Primary surface rupture of the 1950 tibet-assam great earthquake along the eastern himalayan front, india
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Abstract
The pattern of strain accumulation and its release during earthquakes along the eastern Himalayan syntaxis is unclear due to its structural complexity and lack of primary surface signatures associated with large-to-great earthquakes. This led to a consensus that these earthquakes occurred on blind faults. Toward understanding this issue, palaeoseismic trenching was conducted across a ~3.1 m high fault scarp preserved along the mountain front at Pasighat (95.33°E, 28.07°N). Multi-proxy radiometric dating employed to the stratigraphic units and detrital charcoals obtained from the trench exposures provide chronological constraint on the discovered palaeoearthquake surface rupture clearly suggesting that the 15th August, 1950 Tibet-Assam earthquake (Mw ~ 8.6) did break the eastern Himalayan front producing a co-seismic slip of 5.5 ± 0.7 meters. This study corroborates the first instance in using post-bomb radiogenic isotopes to help identify an earthquake rupture.
Introduction
Continued convergence between India and Eurasia has produced large-to-great earthquakes along the ~2500 km long Himalayan front (Fig. 1 inset). As a clear elucidation of their rupture segments has remained enigmatic, recent palaeoseismic investigations have provided insight into the rupture dynamics and recurrence interval of these seismic events and the associated hazards1,2,3,4,5,6,7,8,9,10,11,12,13,14. Some dissonance between historical and palaeoseismic data exists due to lack of precise chronological constraints, and this lacunae makes it difficult to correlate surface rupture events through time. Occurrence of great devastating earthquakes like the A.D. 1934 and 1950 events were ascribed to blind faults that did not reach the surface. Contrastingly, a recent study did locate the 1934 earthquake surface rupture11. With this in view, it was quite unlikely that the 1950 earthquake (with seismic moment M o = 4.0 × 1028 Dyne-cm and moment magnitude M w = 8.6) being the largest known, did not produce any surface rupture15.