Earthquake-triggered landslides by the 1718 Tongwei earthquake in Gansu Province, northwest China

The 1718 great Tongwei earthquake in northwest China caused more than 300 large-scale landslides near the epicenter resulting in more than 70,000 deaths. Although previous studies agree that the earthquake-induced landslides in the area are loess-landslides, which slid along ancient terrain surfaces, there is a dearth of evidence to unravel the actual failure mechanisms of these widely-documented events. To clarify their characteristics and mechanisms, the landslides in Pan'an Town, located in Gansu Province, China were rigorously investigated. The field surveys revealed that the landslides occurred in an area comprising loess deposits of several tens of meters underlain by a basal mudstone layer. It was also revealed that instability in the area is strongly influenced by a river valley with large erosion bank slopes, as well as steep mudstone slopes, which are well developed. The majority of the landslides triggered by the Tongwei earthquake were loess-mudstone composite landslides, with the sliding surface in the deep mudstone. Three huge landslides on the northern mountain located at the river bend of Pan'an Town with a total volume of about 6.06 x 10(8) m(3) are the main mass movements during the 1718 catastrophe. In addition, by taking the typical landslide named the Weijiawan landslide as a case study, the internal structure of the earthquake-triggered landslides and their stability were studied. The new findings not only aided the unraveling of the failure mechanisms but also provided insights and knowledge on early recognition of earthquake-triggered landslides in northwest China towns, especially in the surrounding valley, which shares similar geological conditions with landslides studied in this paper.