Résumé
The unusually large displacements of several meters associated with great Himalayan earthquakes point out the limits of classical trenching studies that do not allow the characterization of a long rupture history and may only document one or two events during the last millennium. We present geomorphologic and paleoseismic studies conducted over a large river-cut exposure along the Main Himalayan Front in southwestern Bhutan near the village of Piping. This site reveals an important 30-m cumulative deformation of almost 30 m characterized by a well-developed uplifted terrace system associated with a pluri-decametric fault-propagation fold. Paleoseismic excavation along with 16 radiocarbon ages chrono-stratigraphic modeling reveal the occurrence of at least five major and great earthquakes between 1070 + or - 270 BC and AD 1714 with an average recurrence interval of 550 + or - 300 yr. The slip-per-event analysis based on these new findings does not suggest a constant magnitude but rather a bimodal distribution, which includes (1) great earthquakes with decametric vertical offsets and (2) relatively smaller ones with metric vertical offsets.