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Instituto de Investigação
em Vulcanologia e Avaliação de Riscos
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Referência Bibliográfica

WALLENSTEIN, N., SILVA, R., RIEDEL, C., LOPES, C., IBÁÑEZ, J., SILVEIRA, D., MONTALVO, A. (2009) - Recent developments in seismic studies in the Fogo Volcano area, São Miguel Island (Azores). In: VOLUME project, EU PF6 (No. 018471). Edited by : C. J. Bean, A. K. Braiden, I. Lokmer, F. Martini, G. S. O’Brien. ISBN 978-1-905254-39-2. VOLUME Project Consortium, Dublin, 207-216.


​Between 2002 and 2006 an important increase in the seismic activity was observed in the Fogo-Congro area, central São Miguel Island in the Azores. It was characterized by low magnitude (Md<3) clustered seismic swarms. Cross-correlation of the 2003-2004 identified six seismic swarms, three of them with more than 140 events each. Relocation with HypoDD showed a source migration with time towards lower depth regions. The very low Vp/Vs values could be explained by the presence of an important geothermal system. It should also be noted that the focal mechanisms determined from the April 2003 swarm indicate that it could be closely related to pore pressure mechanisms. A seasonal variation of the seismicity could be associated with the role played by rainfall in the pressure changes of the geothermal system. Cross-correlation of the background noise recorded with a 12-channel seismic array showed the existence of low frequency coherent sources. A possible interpretation is proposed herein, in which the regional stress field, and a possible intrusion, could have been the driving forces for seismicity that was strongly controlled by the pressure conditions of the geothermal system.