of MCT in Kumaon, it is along and north of it in Garhwal. Deeper seismicity in Garhwal sub-Himalaya seems associated with the deep-seated Delhi Haridwar Ridge (DHR) transverse to the strike of Himalaya.