Tectonics

COMSHELFRISKS: Promoting a Combined Approach to Investigating Risks of Earthquakes, Landslides, and Tsunamis in Coastal, Shelf, and Continental Slope Areas

For the Scientist

2. Preliminary geodynamic interpetation of the new seismological data fom the southwestern part of the Caspian Sea

The southwestern part of the Caspian adjacent to the Dagestan and Azerbaijan coast and limited from the south by the Apsheron sill is known as a unique area in the Caucasus region where deep (up to 100km) mantle seismisity occurs. Traditionally this seismicity was attributed to underthrusting or even subduction of the South Caspian crust under the continental crust of the Central Caspian belonging to the margin of the Scythian Platform [Khallov et al., 1987]. This conception was stongly supported by deep seismic soundings which demonstrated that the crust of the South Caspian basin lacks “granitic layer” and may be of the oceanic type. According to the pre-existing seismological information, a north dipping stismofocal zone was interpreted below the Apsheron sill [Ulomov & Polyakova, 1999].

Recent studies with a set of bottom seismographers (OBS) conducted by Shirshow Institute of Oceonology RAS in the Yalama – Samur area of SW Caspian revealed a number of weak earthquakes with hypocenters up to 150km deep [Fig 6].

Fig. 6. Area where weak earthquakes (up to M-3,5) were registred by bottom seismographers (square) and possible interpretation of observed data. .
1 – fault; 2 – overthrust; 3 – strike-slip fault; 4 – basic (oceanic-type?) crust; 5 – direction of underthrusting; 6 – the same hypothetical.

Using this data a seismofocal zone was constructed dipping to southwest, i.e. in the opposite direction as previously suggested.

A special review of the active strctures in the Caucasus region was attempted to see how the new interpretation fits data on the surface geology. Together with seismological data the structural and geodynamic analysis was published in “Geotectonics”. It was demonstrated that no structural deformation can be directly correlated with the proposed seismofocal zone. Structures of the Southeastern Caucasus and the Apsheron sill have southern vergensy compatible with the northward underthrusting of the Transcaucasus and South Caspian. There is a cosiderable overthrusting of the Caspian basin from the West by the Caucasus structures of Dagestan. However the magnitude of deformatijn here hardly correlates with the deep underthrusting of the lithospheric slab, unless an independent sinking of the lithospheric mantle is accepted. It becomes clear that further seismological studies are necessary to collect more data. Addinional OBS surwey was conducned and the results are being processed now. Solution of the problem is important for re-asssessment of seismicity related risks in the region of active tectonics.

Previous 1 2 3 4 5 Next