Sete Cidades is an active central volcano on the western part of São Miguel. The geological record reveals that subaerial activity started more than 250 ka ago. Stratigraphic units defined for Sete Cidades deposits reflect major events in the history of the volcano and are organized into two main groups: the Inferior Group and the Superior Group. Caldera formation resulted from three major paroxysmal events that occurred at about 36, 29 and 16 ka ago. Analysis of the eruptive history of Sete Cidades shows that effusive or moderately explosive eruptions, of Hawaiian and/or Strombolian styles, were located on the slopes of the central volcano. Conversely, trachytic explosive activity is mostly centred inside the caldera involving, in a first stage, predominantly Plinian and sub-Plinian phenomena, changing about 5 ka ago to a dominant hydromagmatic style. Trachytic effusive eruptions are represented by domes and associated lava flows that crop out in the inner caldera walls and on the western slopes of the volcano. Offshore submarine activity is represented by the historic Surtseyan eruptions of 1638 and 1811. In the last 5 ka Sete Cidades was the most active central volcano in the Azores with 17 explosive eruptions predominantly with hydromagmatic character. Scoria conesThese are the most common volcanic edifices on Sete Cidades massif and were formed as a result of the accumulation of basaltic † Deceased 19 May 2012.
São Miguel Island comprises five active volcanic systems, including three central volcanoes with calderas and two basaltic fissure systems. Volcanic eruptions in São Miguel are of basaltic and trachytic nature (s.l.), including Hawaiian, Strombolian, sub-Plinian, Plinian and Vulcanian events, the more explosive ones frequently including hydromagmatic phases. Large Plinian eruptions are related to caldera-forming events that occurred in the past. With reference to the Fogo A stratigraphic marker, a total of 73 individual volcanic eruptions have been identified in the last 5 ka, giving a recurrence interval of 68.5 years. Taking into account that only six events have occurred in historical times, the recurrence interval increases to 95 years and, clearly, a future event is overdue because the most recent eruption occurred in 1652. It should be noted, however, that some volcanic eruptions in the past have occurred in clusters.The eruptive frequencies of the last 5 ka of activity have been determined for all types of eruptions and related hazards, including lava flows, pyroclastic falls, pyroclastic density currents (PDCs) and lahars. The areas susceptible to volcanic products have been mapped and modelled under different eruptive conditions. † Deceased 19 May 2012.
Abstract. Geological hazards in the Azores archipelago include earthquakes, volcanic eruptions, degassing phenomena and landslides, being the cause of thousands of deaths and severe damage and loss. To reduce the impact of future events it is necessary to improve the emergency response and reinforce land-use planning, and this has given rise to the development of AZORIS, a GIS database for risk analysis in the Azores. At present this computer-based system comprises nine main dynamic data sets where elemental, monitoring and historical data are grouped in layers of first and second order. The logical structure of the database was conceived in order to facilitate interactivity between data sets and to guarantee the evolution of the system, as determined by the input and the generation of new and more detailed information. Archive organization was designed taking into account regional and local aspects of geological hazard. In order to ensure consistency of the database and the quality of the data within it, an internal process of validation was included.
Basaltic volcanism occurs at all the active volcanic systems of São Miguel. For the last 30 ka, the eruption of basaltic magma has been particularly significant in the areas between the three polygenetic volcanoes -Sete Cidades, Fogo and Furnas -and two basaltic fields have developed: the Picos Fissural Volcanic System (PFVS) and the Congro Fissural Volcanic System (CFVS). About 5 ka ago volcanic activity at CFVS ended abruptly and since then almost all basaltic activity has been concentrated at the PFVS, where about 30 eruptions have taken place. Despite the absence of eruptive activity, the Congro system represents one of the most active seismic areas in the archipelago. It shows episodes of volcanic deformation that are indicative of magma ascent, which halted beneath the volcanic system. Two historical eruptions are associated with the PFVS. The 1563 eruption at Pico do Sapateiro was of basaltic composition, whereas that of the 1652 eruption was, for almost two centuries, also considered to be basaltic. This event had, however, a Vulcanian style and involved the production of large amounts of fine ash and the growth of three trachyte domes with associated coulées. It was accurately described in contemporary historical accounts. † Deceased 19 May 2012.
Abstract. Since the settlement of S. Miguel Island (Azores), in the XV century, several earthquakes caused important human losses and severe damages on the island. Sete Cidades Volcano area, located in the westernmost part of the island, was attained by strong seismic crises of tectonic and volcanic origin and major events reached a maximum historical intensity of IX (European Macroseismic Scale 1998) in this zone.Aiming to evaluate the impact of a future major earthquakes, a field survey was carried out in ten parishes of Ponta Delgada County, located on the flanks of Sete Cidades volcano and inside it is caldera. A total of 7019 buildings were identified, being 4351 recognized as dwellings. The total number of inhabitants in the studied area is 11429.In this work, dwellings were classified according to their vulnerability to earthquakes (Classes A to F), using the structure types table of the EMS-98, adapted to the types of constructions made in the Azores. It was concluded that 76% (3306) of the houses belong to Class A, and 17% (740) to Class B, which are the classes of higher vulnerability.If the area is affected by a seismic event with intensity IX it is estimated, that 57% (2480) to 77% (3350) of the dwellings will partially or totally collapse and 15% (652) to 25% (1088) will need to be rehabilitated. In this scenario, considering the average of inhabitants per house for each parish, 82% (9372) to 92% (10 515) of the population will be affected. The number of deaths, injured and dislodged people will pose severe problems to the civil protection authorities and will cause social and economic disruption in the entire archipelago.
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