| Popis: |
The present study looks into the significant role of metamorphic and aqueous fluids in the mid-crustal Main Himalayan Thrust (MHT) in triggering strong to moderate earthquakes in the Kumaon-Garhwal Himalayas. Using seismic velocity tomograms, crack density (ε) and saturation rates (ξ) have been modelled using the crack theory. All three earthquakes in Uttarakhand with Mw ≥ 6.5 (viz., 1803 Mw7.8 Garhwal, 1991 Mw6.8 Uttrakashi and 1999 Mw6.5 Chamoli earthquakes) occurred within the MHT, which is depicted as a shallow north-dipping layer with low velocity. Our models show that these earthquakes occurred between 10 and 20 km depths where ε (∼0.011–0.012), ξ (∼0.386–0.392), and Vp/Vs (∼1.74–1.90) are all relatively high, indicating a greater possibility of aqueous/metamorphic fluids within the MHT than in the surrounding area. These fluids are suspected to be the trigger for earthquakes in the Uttarakhand Himalaya. As we drill deeper into the crust, we find that ‘ε’ decreases as a result of crack closure (≤0.011), while ‘ξ’ is predicted to increase (∼0.3–0.6) as a result of the presence of more wet and saturated cracks caused by the presence of partial melts (higher Vp/Vs: 1.74–1.92). Our modelling further verifies that the earthquakes in the Uttarakhand Himalaya are primarily driven by the existence of a highly conductive and weakened MHT with a greater concentration of metamorphic/aqueous fluid-filled cracks at depths ranging from 10 to 20 km. The ramifications of our research could significantly impact the earthquake hazard and risk in the Uttarakhand Himalayas. |
