Reviewing and updating (1996–2012) ground deformation in nisyros volcano (Greece) determined by GPS and SAR interferometric techniques (1996–2012)

Abstract

The southeastern part of the South Aegean Volcanic Arc (SAVA), including Kos, Yali and Nisyros islands, is geodynamically very active. Intense seismic activity occurred on Nisyros Island during 1996–1998, accompanied with strong ground deformation and temperature increase of the fumaroles. Ground-deformation studies based on GPS and differential Interferometry for the period 1996–2000 revealed an “opening” of the island along the main faulting zones together with intense uplift of up to 90 mm that gradually reduced the following two years (2000–2002). Two Mogi point sources related to magmatic chambers fitted the observed ground deformation that was deduced from differential GPS and conventional Interferometry. One source is located onshore at about the centre of Nisyros, and the other is offshore at about 5 km to the north of Nisyros and 2 km to the southeast of Yali. Additional geophysical and geotectonic data support the existence and the location of these two magma chambers. Remeasurement of the Nisyros-Yali GPS network in 2012, combined with the analysis of SqueeSAR™ Interferometric data from ENVISAT ascending and descending imageries (2003–2010), provided the spatial and temporal ground-deformation pattern after the period of the seismic crisis. The GPS results indicated intense subsidence (5–10 mm/year) in the northern and central parts of Nisyros that caused the western and eastern flanks of the island to “collapse” towards its centre, as is evident from the horizontal component of the GPS vectors. The observed line of sight (LOS) velocity field of the SqueeSAR™ analysis revealed a nearly linear type of ground deformation for that period, exhibiting values ranging between −3 and +3 mm/year with small standard deviations (<0.8 mm/year). The SqueeSAR™ results have generally shown a similar deformational pattern as the GPS one; subsidence is observed along the northern and central parts of Nisyros, while when comparing ascending and descending LOS components of ground velocities, an eastward horizontal component is inferred for the southeastern part of Nisyros. Motions along the main faulting zones that crosscut the island are also clearly identified. Both analyses have shown that the ground deformation on Yali may be associated with a more regional subsidence that takes place to the south and offshore of the islet, as is clearly indicated from the horizontal component of the GPS vectors. The overall pattern of subsidence in the broader area of Nisyros and Yali after 2000 is consistent with the two modelled Mogi sources, and indicates that the pressure inside the two associated magma chambers and the hydrothermal system has likely decreased.