The Azores archipelago is located in the Atlantic Ocean and is composed by nine volcanic islands. S. Miguel is the largest one being formed by three active trachytic central volcanoes with caldera (Sete Cidades, Fogo and Furnas), placed along an E-W direction. Basaltic cinder cones emplaced along NW-SE fractures link that major volcanic structures. The easternmost part of the island comprises an inactive trachytic central volcano (Povoação) and an old basaltic volcanic complex (Nordeste).
Since the settlement of the island, early in the XV century, several destructive landslides occurred in different areas of S. Miguel, triggered by earthquakes, volcanic eruptions and catastrophic rainfall episodes. One unique event killed thousands of people on 1522. Houses and bridges are sometimes destroyed, roads are cut, communications, water and energy supply systems become frequently disrupted and areas of fertile land are often buried by mud.
Most of the landslides are essentially very fluid debris flows, consisting in a mixture of water, pumice and ash. Trees with their root systems, tree trunks and branches are normally present. Depending on the source, some debris flows transport large blocks of lava. Although less frequent, dry debris flows and rock fall events also occur.
Based on (1) aerial photographs, (2) historical documents and (3) field observations, landslide sites were plotted in a topographic map in order to produce a landslides density map for the island. Data obtained showed that landslides hazard is higher on (1) the main central volcanoes, where the thickness of unconsolidated pyroclastic deposits is considerable and (2) the old basaltic volcanic complex, marked by deep gullies developed on thick sequences of lava flows. In these areas, caldera walls, fault scarps, steep valley margins and sea cliffs are potentially hazardous.