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The Fuyun Fault is a typical intraplate, slow-slipping fault, but has been repeatedly ruptured by surface wave magnitude (Ms) ≥ 8.0 earthquakes. The 11 August 1931 Ms 8.0 Fuyun earthquake resulted in more than 10,000 casualties in the sparsely populated Fuyun area. Cosmogenic 10Be dating of offset landforms produced by prehistoric Ms ≥ 8.0 earthquakes yields an average recurrence interval of 9,700 ± 3,300 years, much longer than previously estimated 2,000–4,500 years, clouding our understanding of the timing and recurrence behavior of past earthquakes originating from the Fuyun Fault. Reflection seismic data reveal widely distributed subaquatic faults in Yileimu Lake, implying high sensitivity of the lake sediments to paleoearthquakes originating from the Fuyun Fault. Two new long sediment cores (Y20A: 267 cm; Y20B: 890 cm) together with previously published two short cores (Y19: 71 cm; Y20: 31.5 cm) from the depocenter and nearshore zone of Yileimu Lake are used for stratigraphic correlations and analyses of sedimentary structures, grain-size distributions, magnetic susceptibility, elemental composition and carbon content. The mass-wasting deposits with underlying soft-sediment deformation structures (SSDS) in the Y20B core indicate six siliciclastic-enriched sandy sediment fluxes from earthquake-triggered landslides of granitic rocks, and isolated SSDS record three additional earthquake-induced in situ deformations. Turbidite-like deposits with sorting indices >3 and Si contents >700 counts per second (cps) are comparable to those of the seismic mass-wasting deposits, and are thus interpreted as seismites from earthquake-induced re-deposition of nearshore sediments. There are a total of 20 seismic events recorded by the Y20B core. Seismic intensity calculation results, combined with historical seismic data, provide potential magnitudes of Ms ≥ 8.0, Ms ≥ 7.0, and Ms ≥ 5.5 for the earthquake-triggered mass-wasting deposits, SSDS, and turbidite-like seismites, respectively, in Yileimu Lake, generally consistent with previously published magnitude thresholds. Radiocarbon dating and stratigraphic correlations constrain the timing of these past earthquakes to ∼28 cal kyr BP. This unique, long lacustrine paleoseismic record suggests a weakly periodic pattern with recurrence intervals between 2,317 and 7,830 years and an average of 5,303 years for potential Ms ≥ 8.0 earthquakes, and reveals an unprecedented high frequency of potential Ms ≥ 7.0 earthquakes originating from the Fuyun Fault in the last 5 kyr, demonstrating the urgent need for an improved assessment of seismic hazards and risks in the Fuyun Fault zone.

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Tectonic and climatic process controlling gravel accumulation in tectonically active regions is the subject of active debate. In this study, the formation mechanism of a gravel layer in the Diexi lacustrine section, eastern Tibetan Plateau, was investigated using mutually validated dating methods and detailed analysis of sedimentary processes. Optically stimulated luminescence (OSL) and accelerator mass spectrometry (AMS) 14C dating show that the gravel layer in the Diexi section accumulated at approximately 16.79 ka BP. The sedimentary characteristics of the gravel layer and the contact between upper and lower strata indicate that the formation of the Diexi gravel layer was triggered by an earthquake rather than by a debris flow caused by torrential rain. And the result based on the intensity attenuation model are consistent with the characteristics of frequent large earthquakes in Diexi area. Detailed analysis of satellite images and sedimentary characteristics of the gravel layer provide evidence for an ancient landslide, which may be related to the gravel layer at Muer village (to the north of the Diexi section). Overall, this study reconstructs a gravel event at approximately 16.79 ka BP and has important theoretical and practical significance for understanding the formation mechanism of gravel deposits and analysing seismic events through gravel accumulation.

It has previously been established that tectonic activity influences the evolution of fluviolacustrine sedimentary sequences by affecting the provenance supply, while climate change modifies the sedimentary record through weathering and denudation (Najman, 2006). During the Early Cenozoic, many rifted basins developed in eastern China, while an array of compressional basins formed in western China, mostly due to the India-Eurasia collision and continued convergence (Yin & Harrison, 2000) and/or subduction of the western Pacific Plate (Northrup et al., 1995). Most of these basins are filled with non-marine sediments (Ye et al., 1993) and the documented signals suggest that the environmental evolution has been driven mainly by tectonic activity and/or climate change (Ye et al., 1993).The Qinling-Qilian-Kunlun orogenic system (QQKOS) extends for ∼3,000 km from east to west and formed in response to late Mid-Proterozoic to Cenozoic tectonism (Meng & Zhang, 2000;Peltzer et al., 1985). The QQKOS represents an important tectonic zone in Asia linking the India-Eurasia collision with extensive deformation in eastern China (Meng & Zhang, 2000;Peltzer et al., 1985; Figure 1). Field observations show that the faults of the QQKOS are still active today and may have undergone several tens of kilometers of post-Eocene left-lateral displacement (Peltzer et al., 1985). However, the impact of the India-Eurasia collision on eastern China during the Palaeogene, which represents the critical period of the India-Eurasia collision, has not been determined. On the other hand, a broad belt of aridity stretched across China from west to east during the Palaeogene (X. Sun and Wang, 2005), overlapping spatially with the QQKOS, with only the southern-most and northeastern China being

Abstract. The deposition of fluvial sediments in tectonically active areas is mainly controlled by tectonics, climate, and associated Earth surface processes; consequently, fluvial sediments can provide a valuable record of changes in regional climate and tectonic activity. In this study, we conducted a detailed analysis of the grain-size distribution in modern fluvial sediments from the upper Min River, eastern Tibet. These data, combined with information on regional climate, vegetation, hydrology, geomorphology, lithology, and fault slip rate, indicate that modern regional tectonic activity along upper Min River can be divided into three segments. Specifically, fluvial sediments in the Minjiangyuan–Diexi segment are dominated by silts (< 63 µm, 70.2 %), agreeing with low runoff, low rainfall, and high vegetation cover and revealing a windblown origin influenced by the arid and windy climate. These observations are consistent with the low hillslope angle and low relief, all indicating weak activity along the Minjiang Fault. The coarse-grained fraction (> 250 µm) of fluvial sediments in the Diexi–Wenchuan and Wenchuan–Dujiangyan segments increases stepwise downstream, although runoff and rainfall do not change significantly. These patterns correlate well with increases in both regional relief and hillslope angles. Together, these observations imply that regional tectonic activity along the Maoxian–Wenchuan Fault becomes more pervasive downstream along the Min River. The occurrence of well-sorted and well-rounded pebbles of fluvial sediments downstream of Dujiangyan must be related to the long-time scouring and sorting by rivers. This study marks the first development of a new research approach that can characterize regional tectonic activity by analysis of grain-size distribution of fluvial sediments collected from tectonically active regions.