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Instituto de Investigação
em Vulcanologia e Avaliação de Riscos
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Referência Bibliográfica


TROTA, A., MENDES, V., AMARAL, P. (2009) - Deformation source modelling of a probable magma intrusion in the Fogo/Congro area (S. Miguel Island, Azores). 13th IAIN Wold Congress, Estocolmo, Suécia, 27 – 30 de Outubro (Comunicação Oral).


Resumo


GPS is nowadays an essential tool for understanding crustal deformation in volcanic regions. The monitoring of volcanic activity is particularly important in populated regions, such as the Azores islands. As the result of such monitoring in the S. Miguel Island, we present a case study of a probable siliceous magma intrusion, inferred from the velocity field obtained from the analysis of GPS data spanning the period 2000.0 – 2007.7. The magma inflow has contributed to an important 3D deformation pattern over central S. Miguel Island, in the vicinity of Fogo composite volcano, placed in the boundary between Nubia and Eurasia stable tectonic plates, east of Middle Atlantic Ridge. The horizontal deformation shows a clear outward radial pattern whereas the vertical component shows stepwise inflation/deflation episodes along the intrusion processes. The best simple source deformation modelling of the 3D displacement field points out for a cigar-like shape intruded magma body, with a 110 m radius and a depth of 600 m (height of top and bottom of 1800 m and 2400 m, respectively). This intrusion has contributed to a widespread seismic swarm, correlated in magnitude, time, and space with the deformation field. The very high ratio of the seismic moment (calculated based on the seismic magnitude of registered events) to the magma injection geodetic moment (based on the volume change estimations using the Mogi model) give evidences of an important stress regime prior to the onset of the intrusion, afterwards triggered by the uprising magma. Moreover, underestimation of intrusion volume and very high magma viscosity could also be considered as causes for the high moment ratio. These results give important clues for understanding of the source mechanisms of magma intrusion for the Azores volcanic systems and contribute for risk mitigation in S. Miguel Island, far significant for society.

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