Oceanographic and Geophysical Surveys

The Ocean Floor Mapping System Using Sea Beam
Sea Floor Seismic Reflection Soundings (Generation of a Local Velocity Model)
Seismic Monitoring (The Generation of a Regional Velocity Model)
Seismic Activity and Monitoring within the Kos - Yali - Nisyros - Tilos Volcanic Field
Gravity and magnetic surveys within the Kos - Yali - Nisyros - Tilos volcanic field
The GPS network

The GPS network

The Global Positioning System (GPS) has also been applied on Nisyros for the surveillance of the Kos - Yali - Nisyros - Tilos volcanic field. Since Nisyros is a small island, an accuracy of ±5 mm for the horizontal, and ±10 mm for the vertical direction is expected.

Fig.18 GPS Network for the Nisyros Volcano surveillance. National Kapodistrian University of Athens (Greece), Department of Geophysics and Geothermics (NKUA)
(Click on image to enlarge).

A geodetic GPS network consisting of 17 stations was established in the area in June 1997. Fifteen of these were established on Nisyros and two on Kos (Fig. 17 and 18). The settlement of the GPS stations in Nisyros was principally based on the need for a homogeneous coverage.

The mean distance between two stations is approximately 2 km. The stations established on Kos served two purposes: First, they contribute to the study of the ground deformation in a regional sense, and second, one station on the eastern part was used as the reference base for the rest of the stations.

Magma Chamber Inflation/Deflation Processes - The Mogi Point Source Model

In active volcanic environments, a magma chamber inflation/deflation process model is usually applied to the data. The “Mogi” point source model, which is based on an elastically expanded point source in a half space (called a “Mogi point source”), usually serves to explain the observed deformation in such cases. In the case of Nisyros, the "Mogi model" was applied in order to explain the observed deformation deduced by DGPS measurements (Fig. 19).

Theoretical relative displacements were calculated from the GPS points at varying depths and locations around the centre of the island. The residual of the observed and theoretical displacements was minimized by least squares fitting to find the best fit to the data.

The resulting source according to the best fit of the model was located at a point slightly shifted to the NW from the centre of the island, at a depth of 5.5 ± 0.5 km

The Mogi (red) arrows are expected to point radially outwards from the Mogi source (Fig. 19). It was found that the calculated Mogi displacements for the horizontal component (red arrows) seemed to generally fit quite well the observed displacements (black arrows) in the southern part of the island.

Fig. 19 The Horizontal deformation is represented with the arrows, while the vertical displacement is presented with colour contouring. National Kapodistrian University of Athens (NKUA)
(Click on image to enlarge).

When using the year 1997 as the reference point, the shape of the ground deformation does not significantly change, indicating an "opening up" of the island along the two major faulting zones F1 and F2. The deformation at the NE half of the island was almost to ESE, turning progressively to the SE at its southern part.

At the western half, the direction of deformation was clearly to the SW. Summarizing the results for the whole period of the measurements 1997-2002, a total horizontal displacement of 10-60 mm and vertical displacement ranging from 30-150 mm was observed.

A second Mogi point source was then considered off-shore to explain the observed discrepancies between the NE and NW part of the island.

Taking into consideration the tectonic structure of the sub-marine area north of Nisyros, the presence of major faulting zones north of Nisyros and south of Yali island, and the concentration of shallow seismic activity in that area, a second Mogi point source was finally adopted.

It is located at a depth of 6,500 ± 500 m, and 60 m of the radius of the expanding sphere associated with the source strength parameter. The deformation model at the NW part of the island may be improved by taking into consideration the observed fault motion that took place along the activated fault passing through Mandraki in the 1996 seismic crisis that caused damages to the houses by its distinct displacements.

The observed and calculated horizontal displacement for the period 1997-2001, using the Mogi point sources and fault movements are shown in Figure 19.

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