• Lagios E., Sakkas V., Novali F., Bellotti F., Ferretti A., Vlachou K. & Dietrich V. (2013). SqueeSAR^TM and GPS Ground Deformation Monitoring of Santorini Volcano (1992-2012): Tectonic Implications. Tectonophysics,  doi:10.1016/j.tecto.2013.03.012.

Santorini, one of the largest Quaternary volcanoes of South Aegean, is comprised by Thera, Therassia, Palea & Nea Kammeni Islands and Aspronisi tuff ring. Volcanism in the area started 2 ma ago, while hundreds of eruptions followed. The interior area, bounded by the island system, is covered by a flooded, almost circular caldera, formed during the Minoan eruption (1645 B.C.)

The submarine crater of the Columbo volcano, just 7 kilometers from the island of Thira and part of the same volcanic complex caused a Plinian volcanic eruption in 1650 (Fouqué 1879), causing a powerful tsunami.

The last significant volcano activity of Santorini was observed between 1925 and 1950, with four explosive periods.    

Santorini GPS network was established in summer of 1994, consisting of twenty one (21) stations distributed in the island system of the area, to study the ground deformation associated with probable upward magma motions. The network was remeasured in 1995, 1996, 1998, 2005 and 2011. The results reveal significant ground deformation of both subsidence and uplift of the area. Furthermore, a correlation between the tectonic structure and the observed deformation is finally achieved.  

The system has been in a quiet state the last 60 years, until January 2011 when the volcano reawakened, starting producing higher seismicity rates and showing a radial ground deformation. In the following, the ground deformation is presented since 1992, based on the advanced SqueeSAR analysis, local DGPS network and continuous recording GPS (CGPS) stations on the island. The spatial deformation of the System during the "quiet" period 1992-2010 deduced by joint interpretation of ERS1&2 and ENVISAT images of ascending and descending geometry. This period, Palea Kammeni was getting uplifted (2-3 mm/yr), characterized by an increasing rate of uplift, whilst the adjacent Nea Kammeni was subsided (up to -3 mm/yr) with increasing rates. The above two islets even though they are considered as a single volcanic center they exhibit a different type of vertical motion. The rest of the study area showed a velocity field varying from -1 to +2 mm/yr and sub-millimeter acceleration field values, indicating a linear deformation during this period. Combining ascending and descending radar data, the vertical and horizontal (E-W) component of the velocity field were determined. Several deformation patterns were identified: The two main tectonic features, the Columbo and the Kammeni lines at the northern and central part of the system, the Alpine basement at the SE part of Thera, and a pattern associated with the graben basin at the southern part, consistent to earlier AMT work and the recently compiled Gravity Anomaly Map. The remeasurement periods of the GPS network (established in 1994) resulted in the calculation of the strain field, and velocity deformation patterns consistent to the SqueeSAR analysis results.

The deformation study for the “active” period 2011-2012 was based: (i) on the SqueeSAR analysis of ENVISAT images of ≈39 deg LOS angle, and (ii) the processing of CGPS and DGPS data. Mogi point-source modelling suggests an expanding (magmatic) sphere (radius ≈150m) located ≈1.5km north of Kammeni at a depth of 3.7(±0.3) km, matching the observed displacement vectors. However, the seismicity seems to have lately drastically decreased, and a velocity decrease at some CGPS station components is also noticed. If that situation continues, the volcano may not reach any further state of alert in the near future.

In July 2011 a Continous GPS station was establised in Imeroivgli to better monitor the ground deformation, together with six (6) CGPS sites of the UNAVCO operated by  Georgia Institute of Technology and University of Patras.

 A poster  presenting the results from the ground deforamtion monitoring in Santorini was presented in ESA's conference: 21-23 May 2012

GPS ground displacement maps (mm) for the period Sept. 2011 to June 2012, referred to (a) No. 07 local reference station; (b) ITRF2008; (c) Strain field (μstrain) deduced from the DGPS measurements; (d) Diagram showing comparison between GPS vectors projected on the LOS direction and the corresponding PS/DS vectors (rms=7.3 mm) 

(a) GPS ground deformation map for the period June to Sept. 2012 (ITRF2008); (b) GPS ground deformation map for the period Sept. to Dec. 2012 (ITRF2008).

The daily processing of SANT CGPS station and another 6 CGPS stations (established by Georgia Institute of Technologt (USA) and University of Patras (Gr.), UNAVCO sites)
shows a decrese in the velocities since February 2012.

  

REFERENCES:

• Lagios E., Sakkas V., Novali F., Bellotti F., Ferretti A., Vlachou K. & Dietrich V. (2013). SqueeSAR^TM and GPS Ground Deformation Monitoring of Santorini Volcano (1992-2012): Tectonic Implications. Tectonophysics,  doi:10.1016/j.tecto.2013.03.012

• Novali F., Bellotti F., Giannico C., Lagios E., Sakkas V., Vlachou K. (2012). Ground Deformation of Santorini Volcano (1992-2012) based on SqueeSAR^TM Analysis and GPS Measurements,  International Forum on Satellite Observation for Geo-Hazard Risk Management,  Santorini 21 -23 May 2012,  European Space Agency (ESA)