Large Himalayan Frontal Thrust paleoearthquake at Khayarmara in eastern Nepal

Abstract An exposure created by excavation of a trench across the Himalayan Frontal Thrust provides the basis to interpret that a single earthquake produced vertical separation of ∼7 m at Khayarmara, a small community ∼80 km southwest of Kathmandu. The fault trace at Khayarmara is expresssed by a topographic ridge resulting from folding up-dip and toward the surface at the expense of greater fault slip taking place at depth. Structure, stratigraphy, and radiocarbon data are interpreted to indicate displacement occurred after about 1050 CE to 1200 CE. The timing and displacement at Khayarmara are compared to that reported previously at six contiguous sites that extend from 200 km to the west and 250 km to the east of Khayarmara, respectively. The comparison leads us to conclude that the surface rupture at Khayarmara was part of a ≥250 km long synchronous surface rupture earthquake of magnitude approaching if not surpassing Mw 9. We observe in the exposure no record of surface rupture associated with the great 1934 Bihar-Nepal earthquake.