Rapid onset hazards, fault-controlled landslides and multi-method emergency decision-making

Numerous large-scale fragmented bedrock landslides developed along major fault system is a world-wide phenomenon, which are often characterized with repeated reactivation throughout histories. Due to the large-scale and deep-seated features, it is normally difficult to control such landslides, which in turn pose great threat to local residents and infrastructures. Therefore, monitoring and forecasting these gigantic landslides has become a key protocol for risk reduction. This paper introduces such a typical massive landslide, named Yahuokou landslide, besides Min River in Zhouqu County, Gansu Province, China. Reactivated on July 16, 2019 with a volume of approximately 4×106m3, moving slowly and transitionally starting from top part, its toe had partially blocked the Min River and destroyed roads and houses eventually by August 11, 2019. As to emergency response for such huge slow-moving landslide, there is no standard national protocols. Therefore, how to make effective emergency decision has become a challenge. Based on previous experiences, integrated multi-methods, including UAV imagery interpretation, we applied GNSS monitoring and field investigations in the early stages of landsliding, in order to assist the decision-making. The results show that the movement path of the current displacement is consistent with that of the 1989 reactivation event, and the slide body was separated into three relatively independent blocks with different sliding velocities and responses to rainfall. The upper and lower blocks appeared less affected by rainfall, while the middle block responded more to the changes in precipitations. It proves that the combined approaches using a variety of monitoring techniques can play an effective role in the monitoring of rapidly deformed transitional large-scale landslides, and can also provide a set of reference methods for the emergency disposal of similar landslide hazards.