Harokopio University of Athens Department of Deformation time series monitoring of Volcano () during unrest and rest period. Derdelakos K.1, Papageorgiou E.2, Parcharidis I.1, Benekos G. 1 1 Harokopio University of Athens, Department of Geography, El. Venizelou 70 Kallithea, 17671 Athens, Greece, [email protected], [email protected] 2 National Research Council of Italy, Rome, Italy, [email protected]

Abstract The Hellenic Volcanic Arc, which stretches from and Methana in the west over and and terminates at Kos and Nisyros in the east of , is a result of northeastward-directed subduction (started around 4 Ma at the beginning of Pliocene) of the African plate beneath the Aegean microplate. Santorini volcano in the central sector of the arc and Nisyros in the eastern sector, are the two active volcanoes of the arc. Nisyros shows an unrest phase during 1996-97 accompanied by intensive seismic activity in the broader area at the beginning of 1996 and lasted through the end of 1997 and returned to the background level at the beginning of 1998. Spaceborne radar interferometry has already shown its ability in mapping ground deformation, like co-seismic deformation, as well as long-term movements as is the ground deformation in volcanoes, landslides and subsidence. This technique can be used to identify Fig. 1 Location of the study area ground-surface deformation associated with movement of magma and/or hydrothermal fluids beneath volcanoes. Ground deformation monitoring is one of the main parameters that should be considered to assess volcanic hazard. In the current study the multi-reference synthetic aperture radar (SAR) interferometric technique was applied in order to study the evolution of ground deformation during the unrest period (1996-1997) and the post period of 2003-2010. Two different data sets of common acquisition geometry of radar scenes covering the mention periods were used. Fig. 2 Tectonic map of Nisyros island Specifically, SAR data obtained from ERS-1 & -2 and ENVISAT satellites were used with a total lifespan of 18 years. Concerning the unrest (Source: Geowarn project) period 18 ERS-1 & -2 SAR SLC scenes in ascending orbit, 46 in descending covering the period of 1992-2001. For the rest period 27 ENVISAT ASAR SLC (ERS like) scenes in ascending orbit and 23 in descending covering 2003-2010. Due to the inability to find sufficient number of point-targets, natural or man-made, Persistent Scatterers Interferometry (PSI) technique failed to deliver acceptable results. Therefore, in order to reveal the deformation history Singular Value Decomposition (SVD) method was applied, allowing to measure ground deformation time-series on pixel basis.

UNREST PERIOD (1995-1997) TIME SERIES INTERFEROMETRIC MONITORING

Fig. 3: Unwrapped interferograms Fig. 4: Time series interferograms (28mm color cycle) network

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REST PERIOD (2004-2010) TIME SERIES INTERFEROMETRIC MONITORING

Fig. 5: Unwrapped interferograms Fig. 6: Time series interferograms (28mm color cycle) network

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Our increasing understanding of processes inside volcanoes and their measurable effects like ground deformation during rest and unrest periods put predictions more and more on a deterministic basis. The careful analysis of the history of a volcano is the most important method in assessing the long-term probability of the occurrence of an eruption. Specifically SAR interferometry provides crucial inputs about spatio-temporal evolution of ground deformation over a volcanic area.

Acknowledgements: SAR data were provided by ESA in the frame of CAT-1 project, id:11653, also the authors would like to thank Dr Michael Foumelis, ESA/ESRIN for his support during interferometric processing. Fig.7: Uplift zone during unrest period Fig.8: Downlift zone during rest period