Key Points: • Three areas of tectonic activity are identified by analysis of the fluvial sediments from the Min River in eastern Tibet. • The dominance of fluvial silts, low relief and low slope-angles indicate weak activity of the Minjiang fault. • Stepwise increases in sands, relief, and slope-angle reveal increased tecton-ism in the Diexi-Wenchuan-Dujiangyan segments. Abstract The deposition of fluvial sediments in the tectonically active areas is mainly controlled by climate change and tectonic activity, meaning that fluvial sediments can provide a valuable record of regional climatic and tectonic signals. In this study, a detailed analysis was conducted on the grain-size of modern fluvial sediments from the upper Min River in eastern Tibet, and these data were combined with regional information about the vegetation, hydrology and geomorphology. The results indicate that modern regional tectonic activity in the study area can be divided into three segments. The fluvial sediments in Minjiangyuan-Diexi segment are dominated by fine silts (<63 ï¿¿m: 70.2%), showing low runoff and rainfall conditions and revealing a windblown origin influenced by climate change. This observation is consistent with the small hillslope angle and low relief in this segment, indicating weak activity of Minjiang fault. The coarse-grained fraction (>250 ï¿¿m) of fluvial sediments in Diexi-Wenchuan-Dujiangyan segments increases stepwise (A:6.2%, B:19.4%, C:33.8%) with a stable hydrological conditions, which corresponds well to the increasing trend in the regional relief and the steepness of hillslope angles. These observations indicate the phased enhancement of regional tectonic activity for Maoxian-Wenchuan fault. The fluvial sediments in Dujiangyan-Sichuan basin segment show good sorting and rounding, which is well correlated with significant increases in rainfall and runoff. There is also almost no evidence of tectonic activity in this segment. This study first develops an important research approach for revealing regional tectonic activity through fluvial sediment analysis in tectonically active regions. Plain Language Summary In tectonically active areas, intense tectonic activity tends to create steep ge-omorphology, and deeply affect erosion and sedimentation of river systems. Hence, whether fluvial sediments document changes of regional tectonic activity 1 deserve detailed investigation. In this study, the fluvial sediment of the Min River in the eastern Tibetan Plateau is taken as the research object. Based on grain size analysis of the Min River sediments, we reveal the close correlation between grain size variation and tectonic activity, under the comprehensive comparison of regional topography and geomorphology (slope, height difference) and hydrological conditions. We find that the fine-grained fluvial sediments from the Minjiangyuan to Diexi segment correspond to the weak fault activity of the Minjiang Fault, while the significant increase of coarse-grained composition from Diexi to Dujiangyan segment correspond to the enhancement of fault activity of the Maoxian-Wenchuan fault. This is supported by the greater slope angles and mountain relief from Diexi to Dujiangyan. Novelty of the research methods and reliability of results in this study provide a good reference for revealing regional tectonic activity through fluvial sediments in tectonically active areas.

Key Points: • The recurrence time of large earthquakes (M ï¿¿ 8.1) is 1200 years in the Long-men Shan collision zone. • MMI ï¿¿ ï¿¿½ (M ï¿¿ 5.9, R min ï¿¿ 10 km) is the lower-bound conditions for triggering the deformation in the Lixian lacustrine sediments. • The Maoxian-Wenchuan fault is the main fault that triggered the deformation in Lixian lacustrine sediments. Abstract The thrust collision zone with a low slip rate along the plate boundary is the main stress accumulation area and prone to occur more destructive earthquakes with a longer recurrence interval. Such region is often classified as low seismic risk if lack of continuous records of large earthquakes, such as the eastern Ti-betan Plateau before the 2008 M w 7.9 Wenchuan earthquake. Here, we provide a continuous seismic record in the Longmen Shan thrust fault zone spanning 13000 years based on detailed investigation of the soft-sediment deformation structures and seismites in the Lixian lacustrine sequence. The recurrence time of large earthquakes (M ï¿¿ 8.1) is 1200 years, which is significantly shorter than the previous estimate of 2000-6000 years. The Maoxian-Wenchuan fault is the main fault that triggered the deformation in the Lixian lacustrine sediments. In addition, the frequency of earthquake recurrence in the warm period is more frequent than that in the cold period, which should arouse our attention on the seismic study in the tectonically active regions. Plain Language Summary The Longmen Shan fault zone was evaluated as a low seismic risks area before 2008 because of complete failure of trenching paleoseismic research in the eastern Tibetan Plateau, which led to the severe casualties and property losses caused by the 2008 Wenchuan Ms 8.0 earthquake. Therefore, it is urgent to explore new research methods to reveal the paleoseismic records in the eastern 1 Tibetan Plateau. Continuous lacustrine deposition is a good carrier for revealing tectonic events. The soft-sediment deformation and seismites recorded in lacustrine sediments can be good indicators of paleoseismic event. Here, we redefined the soft-sediment deformation and seismic records of the Lixian lacus-trine sediments in the Longmen Shan fault zone, and discussed the recurrence time and recurrence model of regional earthquakes by linking them with earthquake intensity. We obtain that much shorter recurrence interval (1400 years) of large earthquakes (M ï¿¿ 8.1) are recorded in the Lixian lacustrine sediments compared with earlier studies (2000-6000 years), and the large earthquake have a random earthquake recurrence model of plate boundaries. These results are very important for reassessing the seismic risk of the Longmen Shan fault zone in the eastern Tibetan Plateau.