Revisiting the 1991 Uttarkashi and the 1999 Chamoli, India, earthquakes: Implications of rupture mechanisms in the central Himalaya. (15th August 2018) - Rajendran, Kusala
Graphical abstract: Highlights: The paper presents moment inversion source models for the 1991 Uttarkashi and the 1999 Chamoli earthquakes. Existing source models are based on geodetic data and limited strong-motion networks. The paper explores the role of non-MHT faults in Himalayan seismicity and impending hazard. Abstract: The central Himalaya has attracted much attention due to its potential to host a great earthquake – an inference based on the history of earthquakes and the geodetic slip estimates. The largest and earliest historically documented earthquake in the central Himalaya occurred in 1803 (M ∼ 7.7) near Uttarkashi on its western part, much before the great event of 1934 (Mw 8.2, located farther to the east) - both of which were felt severely in the Gangetic Plain south of the mountain front. The seismotectonic model by Seeber and Armbruster (1981) predicts that large/great earthquakes originate on the Main Himalayan Thrust (MHT) and their ruptures propagate southward to the Main Frontal Thrust (MFT). The rupture of the April 25, 2015 (Mw 7.8) Gorkha (Nepal) earthquake, however, was southeast-directed with a southwest-directed slip, and its Mw 7.3 aftershock on May 12 also showed a similar slip direction. Post-2015, we revisited the 1991 Uttarkashi (Mw 6.7) and the 1999 Chamoli (Mw 6.5) earthquakes in India, both of which had originated on the MHT flat, but show SSW-directed slips, as reported in this study. Ruptures of both these earthquakes fell short of reaching the MFT, just as the historically documented 1803 earthquake, but their shaking effects were felt in the southern plains. These observations compel us to conclude that the varying rupture modes exhibited by the Himalayan earthquakes are controlled primarily by the geometry of the flat-ramp-flat system.