Landslide incidence in the North of Portugal: Analysis of a historical landslide database based on press releases and technical reports

- A regional landslide database based on documental sources is explored.
- Most landslides included in the regional database are damaging landslides.
- The incompleteness of the regional landslide database is demonstrated.
- Regional and local newspapers are more effective in reporting damaging landslides.
- The regional landslide database is not suitable to validate landslide susceptibility.

This work presents and explores the Northern Portugal Landslide Database (NPLD) for the period 1900-2010. NPLD was compiled from press releases (regional and local newspapers) and technical reports (reports by civil protection authorities and academic works); it includes 628 landslides, corresponding to 5.7 landslides per year on average. Although 50% of landslides occurred in the last 35 years of the series, the temporal distribution of landslides does not show any regular increase with time. The relationship between annual precipitation and landslide occurrence shows that reported landslides tend to be more frequent in wetter years. Moreover, landslides occur mostly in the wettest months of the year (December, January and February), which reflects the importance of rainfall in triggering slope instability. Most landslides cause damage that affects people and/or structures; 69.4% of the landslides in Northern Portugal caused 136 fatalities, 173 injured and left 460 persons homeless. More than half of the total landslides (321 landslides) led to railway or motorway closures and 49 landslides destroyed 126 buildings.The NPLD is compared with a landslide database for the whole of Portugal constructed from a single daily national newspaper covering the same reference period. It will be demonstrated that the regional and local newspapers are more effective than the national newspaper in reporting damaging landslides in the North of Portugal. Like other documentary-based landslide inventories, the NPLD does not accurately report non-damaging landslides. Therefore, NPLD was found unsuitable to validate municipal-scale landslide susceptibility models derived from detailed geomorphology-based landslide inventories.