Harokopio University of Athens Department of Geography

Deformation time series monitoring of Nisyros Volcano (Greece) during unrest and rest period.

1 Harokopio University of Athens, Department of Geography, El. Venizelou 70 Kallithea, 17671 Athens, Greece, gs20953@hua.gr, parchar@hua.gr, benekos@hua.gr2 National Research Council of Italy, Rome, Italy, Elena.Papageorgiou@idasc.cnr.it

Derdelakos K.1, Papageorgiou E.2, Parcharidis I.1, Benekos G. 1

AbstractThe Hellenic Volcanic Arc, which stretches from Aegina and Methana in the west over Milos and Santorini and terminates at Kos andNisyros in the east of Aegean Sea, is a result of northeastward-directed subduction (started around 4 Ma at the beginning of Pliocene) ofthe 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 1996and 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 aslong-term movements as is the ground deformation in volcanoes, landslides and subsidence. This technique can be used to identifyground-surface deformation associated with movement of magma and/or hydrothermal fluids beneath volcanoes. Ground deformationmonitoring 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 theevolution of ground deformation during the unrest period (1996-1997) and the post period of 2003-2010. Two different data sets ofcommon acquisition geometry of radar scenes covering the mention periods were used.Specifically, SAR data obtained from ERS-1 & -2 and ENVISAT satellites were used with a total lifespan of 18 years. Concerning the unrestperiod 18 ERS-1 & -2 SAR SLC scenes in ascending orbit, 46 in descending covering the period of 1992-2001. For the rest period 27ENVISAT ASAR SLC (ERS like) scenes in ascending orbit and 23 in descending covering 2003-2010. Due to the inability to find sufficientnumber 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 measureground deformation time-series on pixel basis.

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Fig. 1 Location of the study area

Fig. 2 Tectonic map of Nisyros island (Source: Geowarn project)

UNREST PERIOD (1995-1997) TIME SERIES INTERFEROMETRIC MONITORING

REST PERIOD (2004-2010) TIME SERIES INTERFEROMETRIC MONITORING

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

Acknowledgements: SAR data were provided by ESA in the frame of CAT-1 project, id:11653, also the authorswould like to thank Dr Michael Foumelis, ESA/ESRIN for his support during interferometric processing.

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Fig.7: Uplift zone during unrest period Fig.8: Downlift zone during rest period

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Fig. 3: Unwrapped interferograms(28mm color/cycle)

Fig. 5: Unwrapped interferograms(28mm color/cycle)

Fig. 4: Time series interferogramsnetwork

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Profile graph 1a Profile graph 1b

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Fig. 6: Time series interferogramsnetwork

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Observations: Maximum uplift is observed within the caldera ofabout 5 cm and also uplift is observed in the area of Mandrakivillage. The East and SW coasts show downlift up to 6 cm. Thereference point is located between the caldera and mandrakivillage at the edge of the ring

Observations: Downlift within the caldera showing locally values up to 6cm. The subsided Is extended from the southern coast of the islandcovering the caldera and follows a NNW trend to Mandraki. On the otherhand the SE coast is uplifted.