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.
Highlights
Continued convergence between India and Eurasia has produced large-to-great earthquakes along the ~2500 km long Himalayan front (Fig. 1 inset)
Our study signifies that the 1950 Tibet-Assam earthquake, the highest recorded magnitude (Mw ~ 8.6) in the Himalaya[33, 34] did break the Himalayan front
Considering a dip of 55–60° for the causative fault[15, 16], we interpret that the scarp of ~3.1 m height at Pasighat has been produced due to a co-seismic slip of ~5.5 ± 0.7 m during the 1950 earthquake (Supplementary Table S5)
Summary
The study area Pasighat is located at the hinge zone of Siang antiform[24], where the Quaternary landform comprises well preserved alluvial fans and terraces along the Siang (Brahmaputra) River and its tributaries (Fig. 2a,b,c and e). The T3 terrace is in actual a large fan surface emerging from the Siwaliks, that has been abruptly truncated and terminated at the front due to tectonic activity. The T1 and T2 terraces have been truncated by a NNE-SSW to NS striking tectonic scarp that gradually decreases in height from ~25 m in the southwestern end to ~3.1 m at the excavation site (~28.07°N, 95.33°E) of the trench (~14 m long and 4 m deep, see Figs 3 and S1a) in the northeastern end, and abruptly disappears with a ~E-W striking terrace riser (Fig. 2a and b). Whereas in the northern segment, the fault scarp striking N-S is steep due to sharp transition from terrace T1 to T0 and less anthropogenic activities
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