• Lagios E, Sakkas V., Parcharidis Is. & Dietrich V. (2005). Ground Deformation of Nisyros Volcano (Greece) for the period 1995-2002: Results from DInSAR and DGPS observations.Bull.  Volcanology, 68, 201-214.

Suitable interferometric pairs of SAR images have been selected (table 1) after searching European's Space Agency FRINGE according to the following criteria:

• Bp < 100 m
• Acquisitions of the same season to avoid seasonal land use changes
• Temporal separation between the scenes of the same pair covering the periods 1996/1999 and 1999/2000

Table 1
Technical characteristics of interferometric pairs:

DATE	ORBIT	FRAME	PASS	SATELLITE	BP (m)
22-MAY-1995	20135	2871	DESCENDING	ERS-1	85
22-JAN-1996	23642	2871	DESCENDING	ERS-1
¦ 4 months overlapping
04-SEP-1995	21638	2871	DESCENDING	ERS-1	58
20-AUG-1996	6975	2871	DESCENDING	ERS-2
¦ 2 months overlapping
11-JUN-1996	5973	2871	DESCENDING	ERS-2	44
06-JUL-1999	22005	2871	DESCENDING	ERS-2
¦ 5 months gap
05-DEC-1999	24188	729	ASCENDING	ERS-2	104
10-SEP-2000	28196	729	ASCENDING	ERS-2

May 22, 1995 -> January 22, 1996 Image 1 (600x493 jpg, 90 Kb)	September 04, 1995 -> August 20, 1996 Image 2 (600x476 jpg, 86 Kb)
June 11, 1996 -> July 06, 1999 Image 3 (600x451 jpg, 88 Kb)	December 05, 1999 -> September 10, 2000 Image 4 (600x451 jpg, 90 Kb)

The differential interferometric SAR processing and analyses were based on Atlantis software. At first the DEM of the island was created with a resolution of 4m/pixel by digitizing the contour lines from the topographic 1:5,000 scale map. As a next step, the initial orbit state vectors have been taken from the Delft Institute (NL) for Earth-Oriented Space Research (DEOS) for both pairs. DEOS precise ERS-2 orbits are based on the DGM-E04 gravity field model and the SLR and OPR2 altimeter crossovers and normal points (Scharroo & Visser 1998). In the coregistration step the master and slave scenes were validated; a coregistration refinement has also been performed, between the two scenes for each pair, of bilinear polynomial order, and the orbit/earth geometric analysis has been calculated. The input master scene and the external DEM were coregistered by displayed the external DEM as a simulated SAR image. The finally obtained RMS of the co-registration was almost half of pixel in the column direction and in the row direction. After the generation of a first (raw) interferogram, the flat Earth phase and topographic phase were subtracted. A phase coherence map has been created and the raw interferogram has been enhanced. It is deduced from the coherence images that the south-western and eastern part of the island shows a fairly good coherence, while the coherence is low to very low at the rest of the island. The areas showing good coherence image correspond to Nikia rhyolites, Prophitis Ilias dacites and lavas, while coherence is lost in the other rock-cover types, like pyroclastics, pumice etc. Additionally, the typical volcanic landscape of the island forms steep slopes in the caldera area, where the signal is not depicted by the satellite sensor (areas under shadow) except for the coherence image generated by the ascending pair where a significant part of the caldera show good coherence.

Summary on crustal deformation is as follows;

• Image 1
  The produced interferometric image from the first interferometric pair, covering the period 22-May-1995 to 22-January-1996, coincided with the beginning of the seismic unrest period in Nisyros Island. One interferometric fringe, equivalent to 28 mm of ground deformation along the slant direction, was recognized and evaluated. Most of the island is covered by a circular shape fringe, which extended from the NW part, covers the southern island and dying out (due to poor coherence) on the NE part of Nisyros. A second fringe located on the SW extremity of the island can also be recognized. The two fringes presented different patterns of deformation, without being connected to each other. The contact area of the two fringes coincided with the presence of a fault orientated in N-S direction.

• Image 2
  The interferogram for the period September 04,1995 to August 20, 1996 had an overlapping period of four months with the previous described one. This pair covered the commencement of the seismic activity that started during the summer of 1996. Two fringes (=56 mm of ground deformation) were recognized covering most of the island. Nevertheless, the two fringes presented the full range of colors (RGB= 1 fringe) only on the west and southwest part of the island, while in the southeastern part only 1 ½ of fringes was evident.

• Image 3
  Almost two interferometric fringes on the western and southern part of Nisyros, were respectively recognized and evaluated for the period 11-June-1996 to 06-July-1999, corresponding to 56 mm of ground deformation. These two zones coincide with the main tectonic fractures of the island. At the eastern part only 1 ½ fringe is observed. A very distinctive aspect appeared in the southern part where the fringes presented an abrupt interruption and shift of the color continuation, may due to the presence of the main faulting zone F1 with direction NNE-SSW that crosses the island.

• Image 4
  The picture is different though for the last interferometric pair 05-December-1999 to 10-September-2000. Three interferometric fringes may be identified, with direction almost linear (NNE-SSW) covering the whole of the island. From DGPS measurements during that period it can be stated that the fringes covering the eastern part of the island may be attributed to uplift that took place that period, while the fringes at the central and western part may correspond to subsidence.

(a) Observed (black arrows) and calculated (red arrows) horizontal displacements deduced from DGPS measurements for period 1997-2001. Calculated (red) arrows result from the combination of two Mogi sources (in-land and off-shore) and fault (red line) forward modelling. (b) Same as (a) but for vertical displacements.

Synthetic interferometric image of the Nisyros-Yali area deduced from two Mogi point sources: One located below Nisyros and the other off-shore north of Nisyros.

REFERENCES:
GEOWARN Project (www.geowarn.org)

Surface displacement model of Nisyros Volcanic Field deduced from DInSAR analysis & DGPS measurements.
Lagios E., Sakkas V., Parcharidis Is. & Vassilopoulou Sp.
Proc. 11th General Assembly of Wegener Project, Sept. 2002, Athens, Greece.

Will Nisyros Volcano (GR) Become Active? Seismic Unrest and Crustal Deformation.
Lagios, E., Dietrich, V., Stavrakakis, G., Parcharides, I., Sakkas V. & Vassilopoulou S.
European Geologist 2001, Vol. 12, 44-50.