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Seismically-Tectonic Characteristics
Seismically-Tectonic Characteristics

Dubrovnik region is seismically and tectonically very active.By its seismic and tectonic potential it is also the most striking area in Croatia with estimated maximum possible magnitude of 7.5.

The region around Dubrovnik is very active in seismically-tectonic sense. Relatively high concentration of earthquakes and particularly phenomena of high intensity earthquakes indicate to permanently present tectonic activity. According to geologic classifications this region is located in the area of contact between regional structural units the Adriatic and the Dinaric. Along direct borders of these units the fault Ploče – Dubrovnik is stretched. Between Mljet and Dubrovnik's seabed the unit Adriatic adjoins the Adriatic micro-plate. In the seismically-tectonic sense the whole area of contact of these units is the most active in the structural system.

A significant space compression has been present in recent tectonic activity. It stipulates reverse relations and moves of structures. On the surface two structural units are separated in the enclosed part of the Dinaric and one in the area of the Adriatic. Due to reverse relations within them certain series of reverse structures differ. In seismically-tectonic studies the structural relations in depth are essential. Basic relations discern from gravimetric data. The most important are two data. Before all, these are position and extensions of gravimetric maximum and minimum. The noticeable maximum appears in the area of highest compression in the back of extension of zone at the fault Ploče – Dubrovnik. The biggest minimum denotes zone extension where the rocks' complexes are sinking in depth. The second data is related to the appearance and extension of zone of higher gravimetric gradients. It certainly shows the area of precipitated moves of rocks' complexes with different density in depth. The zone of gradients is parallel to extension of the fault Ploče – Dubrovnik and indicates to the most active sections of the fault and particularly noticeable reverse moves between Pelješac and Dubrovnik. It is necessary to point out that the fault Pelješac – Dubrovnik and the faults parallel to it in depth are joined together with the fault Ploče – Dubrovnik. This seismically-tectonic most active region also includes branches and most probably faults adjacent to series of reverse structures.

It is necessary to point out that certain structures and faults are directly reflected in the relief. Dependence of relief forms and structural relations indicate to permanent presence of tectonic activity. For example, large and local structures are represented in mountainous relief. Reverse relations and moves create asymmetric relief. Certain faults and moves stretch along steep slopes and noticeable inclines.

In the structural system the most important is the fault Ploče – Dubrovnik. It is represented by zone which width is 500-2.500 metres. Faults in zone are reversed in moves of wings towards J and JJZ, with inclination at the surface and 75°. In spurs most frequently there are parallel faults raked in the space.

In tectonic moves the compression of space is dominant. The rocks in depth become creased and faulted and reversely move along the fault which they support to the surface. These processes are accompanied by occurrence of earthquakes.

For observing tectonic activities it is most important to determine moves of big and local structures in the wings of faults. It is necessary to emphasize that in conditions of space compression between rocks' complexes that form certain structure, a field of stress is being established. Maximal compression stress shows basic relations of structures and general state of structural framework. Its orientation is between 7-187? and 15-195?. It is opposite or almost opposite to extension of faults and structures which points to present noticeable space compression and reverse moves of structures in the wings of fault. Opposite orientation of stress points to seismically-tectonic most active sections of fault.

Causes of earthquakes are tectonic movements which starting point is situated in the moves of the Adriatic micro-plate. Additionally the micro-plate pushes forward parts of the unit Adriatic. Due to resistance to these moves it is formed relatively wide space of noticeable compression along the touch of units the Adriatic and the Dinaric. Seismically-tectonic profile has been done on the basis of interpretation of deep seismic reflexive profile. Firstly, positions of certain faults in space are observed then leaps of rocks' complexes in their zones. The highest leap is present along the fault Ploče – Dubrovnik and its largeness is about 4 km in the under surface part of the Earth's crust. In profile added focuses of earthquakes are joined to the certain zones of fault. It has been distinguished the occurrence of earthquakes in depths between 5 and 15 km. The highest concentration of earthquakes' focuses is placed in depths between 3 and 20 km. The largeness of seismically-tectonic active space during the earthquake of higher intensity could be best observed towards the period of activity after the earthquake with epicentre near Slano in 1996, which magnitude was 6,0.