Additional Causes of Seismically-Related Landslides in the Northern Apennines, Italy

The results of a multidisciplinary research on the additional causes in historical landslides induced by earthquakes in the north-western sector of the Northern Apennines (Italy) are discussed. The first investigation phase was based on bibliographic records on earthquakes and landslides. This step led to the collection of 18 well documented landslides induced by seismic shocks. Up to 11 landslides were set in motion by a strong (6.5 magnitude) earthquake wich struck the Tyrrhenian side of the Northern Apennines on September 7th 1920. Other landslides were triggered by earthquakes occurring in 1779, 1832, 1952, 1996 and 2003. The landslides were triggered by earthquakes ranging from 3.3 to 6.5 magnitude (IV to X MCS degrees) with epicentres of 6 to 40 km away. The earthquakes-related landslides studied are mainly complex or slide-type movements. The rock types involved are prevalently calcareous flysch, clay shales and debris. In order to understand the complexity of the relationships between all the parameters affecting slope stability, detailed studies on geology, hydrogeology, geomorphology, soil/rock mechanics and meteorology were carried out in each landslide area. According to the data collected during research, it comes out that earthquakes seem to be just the triggering cause for a great number of these landslides whereas the intrinsic causes mainly result from the amount of precipitation in the preceding periods (soil saturation condition and build-up of pore-water pressures). Out of the 18 landslides investigated, earthquakes undoubtedly played a decisive role in 5 cases only. Also the lithological characteristics and weathering conditions of the bedrock appear to be extremely important since the five cases previously mentioned affected loose debris materials or weak rocks.