The continuity of active fault systems in Greece

Evidence from geomorphology, the distribution of large earthquakes, and geodetic measurements suggests that the active faulting in mainland Greece and the north Aegean Sea is concentrated into a small number of discrete, linear zones that bound relatively rigid blocks. On land, the zones are most clearly identified where the faulting is associated with large escarpments, particularly in hard footwall rocks. The zones can become indistinct and diffuse near their ends, sometimes because they enter high, unstable topography where landsliding obscures the fault morphology, but mostly because the extension rates decrease along strike as a consequence of the relative rotation of the blocks they bound. The main graben systems of Chalkidiki, southern Thessaly and the North Gulf of Evia all seem to connect with the strike-slip faulting in the offshore Aegean and to die out in the west, a configuration related to clockwise rotations in east-central Greece. By contrast, the Gulf of Corinth, which is the fastest opening graben system in Greece, opens more rapidly in the west than the east. Both these features of the Gulf of Corinth are consequences of the motion of the south Aegean and Peloponnese as a single block. Late Pliocene and Quaternary geology and geomorphology indicate that the boundaries of the rigid blocks in central Greece have changed over that time, with faulting migrating into the hanging walls, sometimes changing in orientation. These changes are probably related to the way faulting in the seismogenic layer adapts to block rotation, so as to maintain the general features of a velocity field that is controlled by larger scale effects, such as buoyancy forces and the seaward migration of the subducting slab beneath Crete. The image of Greece as a mosaic of rigid blocks whose boundaries change with time is a useful framework for seismic hazard evaluation, but we emphasize that some moderate-sized earthquakes do occur away from the main fault zones, within the relatively rigid blocks and especially near their diffuse ends.