Seasonal cycles of seismic velocity variations detected using coda wave interferometry at Fogo volcano, São Miguel, Azores, during 2003–2004

Abstract

Fogo volcano is an active central volcano, with a lake filled caldera, in the central part of São Miguel Island, Azores, whose current activity is limited to hydrothermal manifestations such as active fumarolic fields, thermal and CO2 cold springs and soil diffuse degassing areas. It is affected by important active tectonic structures, with high seismic activity and practically continuous micro-seismicity. A recurrent feature from the seismicity observed in volcanic regions is the occurrence of clusters of similar earthquakes, whose origin can be attributed to the repeated action of a similar source mechanism at the same focal area. 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 analysed using the coda-wave interferometry technique. 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 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 during the wettest season or close to episodes of abundant (above average) rainfall. Moreover, temporal and spatial analysis of several swarms highlighted the lack of any mainshock–aftershock 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 steamy fluid associated with the geothermal system. Taken together, these observations suggest that pore pressurisation plays a major role in controlling a considerable part of 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 flanks and flows from south to north on the northern flank. All those elements seem to point to a role played by rainfall in triggering seismicity at São Miguel, possibly through pressure changes at depth in response to surface rain and/or an interaction with the geothermal system.