POTENTIAL OF LARGE EARTHQUAKES IN CENTRAL NEPAL: HIMALAYAN FRONT AND KATHMANDU BASIN

The 2015 Gorkha earthquake ruptured only the deeper northern part of the thrust during the Mw 7.8 earthquake, but did not rupture the surface and shallow portion of the Himalayan Frontal Thrust (HFT) in south. Since then, the evaluation of the potential of the future surface rupture event on the HFT in central Nepal in south of 2015 rupture became an important task for paleoseismology. At the same time, it is necessary to evaluate the risks from upper plate faulting under or near Kathmandu. As to the HFT in central Nepal, several different models have been proposed for the timing of the last surface-rupture event since 2015, but supporting evidence are not enough. Also, the existence and the east termination of 1505 CE Indian central seismic gap event (Malik et al., 2017) are not certain yet. In order to solve the these problems, we excavated two trenches across the HFT around the city of Butwal at Sorauli (27.70N 83.36E) and at Susto (27.48N 83.86E). Around the Sorauli trench site there is no clear continuous scarp along the Siwalik hill front, but ~100 m wide zone of alluvial surface adjacent to the foot of the hills tilt toward south and juxtaposed to flat alluvial surface. A trench here exposed a fine sand layer tilting south towards the boundary. An Baesian model of radiocarbon dates indicates the timing of tilting is between 1328 CE and 1435 CE. This date coincides with previously proposed 1344 event in central Nepal HFT The Susto trench into the foot of ~20 m high fault/flexure scarp exposed 13 degrees south-dipping fine sand layers indicating the fault underneath. The timing of the tilt in Susuto trench is dated as slightly younger than in Sorauli. As to the Kathmandu basin, the Chandragiri fault along the southwest margin of the basin has been known since 1990s but we know very little about the potential of large earthquake. The results from high-resolution topographic survey and radiocarbon dating will be presented.