Repeated catastrophic valley infill following medieval earthquakes in the Nepal Himalaya

Geomorphic footprints of past large Himalayan earthquakes are elusive, though urgently needed for gauging and predicting recovery times of seismically perturbed mountain landscapes. We present evidence of catastrophic valley infill following at least three medieval earthquakes in the Nepal Himalayas. Radiocarbon ages from peat beds, plant macrofossils, and humic silts in fine-grained tributary sediments near Pokhara, Nepal’s second largest city, match the timing of nearby M~8 earthquakes in ~1100, 1255, and 1344 AD. The upstream dip of these valley fills and X-ray fluorescence spectrometry of their provenance rule out any local source. Instead, geomorphic and sedimentary evidence is consistent with catastrophic debris flows that had invaded and plugged several tributaries with tens of meters of calcareous sediment from a Higher Himalayan source >60 km away.