Timing and seismic origin of Nixu rock avalanche in southern Tibet and its implications on Nimu active fault

In tectonically active mountain areas, the identification, dating and determination of origin of large landslides are of special importance in recognizing the seismicity and earthquake magnitude of regional faults, reconstructing the prehistoric landscape evolution and estimating the seismic hazards. Based on field survey, high-resolution satellite image interpretation and C-14 dating of substrate, this paper studied the large, long-runout Nixu rock avalanche (the NXRA) in the Angang graben of the southern Tibetan Plateau. The study revealed that: (1) the deep-seated Nixu rock avalanche straddles the Nimu active normal fault and spreads onto the Angang basin with a deposit volume of 47 Mm(3) and a travelling distance of 4960 m. (2) Humus samples immediately under the deposit of rock avalanche along the natural river-erosion section suggest a C-14 age of 820 +/- 30 BP (1220 +/- 30 CaIAD) for the NXRA event. The age well matches the 1264 CE Chubushi earthquake recorded in Tibetan documents. (3) 23 adjacent landslides have been recognized with co-seismic distribution features, such as clustering and distance effect, ridge-end amplification effect and slope aspect effect. Sand liquefaction and peat deformation were discovered in the substrate beneath and at a distance of -300 m away from the avalanche deposit. (4) This information indicate that the NXRA was triggered most likely by the 1264 Chubushi earthquake with a magnitude of Mw 7.0-7.2 estimated by empirical equations of Keefer (1994) and Wells & Coppersmith (1994). It is suggested that the Nimu fault of the Yadong-Gulu rift may be more hazardous than the northern and southern part due to its high susceptibility of large co-seismic landslides.