A ∼28-kyr Continuous Lacustrine Paleoseismic Record of the Intraplate, Slow-Slipping Fuyun Fault in Northwest China

Abstract

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.