IAVCEI 2013 Scientific Assembly - July 20 - 24, Kagoshima, Japan
Forecasting Volcanic Activity - Reading and translating the messages of nature for society

1W_2F-P34

Date/Time: July 21 Poster

Volcano-tectonic interactions captured by decade-long GPS monitoring in the Azores
Jun Okada1 , Freysteinn Sigmundsson2 , Benedikt G Ofeigsson3 , Teresa JL Ferreira4 , Joao LRB
Gaspar1 , Maria FP Lorenzo4 , Joao PMT Araujo4 , Rita MMTC Rodrigues5
1Centre for Volcanology and Geological Risks Assessment (CVARG), University of Azores, Portugal,
Nordic Volcanological Center (NORDVULK), University of Iceland, Iceland, 3 Icelandic Meteorological
Office (IMO), Iceland, 4 Centre for Information and Seismovolcanic Surveillance of the Azores (CIVISA),
Portugal, 5 Department of geosciences, University of Azores, Ponta Delgada, Portugal
2E-mail: Jun.Okada@azores.gov.pt

The Azores archipelago is located across the Mid-Atlantic Ridge, where three mega tectonic plates meet: Nubia,
North America, and Eurasia. The archipelago has many active volcanic systems. The Fogo - Congro area inS. Miguel Island has been recognized as one of the most active seismo-volcanic fields. This area has been
repeatedly suffering from intense earthquake swarms, at least in last few decades, such as in 1989, 2003-2006,2008-2009, and 2011-2012. No geochemical and hydrothermal evidences for a magmatic intrusion were reported
during these seismic crises. However, geophysical data, both seismic and ground deformation, indicate possible
volcanic sources. To understand these repeating seismic swarms and eruption triggering mechanism, the mutualrelations between the regional stress field and volcanic deformations were studied.
In the scope of tectonic and volcanic monitoring 11 continuous GPS stations are currently operating in thearchipelago. The GPS time series for 2008-2011 shows an extensional stress regime at Monte Escuro - Congro
Area (MECA), between the NE flank of Fogo volcano and the western rim of Furnas Caldera. It plays about 38
percent contribution of the total plate spreading. The rest is maybe taking place in other regions from either tectonic
or volcanic contributions in different periods. The existence of this extensional regime plays important roles for thepresent-day magma ascent (failed and possible eruptions) in the area.
One of the stations inside MECA represents an episodic displacement on late 2008 which was accompanied by
two seismic significances; (1) the seismic migrations (from NE flank of Fogo to Furnas) and (2) the localized deepswarms (at the depth of 4-7km). The former suggests the close link between Fogo and Furnas volcanic systems
through the regional tectonic structures and the latter can be interpreted as minor-scale magmatic injection fromupper mantle. On the other hand, the seismic swarms in 2003-2006 and 2011-2012 were accompanied by theedifice-scale deformation. The time series shows the months to years of gradual pressure accumulation inside
Fogo volcano. These volcanic inflations may have resulted from successive magma injections at the shallow crust.
Decade-long GPS monitoring has revealed volcano-tectonic interactions in the Eurasia-Nubia plate boundary andenabled to distinguish volcanic unrests in S. Miguel Island from the steady-state present-day plate movements.
For better characterizing ongoing volcano-tectonic phenomena and assessing the volcanic risks in real time, it isfundamental to reinforce the present continuous GNSS network and extend it to all the active volcanic systems
in the archipelago. A new network with 30 new GNSS stations is being prepared which will enable high rate
data acquisition (50Hz). Detailed source process of the seismo-volcanic phenomena including co-seismic rupture
propagation and time-dependent magma emplacements will be better constrained in the Azores.

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