Accurate locations of 961 shallow and intermediate depth earthquakes, which occurred between 1956 and 1995 in the Hellenic arc (34-39°N, 19-28°E), are used to define the plate boundaries in this area. Reliable fault plane solutions for 77 shallow and intermediate depth earthquakes are also used in order to define the interaction between the different plates in the arc. An ocean-continent type of interaction occurs on a curved surface, which is defined by the shallow branch (20-100 km) of the Wadati-Benioff zone. The intersection of this zone with the earth's surface is a curve which follows the convex (outer) side of the sedimentary arc (western Peloponnese-west of Cythera-south coast of Crete-east coast of Rhodes) and dips at low angle (∼30°) to the Aegean sea. Coupling between the subducted oceanic crust and the overriding of the Aegean lithospheric plate takes place on this surface. The deep branch (100-180 km) of the Wadati-Benioff zone is dipping freely (without coupling) at a high angle (∼45°) beneath the south Aegean trough and the volcanic arc. The high shallow seismicity ( h≤20 km) which is observed in the southwestern convex (outer) side of the arc (Ionian sea) is attributed to the fast southwestward motion of the Aegean plate. Location of strong deep earthquakes ( h>100 km) in the fore-arc area of the southwestern part of the Hellenic arc (west of Cythera) indicates that oceanic crust is 'destroyed' in this part of the Hellenic trench due to roll-back of the descending lithospheric slab.