Proceedings of the 12th International INQUA meeting on paleoseismology, active tectonic and archaeoseismology

82 PATA Days 2024 framed to the west by the river Balan and a sharp tear fault (15 km- long) and to the east by the Sapta Kosi and its alluvial cone, associated to a topographic step of almost 25 km (Delcaillau, 1992) (Figure 2). This structure lies in the middle of the supposed traces of the great historic earthquakes of 1934 and 1255 or of earthquakes attributed to an earlier medieval period (e.g. Sapkota et al., 2013; Bollinger et al., 2014; Riesner et al., 2023; Rizza et al., 2016; Wesnousky et al., 2017, 2018; Nakata et al., 1998). However, the surface ruptures of these earthquakes have not been described there until now. To fill this gap of information, we undertook a systematic survey of the valleys at the front of the Chandra Bagh in 2012 and 2020. This survey revealed locally the presence of abandoned Holocene fluvial terraces, locally tilted or deformed and fault scarps at the front of the main frontal thrust. CHAPIN KHOLA EAST BANK RIVERCUT CLIFF Well-preserved evidence of recent faulting is still exposed in -at least- one rivercut on the east bank of the Chapin river. The natural rivercut cliff culminates at a height of ~ 30 m. It exposes predominantly light- colored siltstones and sands separated by north-dipping thrust faults. We mainly inspected, refreshed and documented the most frontal – southern part of the rivercut, the inner part being covered by debris that collapsed during themonsoon prior to the survey, obliterating the most internal structures, which included in 2012 a basal decollement exposing a dark centimetric-thick gouge (Figure 3). Fig.2: Map of theHimalayan frontal thrust, west of the SaptaKosi river. Red square, location of the Chapin river cut. Fig.3: Main shear zone at the toe of the cumulative scarp.