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

To the south, the cliff exposes flat beige siltstones and sands, stacked by two main faults (F1 and F2), and locally intensely sheared / deformed close to the faults (Figure 4). This stack is capped by unit U0, a unit markedly different, consisting of irregular thick sand beds and gravels unconformably overlying the units below. U0 dips slightly to the south. We sampled over 100 detrital charcoals in the river-cut east wall and in pits excavated in the abandoned fluvial terraces in the hanging wall of the thrust system (Figure 4). In the F0 shear zone, we have also found pottery sherds that have been broken and transported along the fault. So far, 34 detrital charcoals have been dated by AMS radiocarbon dating, ranging from 6200 to 900 BP. Their distribution indicates that the last earthquake to rupture F0 occurred after the 12th or early 13th century. Two historical earthquakes are possible candidates for a surface rupture in the Fig.4: a) Frontal part of the refreshed Chapin Khola river-cut face (Top) Uninterpreted picture b)Basic log with the main faults and units. Red/green squares respectively for the failed/ succeeded AMS dating of the detrital radiocarbon. area: (1) the 7 June 1255 earthquake, which according to medieval chronicles killed a third of the inhabitants of the Kathmandu Valley, including King Abhaya Malladeva (Pant, 2002; Bollinger et al., 2016), and (2) the 15 January 1934 Bihar-Nepal earthquake. Both earthquakes have been linked by some authors to surface rupture in eastern Nepal. C O N C L U S I O S The main conclusion to be drawn from the results of our study is that the Main Himalayan frontal thrust in the Chandra Bagh region was ruptured during a large Himalayan earthquake at least once since the 12th or 13th century. This confirms and extends the findings of Sapkota et al. (2013) and Bollinger et al. 2014 in the Sir Khola valley, or Rizza et al. (2017) and Riesner et al. (2023) in the Charnath and Khutti valleys, establishing that the large historical earthquakes described in the chronicles (1255 AD and/or 1934) were not blind. A C K N O W L E D G E M E N T S We are grateful to the Department of Mines and Geology in Kathmandu, Nepal, for their continued support, and to DASE- France for funding Magali Riesner’s postdoc, the field campaign and the radiocarbon dating.