Browsing by Author "Machado, Frederico"
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- Excursion guide for field trip V3 : ‘Islands of Fayal and Pico’Publication . Machado, FredericoThe Azores are situated at the intersection of the mid-Atlantic ridge with the Azores-Gibraltar fracture belt which is part of the Alpine orogenic chain. At this triple junction converge the so called American, Eurasian, and African plates. On 6 of the islands there are 9 active volcanoes: the number of independent submarine vents is unknown, but 4 had recorded eruptions since the 15th century. Seismicity of the islands is considerable, which certainly results from plate movement. According to one of the tectonic proposed for the area, the mid-Atlantic rift could cross all the islands with active volcanoes (Fayal, Pico, San Jorge, Graciosa, Terceira and San Miguel). Expansion along the rift would be accompanied by shear at the associated transform faults, this shear being responsible for most of the frequent earthquake swarms. The present tectonic situation is further complicated by the effect of the Alpine movements, which also seem to produce crustal expansion to the west of a no-strain point at longitude 22˚ West. Another tectonic model assumes simply that the mid-Atlantic rift is situated between Fayal and Flores, and is intersected by a minor rift and spreading centre (the Terceira rift) that merges into the Azores-Gibraltar fracture zone.
- Interpretation of ground deformation in the AzoresPublication . Machado, FredericoGeodetic measurements in Fayal, Pico and San Jorge showed some horizontal expansion of these islands. The total expansion during the interval 1936-1979 amounts to· several metres, being considerably greater than the average sea floor spreading for that area of the Atlantic Ocean. This suggests that the spreading could be a discontinuous phenomenon, depending on short lived epochs of quick strain release. A vertical pulsation on the central part of Pico volcano was also detected; the pulsation has amplitude of about one metre and period close to one year. Its-seems due to periodic changes of pressure inside the magma chamber.
- Possíveis causas dos sismos dos AçoresPublication . Machado, FredericoOs abalos sísmicos dos Açores dependem certamente do mecanismo que faz mover as três placas litosféricas que existem na área destas ilhas. Um modelo de mecanismo que parece, adequado para explicar o ritmo das crises sísmicas baseia-se em pulsações de gravitação. Entre as principais perturbações neste ritmo figuram as marés terrestres (especialmente a semi-anual) e as variações de energia elástica disponível, devidas à própria fraturação que produz os abalos.
- Relatividade geral e geotérmicaPublication . Machado, FredericoO movimento galáctico do Sol, juntamente com uma possível oscilação perpendicular ao plano galáctico constituem mecanismos suficientes para produzir os principais fenómenos tectónicos, se admitirmos que ao longo da órbita há variantes da gravitação (ou da correspondente «constante»). A oscilação com um semiperíodo de 11 anos parece produzir as crises sísmicas dos Açores. O efeito do movimento da Terra em torno do Sol, sobreposto àquela oscilação, pode também ser responsável pela pulsação de câmaras magmáticas dos vulcões açorianos.