Discussion-Conclusions
The detailed neotectonic study and complementary seismotectonic analysis show a compatible, almost identical picture of the ongoing active processes that control the normal fault generation on Western Crete, which corresponds to the southwesternmost part of the Hellenic Arc area. After the neotectonic analysis we conclude that two distinctive stress phases took place since Middle Miocene to present. The first one (D1, Mid-Upper Miocene to Lower Pliocene) exhibits a N-S extension and was responsible for the formation of the dominant E-W trending normal faults that control the shape of the large Neogene basins of Western Crete. The second one (D2, Lower Pliocene to present) exhibits a E-W extension, forming mainly N-S faults, but also NE-SW trending smaller faults with significant strike-slip component, often acting as transfer zones between the larger N-S fault zones. Moreover, some large E-W faults from the previous D1 phase are often re-activated, during the later D2 phase, as almost pure strike-slip faults. Neotectonic and seismotectonic fabric is strongly controlled by the dynamics of N-NNE-ward subduction processes of the African plate under the Crete island which belongs to the forearc area of the Hellenic island arc.
This pattern is in excellent agreement with the seismological information, that show mostly N-S normal fault mechanisms, as well as NW-SE and NE-SW strike-slip mechanisms with significant normal component. Moreover, the determined active stress field from fault plane solution data also shows a dominant E-W extension, suggesting that the later D2 phase identified by field fault data is an on-going active process.
The neotectonic information was jointly evaluated together with the available seismological data in order to create a more complete picture about the seismic hazard of the study area. Using a combination of field data and seismicity information, 13 large major fault zones were defined and studied for the area, with nine of them characterized as active or possible active faults. These nine faults were incorporated in a detailed seismic hazard analysis, which also took into account the large thrust fault zones of the outer Hellenic arc, as well as the Benioff-zone intermediate-depth events.
The combination of both a probabilistic and a deterministic approach for seismic hazard evaluation, allowed not only the reliable estimation of the seismic hazard for several selected sites but also the effective de-aggregation of the seismic hazard results. The obtained hazard estimates show a significant spatial variation of seismic hazard, mainly due to the different effect of the studied faults on each site. Furthermore, the role of the on-land, mainly normal, active faults, in comparison to the large thrust fault of the outer Hellenic arc varies significantly. More precisely, the seismic hazard of the westernmost part of study area is mainly controlled by the neighboring higher seismicity neotectonic faults, while the corresponding hazard of the central and easternmost part is mostly affected by the large outer arc thrust faults (e.g. Elafonisos fault). These results suggest that this combined interpretation of field (neotectonic) data with seismological information is essential, not only for the better understanding of the ongoing seismotectonic processes but also for the efficient and reliable estimation of seismic hazard and the corresponding probabilistic or deterministic analysis.