A 2600-year-long paleoseismic record for the Himalayan Main Frontal Thrust (western Bhutan)

Abstract

In spite of an increasing number of paleoseismic studies carried out over the last decade along the Himalayan Arc, the chronology of historical and prehistorical earthquakes is still poorly constrained. In this paper, we present geomorphologic and paleoseismic studies conducted over a large river-cut exposure along the Main Fontal Thrust in southwestern Bhutan. The Piping site reveals a 30m high fault-propagation fold deforming late Holocene alluvial deposits. There, we carried out detailed paleoseismic investigations and built a chronological framework on the basis of 22 detrital charcoal samples submitted to radiocarbon dating. Our analysis reveals the occurrence of at least five large and great earthquakes between 485125 BCE and 1714 CE with an average recurrence interval of 550211 years. Coseismic slip values for most events reach at least 12m and suggest associated magnitudes are in the range of Mw 8.5- 9. The cumulative deformation yields an average slip rate of 24:910:4mmyr1 along the Main Frontal Thrust over the last 2600 years, in agreement with geodetic and geomorphological results obtained nearby.