Dimitris Paradissis

Geodetic estimate of seismic hazard in the Gulf of Korinthos

The recent 15 June 1995, M0 = 6.0 × 1018 N m, Aigion earthquake in the western Gulf of Korinthos has focussed attention on the seismic hazard of the region. Although there have been few large earthquakes in the region during this century, the historical record suggests that there may have been many large earthquakes there in the interval 1750–1900. We present geodetic data that give estimates of the rate of extension of the Gulf of Korinthos during this century and which suggest that less than half of the elastic strain in the central and western Gulf of Korinthos has been released by earthquakes during this century. In contrast, the seismic and geodetic strains in the eastern Gulf of Korinthos are in agreement with each other. If the discrepancy between seismic and geodetic strains in the western Gulf of Korinthos that has accumulated during this century is removed in earthquakes, the moment release will be equivalent to several Ms > 6.5 earthquakes.

Ground deformation at Nisyros volcano (Greece) detected by ERS-2 SAR differential interferometry

Nisyros is a volcanic island in the south-east of the Aegean volcanic arc. Over the period 1995-1998, the volcano exhibited intense seismic activity without eruption. Production and analysis of satellite radar interferograms, spanning period 1995-2000, showed continuous inflation from 1995 until May 1998 and deflation from mid-1998 to 2000, with the same deformation centre situated at the north-west part of the volcano. Assuming the crust to be an elastic homogenous medium, point source modelling of the deformation field indicates the source of deformation to be located at five kilometres depth beneath the north-west edge of the island. Recorded seismicity trend patterns suggest magma intrusion at depth as the source of the inflation sequence. The recorded migration of hypocentres indicates a probable fluid transport affecting the shallow aquifers resulting in the volcanos deflation.

Permanent Scatterer InSAR Analysis and Validation in the Gulf of Corinth.

The Permanent Scatterers Interferometric SAR technique (PSInSAR) is a method that accurately estimates the near vertical terrain deformation rates, of the order of ∼1 mm year-1, overcoming the physical and technical restrictions of classic InSAR. In this paper the method is strengthened by creating a robust processing chain, incorporating PSInSAR analysis together with algorithmic adaptations for Permanent Scatterer Candidates (PSCs) and Permanent Scatterers (PSs) selection. The processing chain, called PerSePHONE, was applied and validated in the geophysically active area of the Gulf of Corinth. The analysis indicated a clear subsidence trend in the north-eastern part of the gulf, with the maximum deformation of ∼2.5 mm year-1 occurring in the region north of the Gulf of Alkyonides. The validity of the results was assessed against geophysical/geological and geodetic studies conducted in the area, which include continuous seismic profiling data and GPS height measurements. All these observations converge to the same deformation pattern as the one derived by the PSInSAR technique.

A method for minimising of low frequency and unwrapping artefacts from interferometric calculations

On 7 September 1999 a magnitude MW = 5.9 earthquake occurred in the Athens area of Greece producing a subsidence of 6–7 cm detected by radar interferometry. This study introduces a processing technique, which produces a clear deformation pattern of the earthquake, mostly released from artefacts due to orbital effects, unwrapping cycle slipping errors and atmospherics disturbances. A set of 17 ERS‐1 and ERS‐2 SAR images acquired between December 1997 and January 2001 has been used. The contribution of each artefact to the interferograms was calculated, both in the frequency and spatial domains.

Inflation/deflation sequence on the Nisyros active volcano, Greece, during 1995-2000 issued from SAR differential interferometry

Nisyros is a poorly known Quarternary volcano, south-east of the Agean volcanic arc. It is characterized by periods of intense seismic activity and paucity in reruptive episodes, sometimes accompanied by hydrothermal explosions. The most recent unrest episode lasted from 1995 to 1998, without eruption. Radar interferometry has been applied in order to study the evolution of the deformation from 1995 to 2000. Observations show a continuous uplift of 140mm during 1995-1997. At mid 1998, the movement trend changes into a slower surface deflation til 2000. Maximum crust deformation is constantly observed at the north-west part of the island, where most earthquake epicenters are located and in agreement to three GPS campaigns conducted between mid 1997 and mid 1998. We solve for the best-fit Mogi point source and best-fit Okada rectangular dislocation of the observed deformation field. Mogi model indicates a source at 5km depth beneath the north-west edge of the island, with a maximum deformation amplitude at surface of 0.14 plus or minus 0.02m and a total volume change of 26 plus or minus 4 × 106 m3, during 1995-1997. The Okada model indicats a dike solution 2km long, 2.2km wide, wiht a 4m opening and a 30° dip. The upper center is at 6.4km depth and the volume change, also during 1995-1997, is 17.6 × 106 m3. Each solution is discussed on the potential controlling mechanism resulting to the volcanos inflation/deflation sequence.