Cristian Araya-Cornejo, Matías Carvajal, Jasper Moernaut, Felipe González, Marco Cisternas

We present evidence that suggests a new risk scenario for the Valdivia basin in south Chile, located in the area of the magnitude 9.5 1960 earthquake. In 1960, three mass movements, triggered by the earthquake shaking, dammed the upper course of the San Pedro River and threatened Valdivia City until it was opened in a controlled manner by its inhabitants. Published historical accounts indicate that the 1575 earthquake, predecessor of the 1960 event, also triggered a mass movement that dammed the upper course of the river. However, here we reinterpret the published account and present new historical records, which we combined with satellite imagery and field surveys to show that the volume of the landslide in 1575 was smaller than the smallest of those of 1960, yet its outburst flood killed thousands of natives located downstream. Additionally, we characterized different mass movement deposits in the upper course of the San Pedro River, including both ancient and those formed in 1960, and we evaluated the mechanisms that could contribute to their generation at present (e.g. land use). Our results suggest that in the present-day conditions a moderately-sized (Mw ~8) earthquake can be sufficient to cause damming the San Pedro River, which challenges the previous assumption that such phenomena are exclusively related to giant 1960-like earthquakes.