Landslides on São Miguel Island (Azores-Portugal) in the period 1900-2020: Analysis of the spatio-temporal distribution, triggering factors and impact based on newspapers press articles

Since the settlement of the São Miguel Island (Azores-Portugal), in the middle of the fifteenth century, there is a record of occurrence of landslides, some with high socio-economic impact. In this work, we carried out a spatial, temporal and impact analysis of landslide events that were registered in the NATHA (Natural Hazards in Azores) database for the period 1900-2020, based on newspapers descriptions. A total of 236 landslide events (a day with one or more landslides identified) that caused human losses, damage to houses or obstruction of roads on São Miguel Island were catalogued. Based on the recorded events, it is verified that there is not a regular increment and/or pattern in the distribution of the events over time, although two main periods can be distinguished: 1900–1994 (1.0 events per year) and 1995–2020 (5.3 events per year). The events were responsible for 82 fatalities, 41 injuries, 66 houses partially or totally destroyed and 305 homeless people. The municipality of Povoação registered 76 landslide events, followed by the municipalities of Ribeira Grande (71 events), Ponta Delgada (69 events), Vila Franca do Campo (47 events), Nordeste (26 events) and Lagoa (21 events). Although there is a relative homogeneity on the distribution of landslide events in the municipalities of Povoação, Ribeira Grande and Ponta Delgada, the same does not apply to the impact caused. In the municipality of Povoação were counted 48 fatalities, 20 injuries, 17 houses destroyed and 109 homeless people, in Ponta Delgada 14 fatalities, 14 injuries, 24 houses destroyed and 173 homeless people and in Ribeira Grande 8 fatalities, 5 injuries, 16 houses destroyed and 21 homeless people. In the municipality of Vila Franca do Campo were counted 7 fatalities and 2 houses destroyed, in Nordeste 3 fatalities and 2 injuries, and in Lagoa 2 fatalities, 7 houses destroyed and 2 were homeless people. Rainfall was the triggering factor responsible for 70% of the catalogued landslide events, followed by sea erosion (8%), anthropogenic actions (4%) and earthquakes (2%). The triggering factor was not possible to identify in 16% of the landslide events. Landslides occurred mostly during the rainiest season (from November to March), which comprise about 78% of the catalogued landslide events.