HR: 0800h
AN: V11C-0748    [Abstracts]
TI: The Role Played by Rainfall on the Seismicity at Fogo Volcano, Azores
AU: * Martini, F
EM: Francesca.Martini@ucd.ie
AF: Seismology and Computational Rock Physics Lab., School of Geological Sciences, University College Dublin, Belfield, Dublin, 4, Ireland
AU: Saccorotti, G
EM: saccorotti@pi.ingv.it
AF: Istituto Nazionale di Geofisica e Vulcanologia, Via U. della Faggiola, 32, Pisa, 56126, Italy
AU: Riedel, C
EM: Carsten.Riedel@azores.gov.pt
AF: Centro de Vulcanologia e Avaliao de Riscos Geologicos, Universidad dos Acores, Rua da Mae de Deus, Apartado 1422, Ponta Delgada, 9501-801, Portugal
AU: Bean, C J
EM: Chris.Bean@ucd.ie
AF: Seismology and Computational Rock Physics Lab., School of Geological Sciences, University College Dublin, Belfield, Dublin, 4, Ireland
AU: Wallenstein, N M
EM: Nicolau.MB.Wallenstein@azores.gov.pt
AF: Centro de Vulcanologia e Avaliao de Riscos Geologicos, Universidad dos Acores, Rua da Mae de Deus, Apartado 1422, Ponta Delgada, 9501-801, Portugal
AU: Viveiros, F M
EM: Maria.FB.Viveiros@azores.gov.pt
AF: Centro de Vulcanologia e Avaliao de Riscos Geologicos, Universidad dos Acores, Rua da Mae de Deus, Apartado 1422, Ponta Delgada, 9501-801, Portugal
AB: Fogo volcano is an active central volcano, with a lake filled caldera, in the central part of Sao Miguel Island, Azores, whose current activity is limited to fumarolic and hydrothermal activity. It is affected by important active tectonic structures, with extremely high seismic activity and micro-seismicity concentrated in very frequent swarms. A recurrent feature of the seismicity observed in volcanic regions is the occurrence of seismic families, whose origins can be attributed to the same source mechanism, acting in the same small rock volume. Doublets/multiplets were identified in this study within a catalogue of small magnitude (usually <3) volcano- tectonic events recorded in 2003-2004 by a selection of stations around Fogo volcano. All events have been cross-correlated and pairs whose waveforms exhibited a cross-correlation coefficient equal to or higher than 0.9 were analyzed through a cross-correlation based technique (CWI, Snieder et al. 2002). Subtle velocity variations found between events highlight a seasonal cycle of the velocity patterns, with lower velocity in winter time and higher velocity during summer months. Those results, together with quantitative differences between the same doublets at different stations, exhibit an excellent correlation with the rainfall. A seasonal effect can also be broadly seen in the seismicity occurrence, and some of the swarms recorded over the two year period occur in close temporal association with higher than average rainfall. Moreover, temporal and spatial analysis of several swarms highlighted the lack of any main-shock after-shock sequence and organized migration of the hypocenters. This is suggestive of a very heterogeneous stress field. Vp/Vs is found to be lower than usually observed in volcanic areas, an occurrence likely related to the presence of fluid associated with the geothermal system (Saccorotti et al., 2004). Taken together, these two observations suggest that fluid migration/pressurization play a major role in triggering the recorded seismicity. The geothermal fluids around Fogo massif have been identified as derived from meteoric water, which infiltrates through Fogo Lake and the volcano flank and flows from south to north on the northern flank (Carvalho, 1999). These results point to the important role played by rainfall in triggering seismicity at Sao Miguel, possibly through pressure changes at depth in response to surface rain and/or interaction with the geothermal system.
DE: 7280 Volcano seismology (8419)
DE: 8408 Volcano/climate interactions (1605, 3309)
DE: 8419 Volcano monitoring (7280)
DE: 8424 Hydrothermal systems (0450, 1034, 3017, 3616, 4832, 8135)
DE: 8488 Volcanic hazards and risks
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting