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

The Main Himalayan Thrust (MHT) is a dominant feature that controls seismogenesis and earthquake hazard in the Himalaya. This study images the geometry of the MHT beneath the Garhwal-Kumaon Himalaya, with a high resolution, using P and S wave arrival times of local earthquakes. The nature of the MHT, expressed as a low-velocity layer, is found to be distinctly different in the Garhwal and Kumaon segments. It is steeper with a narrow width of ∼60 km in the former and gentler with a larger width of ∼ 85 km in the latter. The MHT in Garhwal has a 30 km wide shallow (∼2°) ramp 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 geometry is not evident in Kumaon, with the MHT remaining flat. Precise location of earthquakes using the 3D tomographic model estimated in this study reveals that 85 90 % of them occur in the upper crust, straddling within a narrow zone of 30–50 km around the MCT. Deeper seismicity in Garhwal sub-Himalaya seems associated with the deep-seated Delhi Haridwar Ridge (DHR) that cuts across the strike of Himalaya.

Schematic cartoon of the Main Himalayan Thrust in the Garhwal-Kumaon Himalaya. The ramp is clearly imaged only in the Garhwal region. The transition of MHT from Garhwal to Kumaon is marked by a slab break-off or a lateral ramp. Pink ellipse is the region where most of the local seismicity occurs in the Himalayan seismicity belt (HSB). Topography map on top shows the regional faults. MHT corresponding to the shaded box region is represented in the cartoon. LSB: Lateral Slab Break; MHT: Main Himalayan Thrust, MFT: Main Frontal Thrust, MBT: Main Boundary Thrust, MT: Munsiari Thrust, VT: Vaikrita Thrust, TT: Tons Thrust, NAT: North Almora Thrust.

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