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

Seismic Monitoring (The Generation of a Regional Velocity Model)

Seismic monitoring is the key method in any geodynamically active volcanic field because of its wide range of application. For this reason, a continuous seismic monitoring system was established in the Kos - Yali - Nisyros - Tilos volcanic field consisting of nine land and six ocean bottom seismic stations (see Fig. 1).

The following aspects were considered as part of this section of the project:

1. Location of earthquake sources with an accuracy of better than a few hundred meters, in order to relate eruptive-tectonic behaviours. This resolution requirement constrains the type of sensors, i.e. to several three-component velocity-meters and ideally a spatial array of sensors.

2. Instrumentation was chosen to maximize the dynamic range, in order to be able to detect the faintest, negative magnitude earthquakes and correctly record larger earthquakes (magnitude 3 and over), or earthquakes closer to the location of the monitoring equipment.

3. Data, waveform or event characterizations were transferred to a central station and sent via dedicated telephone line to the Institute of Geodynamics in Athens. High precision algorithms were used to determine the earthquake parameters. Volcanic and tectonic shocks are discriminated in terms of their frequency content.

4. Analysis and interpretation facilities were set up to include near real-time automated procedures in order to follow the high data flow rates.

5. The minimum equipment proposed to be established permanently allows for the detection of earthquakes and, through continuous monitoring, the ability to follow space-time changes in activity. However, the understanding of the behaviour of the volcano and its plumbing system necessitates a characterization of sources such as focal mechanisms and source processes. The latter has been achieved by temporary deployment (several months) of over 15 seismic stations in addition to the permanent array.

Fig. 14 Regional velocity model down to 30 km depth, shown as cross-section of the Kos-Yali-Nisyros-Tilos volcanic field. University of Hamburg (UHIG), GeoPro and NOAIG
(Click on image to enlarge).

The Regional Seismic Velocity Model (Fig. 14)

The sedimentary succession is typically 4 to 5 km thick. The sedimentary layers are flat laying but show significant dips in the Kos-Nisyros volcanic area. The 160,000 year old Kos Plateau Tuff volcanic caldera could be conclusively identified by the geometry of the sedimentary sequences, which cover an area of 30 x 40 km with a depth between 0.8 to 1.0 km.

The volcaniclastic sedimentary filling ranges from 3.5 to 5 km. The basement geometry is strongly influenced by the volcanic activity and is strongly depressed below the caldera region. The crust/mantle boundary (Moho discontinuity) varies between 22 and 25 km.

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