It is unclear where plate boundary thrusts generate giant rather than great earthquakes. Along the Himalayas, the source sizes and recurrence times of large seismic events are particularly uncertain, since no surface signatures were found for those that shook the range in the twentieth century. Here we challenge the consensus that these events remained blind and did not rupture the surface. We use geomorphological mapping of fluvial deposits, palaeo-seismological logging of river-cut cliffs and trench walls, and modelling of calibrated 14C ages, to show that the Mw 8.2 Bihar–Nepal earthquake on 15 January 1934 did break the surface: traces of the rupture are clear along at least 150 km of the Main Frontal Thrust fault in Nepal, between 85° 50′ and 87° 20′ E. Furthermore, we date collapse wedges in the Sir Valley and find that the 7 June AD 1255 earthquake, an event that devastated Kathmandu and mortally wounded the Nepalese King Abhaya Malla, also ruptured the surface along this stretch of the mega-thrust. Thus, in the past 1,000 years, two great earthquakes, 679 years apart, rather than one giant eleventh-century AD event, contributed to the frontal uplift of young river terraces in eastern Nepal. The rare surface expression of these earthquakes implies that surface ruptures of other reputedly blind great Himalayan events might exist.
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The research was principally funded by EOS (NTU, Singapore), with contributions from CEA/DASE and Project PAKSIS (ANR Cattel) in France. We thank M. Goh (EOS) for technical assistance with the Lidar data acquisition and processing, N. Shrestha (Kathmandu) for total station topographic surveying, and G. Cook (SUERC Radiocarbon Dating Laboratory, University of Glasgow, UK) for the charcoal samples analyses. We are grateful to the young men of Cheru for refreshing and cleaning the river-cut cliff and trench walls. We also thank the Department of Mines and Geology in Kathmandu, Nepal, for constant logistical support, and F. Perrier and B. N. Upreti for excellent guidance during a first exploratory field trip. We are particularly indebted to F. Perrier for having inspired this study from the start. S. Wesnousky and T. Rockwell provided thoughtful, constructive comments that helped improve the original manuscript. This is EOS/NTU contribution number 40.
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Scientific Reports (2017)