In situ recurrent Hongshiyan paleolandslides at Ludian, China, and Implications on fault activity and hydro-project risk reduction

Large landslides often cause catastrophic life losses and infrastructure damage. Identification of the driving forces of large ancient landslides is of utmost importance for the understanding of geohazard assessment and regional geomorphologic evolution and for the understanding of regional paleoclimate and paleoseismology. Through field geological survey, multi-temporal satellite image interpretation, sedimentological observation, and static and dynamic numerical simulation, the paper studied the geo-environments and deposit succession of the Hongshiyan paleolandslide (HSYPL), over against the Hongshiyan landslide (HSYL) triggered by the 2014 Ludian M_S (surface wave magnitude) 6.5 earthquake. The study reveals that (1) the HSYPL and HSYL are symmetrically distributed on the opposite banks of the Niulan River and on the opposite wings of a vertical anticline plunging west. Both landslides involved an anti-dip slope structure of upper hard rock while lower soft rock. (2) Two phases of deposit succession in the paleolandslide accumulations were recognized from their surficial appearances, planar distribution, spatial superimposition relationship, permeability test, and borehole survey. (3) The deposit did not result from one single paleolandslide event but two long-interval individual events, i.e., penultimate landslide (PL) and last landslide (LL), whose source volumes were estimated to be ~ 11.8 Mm³ and ~ 113.5 Mm³, respectively. (4) These two landslides kept stable under static conditions but failed when the SN component acceleration reached 1.4 and 1.2 times the value of the 2014 Ludian M_S 6.5 earthquake. The ground motions basically correspond to the earthquake magnitudes that are back-analyzed by their volumes. (5) Both the penultimate landslide and last landslide were seismically triggered with high probability. The former was more likely due to the seismic activity of the Zhaotong-Ludian fault than the Baogunao-Xiaohe fault, while the latter might be induced by either fault which was active since the Holocene. Our findings present new insights into the regional seismological history and considerations on the risk reduction of the new hydro-project constructed from the Hongshiyan co-seismic landslide dam.